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Map 数据结构</a></li><li><a href="/blog/pages/f56ec2ab97d60483/" class="sidebar-link">Proxy</a></li><li><a href="/blog/pages/74de3e45e4491e95/" class="sidebar-link">Reflect</a></li><li><a href="/blog/pages/2810ae8985e9bd52/" class="sidebar-link">Promise 对象</a></li><li><a href="/blog/pages/48df907ad3570f3d/" class="sidebar-link">Iterator 和 for-of 循环</a></li><li><a href="/blog/pages/718b48ed9ce0adce/" class="sidebar-link">Generator 函数的语法</a></li><li><a href="/blog/pages/75af7031eb66847b/" class="sidebar-link">Generator 函数的异步应用</a></li><li><a href="/blog/pages/3777253e65bac487/" class="sidebar-link">async 函数</a></li><li><a href="/blog/pages/e831e1593c82bbe0/" class="sidebar-link">Class 的基本语法</a></li><li><a href="/blog/pages/83f8c3a0cd87dd83/" class="sidebar-link">Class 的继承</a></li><li><a href="/blog/pages/efe2fb04eb8ac5fb/" class="sidebar-link">Module 的语法</a></li><li><a href="/blog/pages/a79ca2e64ceae213/" class="sidebar-link">Module 的加载实现</a></li><li><a href="/blog/pages/984bf549204bb266/" class="sidebar-link">编程风格</a></li><li><a href="/blog/pages/32c35f7651d6e58e/" class="sidebar-link">读懂 ECMAScript 规格</a></li><li><a href="/blog/pages/16121351be68691b/" class="sidebar-link">异步遍历器</a></li><li><a href="/blog/pages/a2ba314746bfdbdd/" class="sidebar-link">ArrayBuffer</a></li><li><a href="/blog/pages/7188882b8d65af1b/" class="sidebar-link">最新提案</a></li><li><a href="/blog/pages/e97bc1e5626b082c/" class="sidebar-link">装饰器</a></li><li><a href="/blog/pages/1cf50330655efc69/" class="sidebar-link">函数式编程</a></li><li><a href="/blog/pages/6a8e2dc558da1b39/" class="sidebar-link">Mixin</a></li><li><a href="/blog/pages/8e8f80f69b775a56/" class="sidebar-link">SIMD</a></li><li><a href="/blog/pages/ea6f3b870f6dab69/" class="sidebar-link">参考链接</a></li></ul> </aside> <div><main class="page"><div class="theme-vdoing-wrapper "><div class="articleInfo-wrap" data-v-06970110><div class="articleInfo" data-v-06970110><ul class="breadcrumbs" data-v-06970110><li data-v-06970110><a href="/blog/" title="首页" class="iconfont icon-home router-link-active" data-v-06970110></a></li> <li data-v-06970110><a href="/blog/note/es6/#《ES6 教程》笔记" data-v-06970110>《ES6 教程》笔记</a></li></ul> <div class="info" data-v-06970110><div title="作者" class="author iconfont icon-touxiang" data-v-06970110><a href="javascript:;" data-v-06970110>阮一峰</a></div> <div title="创建时间" class="date iconfont icon-riqi" data-v-06970110><a href="javascript:;" data-v-06970110>2020-02-09</a></div> <!----></div></div></div> <!----> <div class="content-wrapper"><div class="right-menu-wrapper"><div class="right-menu-margin"><div class="right-menu-title">目录</div> <div class="right-menu-content"></div></div></div> <h1><img src="data:image/png;base64,iVBORw0KGgoAAAANSUhEUgAAAB4AAAAeCAYAAAA7MK6iAAAAAXNSR0IArs4c6QAABGpJREFUSA3tVVtoXFUU3fvOI53UlmCaKIFmwEhsE7QK0ipFEdHEKpXaZGrp15SINsXUWvBDpBgQRKi0+KKoFeJHfZA+ED9KKoIU2gYD9UejTW4rVIzm0VSTziPzuNu1z507dibTTjL4U/DAzLn3nL3X2o91ziX6f9wMFdh6Jvbm9nNSV0msViVO6tN1Rm7NMu2OpeJ9lWBUTDxrJbYTS0hInuwciu9eLHlFxCLCZEk3MegsJmZ5K/JD6t7FkFdEvGUo1g7qJoG3MHImqRIn8/nzY1K9UPKKiJmtnUqHVE3Gbuay6vJE/N2FEmuxFjW2nUuE0yQXRRxLiTUAzs36zhZvOXJPdX850EVnnLZkB8prodQoM5JGj7Xk2mvC7JB8tG04Ef5PiXtG0UtxupRQSfTnBoCy554x18yJHI6I+G5Eru4LHmPJZEQsrvPUbMiA8G/WgMK7w7I+ez7++o2ANfbrjvaOl1tFMs+htG3IrZH9/hDX1Pr8Tc0UvH8tcX29KzAgIGcEkINyW5BF9x891hw6VYqgJHEk0huccS7vh3C6gTiODL+26huuBtbct8eZnqLML8PkxGYpuPZBqtqwkSjgc4mB5gbgig5i+y0UDK35LMxXisn9xQtK+nd26gTIHsHe/oblK/b29fUmN/8Y+9jAQrnBp56m1LcDlDp9irKTExSKduXJVWSqdBMA08pEJnEIOB3FPPMybu/oeV8zFeYN3xx576Q6RH+VmplE4ncQV5v+5rzSoyOU7PuEAg8g803PwBJ0CExno/jcMbN8tONYeOmHiuUNryvm3fRUy4tMPVLdAGkUhNWuggGrJcXPv+ouCjz0MKUHz1J2/E8IC9nqTabcxgaBYM0hPhD5Y65FsbxRQKxCQrDjDctW7PUM3HuZunFyifSAqEfuzCp48Il24luWUWZoyJCaPR82jE0+kFA643wRFVni4RYSq3ohJO2pZ7B5dO4xkDWbEpossJPLSrPjYID8rS2UHTlvyNxqIGsg674XJJ7vnh5L7PNwC4hh2sjCI96mzszOTpxLF0T7l88Yz7lAuK6OnL8gXLOnTvpzSb22YG8W7us3jSebFHeeqnXRG1vt+MoUM84LQIBmMsCTAcOauTh0T0l0neQK7m2bLMt2mGxU3HYssS0J2cdv5wljlPsrIuZLAG/2DOZIXgCYT8uMGZN+e2kSirfxZOPCsC0f24nTZzspnVn9VePS1Z5vubmAGGXG8ZFno9Hel0yfA5ZPhF7Dh972BQJ2qCpgH67lmWtBYbvk6sz02wjky2vXyz0XErP/kFB619js1BtwfOV4OPRqOQBjy3Qbk18vigUPPSD5ceHnwck7W9bhAqZdd7SuG7w4/P2F/GaJh8c7e9qgow+Q7cGBo+98WsLkuktFqiZabtXuQTu/Y5ETbR0v7tNSFnvrmu6pjdoan2KjMu8q/Hmj1EfCO2ZGfEIbIXKUlw8qaX9/b2oeSJmFksSeT/Fn0V3nSypChh4Gjh74ybO9aeZ/AN2dwciu2/MhAAAAAElFTkSuQmCC">数值的扩展<!----></h1>  <div class="theme-vdoing-content content__default"><h1 id="数值的扩展"><a href="#数值的扩展" class="header-anchor">#</a> 数值的扩展</h1> <h2 id="二进制和八进制表示法"><a href="#二进制和八进制表示法" class="header-anchor">#</a> 二进制和八进制表示法</h2> <p>ES6 提供了二进制和八进制数值的新的写法，分别用前缀<code>0b</code>（或<code>0B</code>）和<code>0o</code>（或<code>0O</code>）表示。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token number">0b111110111</span> <span class="token operator">===</span> <span class="token number">503</span> <span class="token comment">// true</span>
    26. 

  26. <span class="token number">0o767</span> <span class="token operator">===</span> <span class="token number">503</span> <span class="token comment">// true</span>
    27. 

  27. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br></div></div> <p>从 ES5 开始，在严格模式之中，八进制就不再允许使用前缀<code>0</code>表示，ES6 进一步明确，要使用前缀<code>0o</code>表示。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token comment">// 非严格模式</span>
    28. 

  28. <span class="token punctuation">(</span><span class="token keyword">function</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">{</span>
    29. 

  29.   console<span class="token punctuation">.</span><span class="token function">log</span><span class="token punctuation">(</span><span class="token number">0o11</span> <span class="token operator">===</span> <span class="token number">011</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    30. 

  30. <span class="token punctuation">}</span><span class="token punctuation">)</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token comment">// true</span>
    31. 

  31. 

  32. <span class="token comment">// 严格模式</span>
    33. 

  33. <span class="token punctuation">(</span><span class="token keyword">function</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">{</span>
    34. 

  34.   <span class="token string">'use strict'</span><span class="token punctuation">;</span>
    35. 

  35.   console<span class="token punctuation">.</span><span class="token function">log</span><span class="token punctuation">(</span><span class="token number">0o11</span> <span class="token operator">===</span> <span class="token number">011</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    36. 

  36. <span class="token punctuation">}</span><span class="token punctuation">)</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token comment">// Uncaught SyntaxError: Octal literals are not allowed in strict mode.</span>
    37. 

  37. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br><span class="line-number">7</span><br><span class="line-number">8</span><br><span class="line-number">9</span><br><span class="line-number">10</span><br></div></div><p>如果要将<code>0b</code>和<code>0o</code>前缀的字符串数值转为十进制，要使用<code>Number</code>方法。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token function">Number</span><span class="token punctuation">(</span><span class="token string">'0b111'</span><span class="token punctuation">)</span>  <span class="token comment">// 7</span>
    38. 

  38. <span class="token function">Number</span><span class="token punctuation">(</span><span class="token string">'0o10'</span><span class="token punctuation">)</span>  <span class="token comment">// 8</span>
    39. 

  39. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br></div></div><h2 id="number-isfinite-是否为有限-number-isnan-是否为nan"><a href="#number-isfinite-是否为有限-number-isnan-是否为nan" class="header-anchor">#</a> Number.isFinite() 是否为有限, Number.isNaN() 是否为NaN</h2> <p>ES6 在<code>Number</code>对象上，新提供了<code>Number.isFinite()</code>和<code>Number.isNaN()</code>两个方法。</p> <p><code>Number.isFinite()</code>用来检查一个数值是否为有限的（finite），即不是<code>Infinity</code>。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code>Number<span class="token punctuation">.</span><span class="token function">isFinite</span><span class="token punctuation">(</span><span class="token number">15</span><span class="token punctuation">)</span><span class="token punctuation">;</span> <span class="token comment">// true</span>
    40. 

  40. Number<span class="token punctuation">.</span><span class="token function">isFinite</span><span class="token punctuation">(</span><span class="token number">0.8</span><span class="token punctuation">)</span><span class="token punctuation">;</span> <span class="token comment">// true</span>
    41. 

  41. Number<span class="token punctuation">.</span><span class="token function">isFinite</span><span class="token punctuation">(</span><span class="token number">NaN</span><span class="token punctuation">)</span><span class="token punctuation">;</span> <span class="token comment">// false</span>
    42. 

  42. Number<span class="token punctuation">.</span><span class="token function">isFinite</span><span class="token punctuation">(</span><span class="token number">Infinity</span><span class="token punctuation">)</span><span class="token punctuation">;</span> <span class="token comment">// false</span>
    43. 

  43. Number<span class="token punctuation">.</span><span class="token function">isFinite</span><span class="token punctuation">(</span><span class="token operator">-</span><span class="token number">Infinity</span><span class="token punctuation">)</span><span class="token punctuation">;</span> <span class="token comment">// false</span>
    44. 

  44. Number<span class="token punctuation">.</span><span class="token function">isFinite</span><span class="token punctuation">(</span><span class="token string">'foo'</span><span class="token punctuation">)</span><span class="token punctuation">;</span> <span class="token comment">// false</span>
    45. 

  45. Number<span class="token punctuation">.</span><span class="token function">isFinite</span><span class="token punctuation">(</span><span class="token string">'15'</span><span class="token punctuation">)</span><span class="token punctuation">;</span> <span class="token comment">// false</span>
    46. 

  46. Number<span class="token punctuation">.</span><span class="token function">isFinite</span><span class="token punctuation">(</span><span class="token boolean">true</span><span class="token punctuation">)</span><span class="token punctuation">;</span> <span class="token comment">// false</span>
    47. 

  47. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br><span class="line-number">7</span><br><span class="line-number">8</span><br></div></div><p>注意，如果参数类型不是数值，<code>Number.isFinite</code>一律返回<code>false</code>。</p> <p><code>Number.isNaN()</code>用来检查一个值是否为<code>NaN</code>。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code>Number<span class="token punctuation">.</span><span class="token function">isNaN</span><span class="token punctuation">(</span><span class="token number">NaN</span><span class="token punctuation">)</span> <span class="token comment">// true</span>
    48. 

  48. Number<span class="token punctuation">.</span><span class="token function">isNaN</span><span class="token punctuation">(</span><span class="token number">15</span><span class="token punctuation">)</span> <span class="token comment">// false</span>
    49. 

  49. Number<span class="token punctuation">.</span><span class="token function">isNaN</span><span class="token punctuation">(</span><span class="token string">'15'</span><span class="token punctuation">)</span> <span class="token comment">// false</span>
    50. 

  50. Number<span class="token punctuation">.</span><span class="token function">isNaN</span><span class="token punctuation">(</span><span class="token boolean">true</span><span class="token punctuation">)</span> <span class="token comment">// false</span>
    51. 

  51. Number<span class="token punctuation">.</span><span class="token function">isNaN</span><span class="token punctuation">(</span><span class="token number">9</span><span class="token operator">/</span><span class="token number">NaN</span><span class="token punctuation">)</span> <span class="token comment">// true</span>
    52. 

  52. Number<span class="token punctuation">.</span><span class="token function">isNaN</span><span class="token punctuation">(</span><span class="token string">'true'</span> <span class="token operator">/</span> <span class="token number">0</span><span class="token punctuation">)</span> <span class="token comment">// true</span>
    53. 

  53. Number<span class="token punctuation">.</span><span class="token function">isNaN</span><span class="token punctuation">(</span><span class="token string">'true'</span> <span class="token operator">/</span> <span class="token string">'true'</span><span class="token punctuation">)</span> <span class="token comment">// true</span>
    54. 

  54. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br><span class="line-number">7</span><br></div></div><p>如果参数类型不是<code>NaN</code>，<code>Number.isNaN</code>一律返回<code>false</code>。</p> <p>它们与传统的全局方法<code>isFinite()</code>和<code>isNaN()</code>的区别在于，传统方法先调用<code>Number()</code>将非数值的值转为数值，再进行判断，而这两个新方法只对数值有效，<code>Number.isFinite()</code>对于非数值一律返回<code>false</code>, <code>Number.isNaN()</code>只有对于<code>NaN</code>才返回<code>true</code>，非<code>NaN</code>一律返回<code>false</code>。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token function">isFinite</span><span class="token punctuation">(</span><span class="token number">25</span><span class="token punctuation">)</span> <span class="token comment">// true</span>
    55. 

  55. <span class="token function">isFinite</span><span class="token punctuation">(</span><span class="token string">&quot;25&quot;</span><span class="token punctuation">)</span> <span class="token comment">// true</span>
    56. 

  56. Number<span class="token punctuation">.</span><span class="token function">isFinite</span><span class="token punctuation">(</span><span class="token number">25</span><span class="token punctuation">)</span> <span class="token comment">// true</span>
    57. 

  57. Number<span class="token punctuation">.</span><span class="token function">isFinite</span><span class="token punctuation">(</span><span class="token string">&quot;25&quot;</span><span class="token punctuation">)</span> <span class="token comment">// false</span>
    58. 

  58. 

  59. <span class="token function">isNaN</span><span class="token punctuation">(</span><span class="token number">NaN</span><span class="token punctuation">)</span> <span class="token comment">// true</span>
    60. 

  60. <span class="token function">isNaN</span><span class="token punctuation">(</span><span class="token string">&quot;NaN&quot;</span><span class="token punctuation">)</span> <span class="token comment">// true</span>
    61. 

  61. Number<span class="token punctuation">.</span><span class="token function">isNaN</span><span class="token punctuation">(</span><span class="token number">NaN</span><span class="token punctuation">)</span> <span class="token comment">// true</span>
    62. 

  62. Number<span class="token punctuation">.</span><span class="token function">isNaN</span><span class="token punctuation">(</span><span class="token string">&quot;NaN&quot;</span><span class="token punctuation">)</span> <span class="token comment">// false</span>
    63. 

  63. Number<span class="token punctuation">.</span><span class="token function">isNaN</span><span class="token punctuation">(</span><span class="token number">1</span><span class="token punctuation">)</span> <span class="token comment">// false</span>
    64. 

  64. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br><span class="line-number">7</span><br><span class="line-number">8</span><br><span class="line-number">9</span><br><span class="line-number">10</span><br></div></div><h2 id="number-parseint-number-parsefloat"><a href="#number-parseint-number-parsefloat" class="header-anchor">#</a> Number.parseInt(), Number.parseFloat()</h2> <p>ES6 将全局方法<code>parseInt()</code>和<code>parseFloat()</code>，移植到<code>Number</code>对象上面，行为完全保持不变。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token comment">// ES5的写法</span>
    65. 

  65. <span class="token function">parseInt</span><span class="token punctuation">(</span><span class="token string">'12.34'</span><span class="token punctuation">)</span> <span class="token comment">// 12</span>
    66. 

  66. <span class="token function">parseFloat</span><span class="token punctuation">(</span><span class="token string">'123.45#'</span><span class="token punctuation">)</span> <span class="token comment">// 123.45</span>
    67. 

  67. 

  68. <span class="token comment">// ES6的写法</span>
    69. 

  69. Number<span class="token punctuation">.</span><span class="token function">parseInt</span><span class="token punctuation">(</span><span class="token string">'12.34'</span><span class="token punctuation">)</span> <span class="token comment">// 12</span>
    70. 

  70. Number<span class="token punctuation">.</span><span class="token function">parseFloat</span><span class="token punctuation">(</span><span class="token string">'123.45#'</span><span class="token punctuation">)</span> <span class="token comment">// 123.45</span>
    71. 

  71. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br><span class="line-number">7</span><br></div></div><p>这样做的目的，是<strong>逐步减少全局性方法，使得语言逐步模块化</strong>。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code>Number<span class="token punctuation">.</span>parseInt <span class="token operator">===</span> parseInt <span class="token comment">// true</span>
    72. 

  72. Number<span class="token punctuation">.</span>parseFloat <span class="token operator">===</span> parseFloat <span class="token comment">// true</span>
    73. 

  73. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br></div></div><h2 id="number-isinteger-是否为整数"><a href="#number-isinteger-是否为整数" class="header-anchor">#</a> Number.isInteger() 是否为整数</h2> <p><code>Number.isInteger()</code>用来判断一个数值是否为整数。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code>Number<span class="token punctuation">.</span><span class="token function">isInteger</span><span class="token punctuation">(</span><span class="token number">25</span><span class="token punctuation">)</span> <span class="token comment">// true</span>
    74. 

  74. Number<span class="token punctuation">.</span><span class="token function">isInteger</span><span class="token punctuation">(</span><span class="token number">25.1</span><span class="token punctuation">)</span> <span class="token comment">// false</span>
    75. 

  75. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br></div></div><p>JavaScript 内部，整数和浮点数采用的是同样的储存方法，所以 25  和 25.0 被视为同一个值。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code>Number<span class="token punctuation">.</span><span class="token function">isInteger</span><span class="token punctuation">(</span><span class="token number">25</span><span class="token punctuation">)</span> <span class="token comment">// true</span>
    76. 

  76. Number<span class="token punctuation">.</span><span class="token function">isInteger</span><span class="token punctuation">(</span><span class="token number">25.0</span><span class="token punctuation">)</span> <span class="token comment">// true</span>
    77. 

  77. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br></div></div><p>如果参数不是数值，<code>Number.isInteger</code>返回<code>false</code>。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code>Number<span class="token punctuation">.</span><span class="token function">isInteger</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token comment">// false</span>
    78. 

  78. Number<span class="token punctuation">.</span><span class="token function">isInteger</span><span class="token punctuation">(</span><span class="token keyword">null</span><span class="token punctuation">)</span> <span class="token comment">// false</span>
    79. 

  79. Number<span class="token punctuation">.</span><span class="token function">isInteger</span><span class="token punctuation">(</span><span class="token string">'15'</span><span class="token punctuation">)</span> <span class="token comment">// false</span>
    80. 

  80. Number<span class="token punctuation">.</span><span class="token function">isInteger</span><span class="token punctuation">(</span><span class="token boolean">true</span><span class="token punctuation">)</span> <span class="token comment">// false</span>
    81. 

  81. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br></div></div><p>注意，由于 JavaScript 采用 IEEE 754 标准，数值存储为64位双精度格式，数值精度最多可以达到 53 个二进制位（1 个隐藏位与 52 个有效位）。如果数值的精度超过这个限度，第54位及后面的位就会被丢弃，这种情况下，<code>Number.isInteger</code>可能会误判。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code>Number<span class="token punctuation">.</span><span class="token function">isInteger</span><span class="token punctuation">(</span><span class="token number">3.0000000000000002</span><span class="token punctuation">)</span> <span class="token comment">// true</span>
    82. 

  82. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br></div></div><p>上面代码中，<code>Number.isInteger</code>的参数明明不是整数，但是会返回<code>true</code>。原因就是这个小数的精度达到了小数点后16个十进制位，转成二进制位超过了53个二进制位，导致最后的那个<code>2</code>被丢弃了。</p> <p>类似的情况还有，如果一个数值的绝对值小于<code>Number.MIN_VALUE</code>（5E-324），即小于 JavaScript 能够分辨的最小值，会被自动转为 0。这时，<code>Number.isInteger</code>也会误判。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code>Number<span class="token punctuation">.</span><span class="token function">isInteger</span><span class="token punctuation">(</span><span class="token number">5E-324</span><span class="token punctuation">)</span> <span class="token comment">// false</span>
    83. 

  83. Number<span class="token punctuation">.</span><span class="token function">isInteger</span><span class="token punctuation">(</span><span class="token number">5E-325</span><span class="token punctuation">)</span> <span class="token comment">// true</span>
    84. 

  84. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br></div></div><p>上面代码中，<code>5E-325</code>由于值太小，会被自动转为0，因此返回<code>true</code>。</p> <p>总之，如果对数据精度的要求较高，不建议使用<code>Number.isInteger()</code>判断一个数值是否为整数。</p> <h2 id="number-epsilon-用于浮点数误差检查"><a href="#number-epsilon-用于浮点数误差检查" class="header-anchor">#</a> Number.EPSILON 用于浮点数误差检查</h2> <p>ES6 在<code>Number</code>对象上面，新增一个极小的常量<code>Number.EPSILON</code>。根据规格，它表示 1 与大于 1 的最小浮点数之间的差。</p> <p>对于 64 位浮点数来说，大于 1 的最小浮点数相当于二进制的<code>1.00..001</code>，小数点后面有连续 51 个零。这个值减去 1 之后，就等于 2 的 -52 次方。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code>Number<span class="token punctuation">.</span><span class="token constant">EPSILON</span> <span class="token operator">===</span> Math<span class="token punctuation">.</span><span class="token function">pow</span><span class="token punctuation">(</span><span class="token number">2</span><span class="token punctuation">,</span> <span class="token operator">-</span><span class="token number">52</span><span class="token punctuation">)</span>
    85. 

  85. <span class="token comment">// true</span>
    86. 

  86. Number<span class="token punctuation">.</span><span class="token constant">EPSILON</span>
    87. 

  87. <span class="token comment">// 2.220446049250313e-16</span>
    88. 

  88. Number<span class="token punctuation">.</span><span class="token constant">EPSILON</span><span class="token punctuation">.</span><span class="token function">toFixed</span><span class="token punctuation">(</span><span class="token number">20</span><span class="token punctuation">)</span>
    89. 

  89. <span class="token comment">// &quot;0.00000000000000022204&quot;</span>
    90. 

  90. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br></div></div><p><code>Number.EPSILON</code>实际上是 JavaScript 能够表示的最小精度。误差如果小于这个值，就可以认为已经没有意义了，即不存在误差了。</p> <p>引入一个这么小的量的目的，在于为浮点数计算，设置一个误差范围。我们知道浮点数计算是不精确的。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token number">0.1</span> <span class="token operator">+</span> <span class="token number">0.2</span>
    91. 

  91. <span class="token comment">// 0.30000000000000004</span>
    92. 

  92. 

  93. <span class="token number">0.1</span> <span class="token operator">+</span> <span class="token number">0.2</span> <span class="token operator">-</span> <span class="token number">0.3</span>
    94. 

  94. <span class="token comment">// 5.551115123125783e-17</span>
    95. 

  95. 

  96. <span class="token number">5.551115123125783e-17</span><span class="token punctuation">.</span><span class="token function">toFixed</span><span class="token punctuation">(</span><span class="token number">20</span><span class="token punctuation">)</span>
    97. 

  97. <span class="token comment">// '0.00000000000000005551'</span>
    98. 

  98. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br><span class="line-number">7</span><br><span class="line-number">8</span><br></div></div><p>上面代码解释了，为什么比较<code>0.1 + 0.2</code>与<code>0.3</code>得到的结果是<code>false</code>。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token number">0.1</span> <span class="token operator">+</span> <span class="token number">0.2</span> <span class="token operator">===</span> <span class="token number">0.3</span> <span class="token comment">// false</span>
    99. 

  99. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br></div></div><p><code>Number.EPSILON</code>可以用来设置“能够接受的误差范围”。比如，误差范围设为 2 的-50 次方（即<code>Number.EPSILON * Math.pow(2, 2)</code>），即如果两个浮点数的差小于这个值，我们就认为这两个浮点数相等。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token number">5.551115123125783e-17</span> <span class="token operator">&lt;</span> Number<span class="token punctuation">.</span><span class="token constant">EPSILON</span> <span class="token operator">*</span> Math<span class="token punctuation">.</span><span class="token function">pow</span><span class="token punctuation">(</span><span class="token number">2</span><span class="token punctuation">,</span> <span class="token number">2</span><span class="token punctuation">)</span>
    100. 

  100. <span class="token comment">// true</span>
    101. 

  101. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br></div></div><p>因此，<code>Number.EPSILON</code>的实质是一个可以接受的最小误差范围。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token keyword">function</span> <span class="token function">withinErrorMargin</span> <span class="token punctuation">(</span><span class="token parameter">left<span class="token punctuation">,</span> right</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
    102. 

  102.   <span class="token keyword">return</span> Math<span class="token punctuation">.</span><span class="token function">abs</span><span class="token punctuation">(</span>left <span class="token operator">-</span> right<span class="token punctuation">)</span> <span class="token operator">&lt;</span> Number<span class="token punctuation">.</span><span class="token constant">EPSILON</span> <span class="token operator">*</span> Math<span class="token punctuation">.</span><span class="token function">pow</span><span class="token punctuation">(</span><span class="token number">2</span><span class="token punctuation">,</span> <span class="token number">2</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    103. 

  103. <span class="token punctuation">}</span>
    104. 

  104. 

  105. <span class="token number">0.1</span> <span class="token operator">+</span> <span class="token number">0.2</span> <span class="token operator">===</span> <span class="token number">0.3</span> <span class="token comment">// false</span>
    106. 

  106. <span class="token function">withinErrorMargin</span><span class="token punctuation">(</span><span class="token number">0.1</span> <span class="token operator">+</span> <span class="token number">0.2</span><span class="token punctuation">,</span> <span class="token number">0.3</span><span class="token punctuation">)</span> <span class="token comment">// true</span>
    107. 

  107. 

  108. <span class="token number">1.1</span> <span class="token operator">+</span> <span class="token number">1.3</span> <span class="token operator">===</span> <span class="token number">2.4</span> <span class="token comment">// false</span>
    109. 

  109. <span class="token function">withinErrorMargin</span><span class="token punctuation">(</span><span class="token number">1.1</span> <span class="token operator">+</span> <span class="token number">1.3</span><span class="token punctuation">,</span> <span class="token number">2.4</span><span class="token punctuation">)</span> <span class="token comment">// true</span>
    110. 

  110. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br><span class="line-number">7</span><br><span class="line-number">8</span><br><span class="line-number">9</span><br></div></div><p>上面的代码为浮点数运算，部署了一个<strong>误差检查函数</strong>。</p> <h2 id="安全整数和-number-issafeinteger"><a href="#安全整数和-number-issafeinteger" class="header-anchor">#</a> 安全整数和 Number.isSafeInteger()</h2> <p>JavaScript 能够准确表示的整数范围在<code>-2^53</code>到<code>2^53</code>之间（不含两个端点），超过这个范围，无法精确表示这个值。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code>Math<span class="token punctuation">.</span><span class="token function">pow</span><span class="token punctuation">(</span><span class="token number">2</span><span class="token punctuation">,</span> <span class="token number">53</span><span class="token punctuation">)</span> <span class="token comment">// 9007199254740992</span>
    111. 

  111. 

  112. <span class="token number">9007199254740992</span>  <span class="token comment">// 9007199254740992</span>
    113. 

  113. <span class="token number">9007199254740993</span>  <span class="token comment">// 9007199254740992</span>
    114. 

  114. 

  115. Math<span class="token punctuation">.</span><span class="token function">pow</span><span class="token punctuation">(</span><span class="token number">2</span><span class="token punctuation">,</span> <span class="token number">53</span><span class="token punctuation">)</span> <span class="token operator">===</span> Math<span class="token punctuation">.</span><span class="token function">pow</span><span class="token punctuation">(</span><span class="token number">2</span><span class="token punctuation">,</span> <span class="token number">53</span><span class="token punctuation">)</span> <span class="token operator">+</span> <span class="token number">1</span>
    116. 

  116. <span class="token comment">// true</span>
    117. 

  117. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br><span class="line-number">7</span><br></div></div><p>上面代码中，超出 2 的 53 次方之后，一个数就不精确了。</p> <p>ES6 引入了<code>Number.MAX_SAFE_INTEGER</code>和<code>Number.MIN_SAFE_INTEGER</code>这两个常量，用来表示这个范围的上下限。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code>Number<span class="token punctuation">.</span><span class="token constant">MAX_SAFE_INTEGER</span> <span class="token operator">===</span> Math<span class="token punctuation">.</span><span class="token function">pow</span><span class="token punctuation">(</span><span class="token number">2</span><span class="token punctuation">,</span> <span class="token number">53</span><span class="token punctuation">)</span> <span class="token operator">-</span> <span class="token number">1</span>
    118. 

  118. <span class="token comment">// true</span>
    119. 

  119. Number<span class="token punctuation">.</span><span class="token constant">MAX_SAFE_INTEGER</span> <span class="token operator">===</span> <span class="token number">9007199254740991</span>
    120. 

  120. <span class="token comment">// true</span>
    121. 

  121. 

  122. Number<span class="token punctuation">.</span><span class="token constant">MIN_SAFE_INTEGER</span> <span class="token operator">===</span> <span class="token operator">-</span>Number<span class="token punctuation">.</span><span class="token constant">MAX_SAFE_INTEGER</span>
    123. 

  123. <span class="token comment">// true</span>
    124. 

  124. Number<span class="token punctuation">.</span><span class="token constant">MIN_SAFE_INTEGER</span> <span class="token operator">===</span> <span class="token operator">-</span><span class="token number">9007199254740991</span>
    125. 

  125. <span class="token comment">// true</span>
    126. 

  126. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br><span class="line-number">7</span><br><span class="line-number">8</span><br><span class="line-number">9</span><br></div></div><p>上面代码中，可以看到 JavaScript 能够精确表示的极限。</p> <p><code>Number.isSafeInteger()</code>则是用来判断一个整数是否落在这个范围之内。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code>Number<span class="token punctuation">.</span><span class="token function">isSafeInteger</span><span class="token punctuation">(</span><span class="token string">'a'</span><span class="token punctuation">)</span> <span class="token comment">// false</span>
    127. 

  127. Number<span class="token punctuation">.</span><span class="token function">isSafeInteger</span><span class="token punctuation">(</span><span class="token keyword">null</span><span class="token punctuation">)</span> <span class="token comment">// false</span>
    128. 

  128. Number<span class="token punctuation">.</span><span class="token function">isSafeInteger</span><span class="token punctuation">(</span><span class="token number">NaN</span><span class="token punctuation">)</span> <span class="token comment">// false</span>
    129. 

  129. Number<span class="token punctuation">.</span><span class="token function">isSafeInteger</span><span class="token punctuation">(</span><span class="token number">Infinity</span><span class="token punctuation">)</span> <span class="token comment">// false</span>
    130. 

  130. Number<span class="token punctuation">.</span><span class="token function">isSafeInteger</span><span class="token punctuation">(</span><span class="token operator">-</span><span class="token number">Infinity</span><span class="token punctuation">)</span> <span class="token comment">// false</span>
    131. 

  131. 

  132. Number<span class="token punctuation">.</span><span class="token function">isSafeInteger</span><span class="token punctuation">(</span><span class="token number">3</span><span class="token punctuation">)</span> <span class="token comment">// true</span>
    133. 

  133. Number<span class="token punctuation">.</span><span class="token function">isSafeInteger</span><span class="token punctuation">(</span><span class="token number">1.2</span><span class="token punctuation">)</span> <span class="token comment">// false</span>
    134. 

  134. Number<span class="token punctuation">.</span><span class="token function">isSafeInteger</span><span class="token punctuation">(</span><span class="token number">9007199254740990</span><span class="token punctuation">)</span> <span class="token comment">// true</span>
    135. 

  135. Number<span class="token punctuation">.</span><span class="token function">isSafeInteger</span><span class="token punctuation">(</span><span class="token number">9007199254740992</span><span class="token punctuation">)</span> <span class="token comment">// false</span>
    136. 

  136. 

  137. Number<span class="token punctuation">.</span><span class="token function">isSafeInteger</span><span class="token punctuation">(</span>Number<span class="token punctuation">.</span><span class="token constant">MIN_SAFE_INTEGER</span> <span class="token operator">-</span> <span class="token number">1</span><span class="token punctuation">)</span> <span class="token comment">// false</span>
    138. 

  138. Number<span class="token punctuation">.</span><span class="token function">isSafeInteger</span><span class="token punctuation">(</span>Number<span class="token punctuation">.</span><span class="token constant">MIN_SAFE_INTEGER</span><span class="token punctuation">)</span> <span class="token comment">// true</span>
    139. 

  139. Number<span class="token punctuation">.</span><span class="token function">isSafeInteger</span><span class="token punctuation">(</span>Number<span class="token punctuation">.</span><span class="token constant">MAX_SAFE_INTEGER</span><span class="token punctuation">)</span> <span class="token comment">// true</span>
    140. 

  140. Number<span class="token punctuation">.</span><span class="token function">isSafeInteger</span><span class="token punctuation">(</span>Number<span class="token punctuation">.</span><span class="token constant">MAX_SAFE_INTEGER</span> <span class="token operator">+</span> <span class="token number">1</span><span class="token punctuation">)</span> <span class="token comment">// false</span>
    141. 

  141. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br><span class="line-number">7</span><br><span class="line-number">8</span><br><span class="line-number">9</span><br><span class="line-number">10</span><br><span class="line-number">11</span><br><span class="line-number">12</span><br><span class="line-number">13</span><br><span class="line-number">14</span><br><span class="line-number">15</span><br></div></div><p>这个函数的实现很简单，就是跟安全整数的两个边界值比较一下。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code>Number<span class="token punctuation">.</span><span class="token function-variable function">isSafeInteger</span> <span class="token operator">=</span> <span class="token keyword">function</span> <span class="token punctuation">(</span><span class="token parameter">n</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
    142. 

  142.   <span class="token keyword">return</span> <span class="token punctuation">(</span><span class="token keyword">typeof</span> n <span class="token operator">===</span> <span class="token string">'number'</span> <span class="token operator">&amp;&amp;</span>
    143. 

  143.     Math<span class="token punctuation">.</span><span class="token function">round</span><span class="token punctuation">(</span>n<span class="token punctuation">)</span> <span class="token operator">===</span> n <span class="token operator">&amp;&amp;</span>
    144. 

  144.     Number<span class="token punctuation">.</span><span class="token constant">MIN_SAFE_INTEGER</span> <span class="token operator">&lt;=</span> n <span class="token operator">&amp;&amp;</span>
    145. 

  145.     n <span class="token operator">&lt;=</span> Number<span class="token punctuation">.</span><span class="token constant">MAX_SAFE_INTEGER</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    146. 

  146. <span class="token punctuation">}</span>
    147. 

  147. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br></div></div><p>实际使用这个函数时，需要注意。验证运算结果是否落在安全整数的范围内，不要只验证运算结果，而要同时验证参与运算的每个值。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code>Number<span class="token punctuation">.</span><span class="token function">isSafeInteger</span><span class="token punctuation">(</span><span class="token number">9007199254740993</span><span class="token punctuation">)</span>
    148. 

  148. <span class="token comment">// false</span>
    149. 

  149. Number<span class="token punctuation">.</span><span class="token function">isSafeInteger</span><span class="token punctuation">(</span><span class="token number">990</span><span class="token punctuation">)</span>
    150. 

  150. <span class="token comment">// true</span>
    151. 

  151. Number<span class="token punctuation">.</span><span class="token function">isSafeInteger</span><span class="token punctuation">(</span><span class="token number">9007199254740993</span> <span class="token operator">-</span> <span class="token number">990</span><span class="token punctuation">)</span>
    152. 

  152. <span class="token comment">// true</span>
    153. 

  153. <span class="token number">9007199254740993</span> <span class="token operator">-</span> <span class="token number">990</span>
    154. 

  154. <span class="token comment">// 返回结果 9007199254740002</span>
    155. 

  155. <span class="token comment">// 正确答案应该是 9007199254740003</span>
    156. 

  156. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br><span class="line-number">7</span><br><span class="line-number">8</span><br><span class="line-number">9</span><br></div></div><p>上面代码中，<code>9007199254740993</code>不是一个安全整数，但是<code>Number.isSafeInteger</code>会返回结果，显示计算结果是安全的。这是因为，这个数超出了精度范围，导致在计算机内部，以<code>9007199254740992</code>的形式储存。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token number">9007199254740993</span> <span class="token operator">===</span> <span class="token number">9007199254740992</span>
    157. 

  157. <span class="token comment">// true</span>
    158. 

  158. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br></div></div><p>所以，如果只验证运算结果是否为安全整数，很可能得到错误结果。下面的函数可以同时验证两个运算数和运算结果。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token keyword">function</span> <span class="token function">trusty</span> <span class="token punctuation">(</span><span class="token parameter">left<span class="token punctuation">,</span> right<span class="token punctuation">,</span> result</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
    159. 

  159.   <span class="token keyword">if</span> <span class="token punctuation">(</span>
    160. 

  160.     Number<span class="token punctuation">.</span><span class="token function">isSafeInteger</span><span class="token punctuation">(</span>left<span class="token punctuation">)</span> <span class="token operator">&amp;&amp;</span>
    161. 

  161.     Number<span class="token punctuation">.</span><span class="token function">isSafeInteger</span><span class="token punctuation">(</span>right<span class="token punctuation">)</span> <span class="token operator">&amp;&amp;</span>
    162. 

  162.     Number<span class="token punctuation">.</span><span class="token function">isSafeInteger</span><span class="token punctuation">(</span>result<span class="token punctuation">)</span>
    163. 

  163.   <span class="token punctuation">)</span> <span class="token punctuation">{</span>
    164. 

  164.     <span class="token keyword">return</span> result<span class="token punctuation">;</span>
    165. 

  165.   <span class="token punctuation">}</span>
    166. 

  166.   <span class="token keyword">throw</span> <span class="token keyword">new</span> <span class="token class-name">RangeError</span><span class="token punctuation">(</span><span class="token string">'Operation cannot be trusted!'</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    167. 

  167. <span class="token punctuation">}</span>
    168. 

  168. 

  169. <span class="token function">trusty</span><span class="token punctuation">(</span><span class="token number">9007199254740993</span><span class="token punctuation">,</span> <span class="token number">990</span><span class="token punctuation">,</span> <span class="token number">9007199254740993</span> <span class="token operator">-</span> <span class="token number">990</span><span class="token punctuation">)</span>
    170. 

  170. <span class="token comment">// RangeError: Operation cannot be trusted!</span>
    171. 

  171. 

  172. <span class="token function">trusty</span><span class="token punctuation">(</span><span class="token number">1</span><span class="token punctuation">,</span> <span class="token number">2</span><span class="token punctuation">,</span> <span class="token number">3</span><span class="token punctuation">)</span>
    173. 

  173. <span class="token comment">// 3</span>
    174. 

  174. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br><span class="line-number">7</span><br><span class="line-number">8</span><br><span class="line-number">9</span><br><span class="line-number">10</span><br><span class="line-number">11</span><br><span class="line-number">12</span><br><span class="line-number">13</span><br><span class="line-number">14</span><br><span class="line-number">15</span><br><span class="line-number">16</span><br></div></div><h2 id="math-对象的扩展"><a href="#math-对象的扩展" class="header-anchor">#</a> Math 对象的扩展</h2> <p>ES6 在 Math 对象上新增了 <strong>17 个与数学相关的方法</strong>。所有这些方法都是静态方法，只能在 Math 对象上调用。</p> <h3 id="math-trunc-去除一个数的小数部分-返回整数部分"><a href="#math-trunc-去除一个数的小数部分-返回整数部分" class="header-anchor">#</a> Math.trunc() 去除一个数的小数部分，返回整数部分</h3> <p><code>Math.trunc</code>方法<strong>用于去除一个数的小数部分，返回整数部分</strong>。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code>Math<span class="token punctuation">.</span><span class="token function">trunc</span><span class="token punctuation">(</span><span class="token number">4.1</span><span class="token punctuation">)</span> <span class="token comment">// 4</span>
    175. 

  175. Math<span class="token punctuation">.</span><span class="token function">trunc</span><span class="token punctuation">(</span><span class="token number">4.9</span><span class="token punctuation">)</span> <span class="token comment">// 4</span>
    176. 

  176. Math<span class="token punctuation">.</span><span class="token function">trunc</span><span class="token punctuation">(</span><span class="token operator">-</span><span class="token number">4.1</span><span class="token punctuation">)</span> <span class="token comment">// -4</span>
    177. 

  177. Math<span class="token punctuation">.</span><span class="token function">trunc</span><span class="token punctuation">(</span><span class="token operator">-</span><span class="token number">4.9</span><span class="token punctuation">)</span> <span class="token comment">// -4</span>
    178. 

  178. Math<span class="token punctuation">.</span><span class="token function">trunc</span><span class="token punctuation">(</span><span class="token operator">-</span><span class="token number">0.1234</span><span class="token punctuation">)</span> <span class="token comment">// -0</span>
    179. 

  179. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br></div></div><p>对于非数值，<code>Math.trunc</code>内部使用<code>Number</code>方法将其先转为数值。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code>Math<span class="token punctuation">.</span><span class="token function">trunc</span><span class="token punctuation">(</span><span class="token string">'123.456'</span><span class="token punctuation">)</span> <span class="token comment">// 123</span>
    180. 

  180. Math<span class="token punctuation">.</span><span class="token function">trunc</span><span class="token punctuation">(</span><span class="token boolean">true</span><span class="token punctuation">)</span> <span class="token comment">//1</span>
    181. 

  181. Math<span class="token punctuation">.</span><span class="token function">trunc</span><span class="token punctuation">(</span><span class="token boolean">false</span><span class="token punctuation">)</span> <span class="token comment">// 0</span>
    182. 

  182. Math<span class="token punctuation">.</span><span class="token function">trunc</span><span class="token punctuation">(</span><span class="token keyword">null</span><span class="token punctuation">)</span> <span class="token comment">// 0</span>
    183. 

  183. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br></div></div><p>对于空值和无法截取整数的值，返回<code>NaN</code>。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code>Math<span class="token punctuation">.</span><span class="token function">trunc</span><span class="token punctuation">(</span><span class="token number">NaN</span><span class="token punctuation">)</span><span class="token punctuation">;</span>      <span class="token comment">// NaN</span>
    184. 

  184. Math<span class="token punctuation">.</span><span class="token function">trunc</span><span class="token punctuation">(</span><span class="token string">'foo'</span><span class="token punctuation">)</span><span class="token punctuation">;</span>    <span class="token comment">// NaN</span>
    185. 

  185. Math<span class="token punctuation">.</span><span class="token function">trunc</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>         <span class="token comment">// NaN</span>
    186. 

  186. Math<span class="token punctuation">.</span><span class="token function">trunc</span><span class="token punctuation">(</span><span class="token keyword">undefined</span><span class="token punctuation">)</span> <span class="token comment">// NaN</span>
    187. 

  187. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br></div></div><p>对于没有部署这个方法的环境，可以用下面的代码模拟。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code>Math<span class="token punctuation">.</span>trunc <span class="token operator">=</span> Math<span class="token punctuation">.</span>trunc <span class="token operator">||</span> <span class="token keyword">function</span><span class="token punctuation">(</span><span class="token parameter">x</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
    188. 

  188.   <span class="token keyword">return</span> x <span class="token operator">&lt;</span> <span class="token number">0</span> <span class="token operator">?</span> Math<span class="token punctuation">.</span><span class="token function">ceil</span><span class="token punctuation">(</span>x<span class="token punctuation">)</span> <span class="token operator">:</span> Math<span class="token punctuation">.</span><span class="token function">floor</span><span class="token punctuation">(</span>x<span class="token punctuation">)</span><span class="token punctuation">;</span>
    189. 

  189. <span class="token punctuation">}</span><span class="token punctuation">;</span>
    190. 

  190. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br></div></div><h3 id="math-sign-判断一个数是正数、负数、还是零"><a href="#math-sign-判断一个数是正数、负数、还是零" class="header-anchor">#</a> Math.sign() 判断一个数是正数、负数、还是零</h3> <p><code>Math.sign</code>方法用来<strong>判断一个数到底是正数、负数、还是零</strong>。对于非数值，会先将其转换为数值。</p> <p>它会返回五种值。</p> <ul><li>参数为正数，返回<code>+1</code>；</li> <li>参数为负数，返回<code>-1</code>；</li> <li>参数为 0，返回<code>0</code>；</li> <li>参数为-0，返回<code>-0</code>;</li> <li>其他值，返回<code>NaN</code>。</li></ul> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code>Math<span class="token punctuation">.</span><span class="token function">sign</span><span class="token punctuation">(</span><span class="token operator">-</span><span class="token number">5</span><span class="token punctuation">)</span> <span class="token comment">// -1</span>
    191. 

  191. Math<span class="token punctuation">.</span><span class="token function">sign</span><span class="token punctuation">(</span><span class="token number">5</span><span class="token punctuation">)</span> <span class="token comment">// +1</span>
    192. 

  192. Math<span class="token punctuation">.</span><span class="token function">sign</span><span class="token punctuation">(</span><span class="token number">0</span><span class="token punctuation">)</span> <span class="token comment">// +0</span>
    193. 

  193. Math<span class="token punctuation">.</span><span class="token function">sign</span><span class="token punctuation">(</span><span class="token operator">-</span><span class="token number">0</span><span class="token punctuation">)</span> <span class="token comment">// -0</span>
    194. 

  194. Math<span class="token punctuation">.</span><span class="token function">sign</span><span class="token punctuation">(</span><span class="token number">NaN</span><span class="token punctuation">)</span> <span class="token comment">// NaN</span>
    195. 

  195. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br></div></div><p>如果参数是非数值，会自动转为数值。对于那些无法转为数值的值，会返回<code>NaN</code>。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code>Math<span class="token punctuation">.</span><span class="token function">sign</span><span class="token punctuation">(</span><span class="token string">''</span><span class="token punctuation">)</span>  <span class="token comment">// 0</span>
    196. 

  196. Math<span class="token punctuation">.</span><span class="token function">sign</span><span class="token punctuation">(</span><span class="token boolean">true</span><span class="token punctuation">)</span>  <span class="token comment">// +1</span>
    197. 

  197. Math<span class="token punctuation">.</span><span class="token function">sign</span><span class="token punctuation">(</span><span class="token boolean">false</span><span class="token punctuation">)</span>  <span class="token comment">// 0</span>
    198. 

  198. Math<span class="token punctuation">.</span><span class="token function">sign</span><span class="token punctuation">(</span><span class="token keyword">null</span><span class="token punctuation">)</span>  <span class="token comment">// 0</span>
    199. 

  199. Math<span class="token punctuation">.</span><span class="token function">sign</span><span class="token punctuation">(</span><span class="token string">'9'</span><span class="token punctuation">)</span>  <span class="token comment">// +1</span>
    200. 

  200. Math<span class="token punctuation">.</span><span class="token function">sign</span><span class="token punctuation">(</span><span class="token string">'foo'</span><span class="token punctuation">)</span>  <span class="token comment">// NaN</span>
    201. 

  201. Math<span class="token punctuation">.</span><span class="token function">sign</span><span class="token punctuation">(</span><span class="token punctuation">)</span>  <span class="token comment">// NaN</span>
    202. 

  202. Math<span class="token punctuation">.</span><span class="token function">sign</span><span class="token punctuation">(</span><span class="token keyword">undefined</span><span class="token punctuation">)</span>  <span class="token comment">// NaN</span>
    203. 

  203. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br><span class="line-number">7</span><br><span class="line-number">8</span><br></div></div><p>对于没有部署这个方法的环境，可以用下面的代码模拟。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code>Math<span class="token punctuation">.</span>sign <span class="token operator">=</span> Math<span class="token punctuation">.</span>sign <span class="token operator">||</span> <span class="token keyword">function</span><span class="token punctuation">(</span><span class="token parameter">x</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
    204. 

  204.   x <span class="token operator">=</span> <span class="token operator">+</span>x<span class="token punctuation">;</span> <span class="token comment">// convert to a number</span>
    205. 

  205.   <span class="token keyword">if</span> <span class="token punctuation">(</span>x <span class="token operator">===</span> <span class="token number">0</span> <span class="token operator">||</span> <span class="token function">isNaN</span><span class="token punctuation">(</span>x<span class="token punctuation">)</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
    206. 

  206.     <span class="token keyword">return</span> x<span class="token punctuation">;</span>
    207. 

  207.   <span class="token punctuation">}</span>
    208. 

  208.   <span class="token keyword">return</span> x <span class="token operator">&gt;</span> <span class="token number">0</span> <span class="token operator">?</span> <span class="token number">1</span> <span class="token operator">:</span> <span class="token operator">-</span><span class="token number">1</span><span class="token punctuation">;</span>
    209. 

  209. <span class="token punctuation">}</span><span class="token punctuation">;</span>
    210. 

  210. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br><span class="line-number">7</span><br></div></div><h3 id="math-cbrt-计算一个数的立方根"><a href="#math-cbrt-计算一个数的立方根" class="header-anchor">#</a> Math.cbrt() 计算一个数的立方根</h3> <p><code>Math.cbrt</code>方法用于<strong>计算一个数的立方根</strong>。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code>Math<span class="token punctuation">.</span><span class="token function">cbrt</span><span class="token punctuation">(</span><span class="token operator">-</span><span class="token number">1</span><span class="token punctuation">)</span> <span class="token comment">// -1</span>
    211. 

  211. Math<span class="token punctuation">.</span><span class="token function">cbrt</span><span class="token punctuation">(</span><span class="token number">0</span><span class="token punctuation">)</span>  <span class="token comment">// 0</span>
    212. 

  212. Math<span class="token punctuation">.</span><span class="token function">cbrt</span><span class="token punctuation">(</span><span class="token number">1</span><span class="token punctuation">)</span>  <span class="token comment">// 1</span>
    213. 

  213. Math<span class="token punctuation">.</span><span class="token function">cbrt</span><span class="token punctuation">(</span><span class="token number">2</span><span class="token punctuation">)</span>  <span class="token comment">// 1.2599210498948734</span>
    214. 

  214. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br></div></div><p>对于非数值，<code>Math.cbrt</code>方法内部也是先使用<code>Number</code>方法将其转为数值。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code>Math<span class="token punctuation">.</span><span class="token function">cbrt</span><span class="token punctuation">(</span><span class="token string">'8'</span><span class="token punctuation">)</span> <span class="token comment">// 2</span>
    215. 

  215. Math<span class="token punctuation">.</span><span class="token function">cbrt</span><span class="token punctuation">(</span><span class="token string">'hello'</span><span class="token punctuation">)</span> <span class="token comment">// NaN</span>
    216. 

  216. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br></div></div><p>对于没有部署这个方法的环境，可以用下面的代码模拟。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code>Math<span class="token punctuation">.</span>cbrt <span class="token operator">=</span> Math<span class="token punctuation">.</span>cbrt <span class="token operator">||</span> <span class="token keyword">function</span><span class="token punctuation">(</span><span class="token parameter">x</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
    217. 

  217.   <span class="token keyword">var</span> y <span class="token operator">=</span> Math<span class="token punctuation">.</span><span class="token function">pow</span><span class="token punctuation">(</span>Math<span class="token punctuation">.</span><span class="token function">abs</span><span class="token punctuation">(</span>x<span class="token punctuation">)</span><span class="token punctuation">,</span> <span class="token number">1</span><span class="token operator">/</span><span class="token number">3</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    218. 

  218.   <span class="token keyword">return</span> x <span class="token operator">&lt;</span> <span class="token number">0</span> <span class="token operator">?</span> <span class="token operator">-</span>y <span class="token operator">:</span> y<span class="token punctuation">;</span>
    219. 

  219. <span class="token punctuation">}</span><span class="token punctuation">;</span>
    220. 

  220. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br></div></div><h3 id="math-clz32"><a href="#math-clz32" class="header-anchor">#</a> Math.clz32()</h3> <p><code>Math.clz32()</code>方法将<strong>参数转为 32 位无符号整数的形式</strong>，然后返回这个 32 位值里面有多少个前导 0。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code>Math<span class="token punctuation">.</span><span class="token function">clz32</span><span class="token punctuation">(</span><span class="token number">0</span><span class="token punctuation">)</span> <span class="token comment">// 32</span>
    221. 

  221. Math<span class="token punctuation">.</span><span class="token function">clz32</span><span class="token punctuation">(</span><span class="token number">1</span><span class="token punctuation">)</span> <span class="token comment">// 31</span>
    222. 

  222. Math<span class="token punctuation">.</span><span class="token function">clz32</span><span class="token punctuation">(</span><span class="token number">1000</span><span class="token punctuation">)</span> <span class="token comment">// 22</span>
    223. 

  223. Math<span class="token punctuation">.</span><span class="token function">clz32</span><span class="token punctuation">(</span><span class="token number">0b01000000000000000000000000000000</span><span class="token punctuation">)</span> <span class="token comment">// 1</span>
    224. 

  224. Math<span class="token punctuation">.</span><span class="token function">clz32</span><span class="token punctuation">(</span><span class="token number">0b00100000000000000000000000000000</span><span class="token punctuation">)</span> <span class="token comment">// 2</span>
    225. 

  225. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br></div></div><p>上面代码中，0 的二进制形式全为 0，所以有 32 个前导 0；1 的二进制形式是<code>0b1</code>，只占 1 位，所以 32 位之中有 31 个前导 0；1000 的二进制形式是<code>0b1111101000</code>，一共有 10 位，所以 32 位之中有 22 个前导 0。</p> <p><code>clz32</code>这个函数名就来自”count leading zero bits in 32-bit binary representation of a number“（计算一个数的 32 位二进制形式的前导 0 的个数）的缩写。</p> <p>左移运算符（<code>&lt;&lt;</code>）与<code>Math.clz32</code>方法直接相关。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code>Math<span class="token punctuation">.</span><span class="token function">clz32</span><span class="token punctuation">(</span><span class="token number">0</span><span class="token punctuation">)</span> <span class="token comment">// 32</span>
    226. 

  226. Math<span class="token punctuation">.</span><span class="token function">clz32</span><span class="token punctuation">(</span><span class="token number">1</span><span class="token punctuation">)</span> <span class="token comment">// 31</span>
    227. 

  227. Math<span class="token punctuation">.</span><span class="token function">clz32</span><span class="token punctuation">(</span><span class="token number">1</span> <span class="token operator">&lt;&lt;</span> <span class="token number">1</span><span class="token punctuation">)</span> <span class="token comment">// 30</span>
    228. 

  228. Math<span class="token punctuation">.</span><span class="token function">clz32</span><span class="token punctuation">(</span><span class="token number">1</span> <span class="token operator">&lt;&lt;</span> <span class="token number">2</span><span class="token punctuation">)</span> <span class="token comment">// 29</span>
    229. 

  229. Math<span class="token punctuation">.</span><span class="token function">clz32</span><span class="token punctuation">(</span><span class="token number">1</span> <span class="token operator">&lt;&lt;</span> <span class="token number">29</span><span class="token punctuation">)</span> <span class="token comment">// 2</span>
    230. 

  230. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br></div></div><p>对于小数，<code>Math.clz32</code>方法只考虑整数部分。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code>Math<span class="token punctuation">.</span><span class="token function">clz32</span><span class="token punctuation">(</span><span class="token number">3.2</span><span class="token punctuation">)</span> <span class="token comment">// 30</span>
    231. 

  231. Math<span class="token punctuation">.</span><span class="token function">clz32</span><span class="token punctuation">(</span><span class="token number">3.9</span><span class="token punctuation">)</span> <span class="token comment">// 30</span>
    232. 

  232. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br></div></div><p>对于空值或其他类型的值，<code>Math.clz32</code>方法会将它们先转为数值，然后再计算。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code>Math<span class="token punctuation">.</span><span class="token function">clz32</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token comment">// 32</span>
    233. 

  233. Math<span class="token punctuation">.</span><span class="token function">clz32</span><span class="token punctuation">(</span><span class="token number">NaN</span><span class="token punctuation">)</span> <span class="token comment">// 32</span>
    234. 

  234. Math<span class="token punctuation">.</span><span class="token function">clz32</span><span class="token punctuation">(</span><span class="token number">Infinity</span><span class="token punctuation">)</span> <span class="token comment">// 32</span>
    235. 

  235. Math<span class="token punctuation">.</span><span class="token function">clz32</span><span class="token punctuation">(</span><span class="token keyword">null</span><span class="token punctuation">)</span> <span class="token comment">// 32</span>
    236. 

  236. Math<span class="token punctuation">.</span><span class="token function">clz32</span><span class="token punctuation">(</span><span class="token string">'foo'</span><span class="token punctuation">)</span> <span class="token comment">// 32</span>
    237. 

  237. Math<span class="token punctuation">.</span><span class="token function">clz32</span><span class="token punctuation">(</span><span class="token punctuation">[</span><span class="token punctuation">]</span><span class="token punctuation">)</span> <span class="token comment">// 32</span>
    238. 

  238. Math<span class="token punctuation">.</span><span class="token function">clz32</span><span class="token punctuation">(</span><span class="token punctuation">{</span><span class="token punctuation">}</span><span class="token punctuation">)</span> <span class="token comment">// 32</span>
    239. 

  239. Math<span class="token punctuation">.</span><span class="token function">clz32</span><span class="token punctuation">(</span><span class="token boolean">true</span><span class="token punctuation">)</span> <span class="token comment">// 31</span>
    240. 

  240. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br><span class="line-number">7</span><br><span class="line-number">8</span><br></div></div><h3 id="math-imul"><a href="#math-imul" class="header-anchor">#</a> Math.imul()</h3> <p><code>Math.imul</code>方法返回两个数以 32 位带符号整数形式相乘的结果，返回的也是一个 32 位的带符号整数。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code>Math<span class="token punctuation">.</span><span class="token function">imul</span><span class="token punctuation">(</span><span class="token number">2</span><span class="token punctuation">,</span> <span class="token number">4</span><span class="token punctuation">)</span>   <span class="token comment">// 8</span>
    241. 

  241. Math<span class="token punctuation">.</span><span class="token function">imul</span><span class="token punctuation">(</span><span class="token operator">-</span><span class="token number">1</span><span class="token punctuation">,</span> <span class="token number">8</span><span class="token punctuation">)</span>  <span class="token comment">// -8</span>
    242. 

  242. Math<span class="token punctuation">.</span><span class="token function">imul</span><span class="token punctuation">(</span><span class="token operator">-</span><span class="token number">2</span><span class="token punctuation">,</span> <span class="token operator">-</span><span class="token number">2</span><span class="token punctuation">)</span> <span class="token comment">// 4</span>
    243. 

  243. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br></div></div><p>如果只考虑最后 32 位，大多数情况下，<code>Math.imul(a, b)</code>与<code>a * b</code>的结果是相同的，即该方法等同于<code>(a * b)|0</code>的效果（超过 32 位的部分溢出）。之所以需要部署这个方法，是因为 JavaScript 有精度限制，超过 2 的 53 次方的值无法精确表示。这就是说，对于那些很大的数的乘法，低位数值往往都是不精确的，<code>Math.imul</code>方法可以返回正确的低位数值。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token punctuation">(</span><span class="token number">0x7fffffff</span> <span class="token operator">*</span> <span class="token number">0x7fffffff</span><span class="token punctuation">)</span><span class="token operator">|</span><span class="token number">0</span> <span class="token comment">// 0</span>
    244. 

  244. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br></div></div><p>上面这个乘法算式，返回结果为 0。但是由于这两个二进制数的最低位都是 1，所以这个结果肯定是不正确的，因为根据二进制乘法，计算结果的二进制最低位应该也是 1。这个错误就是因为它们的乘积超过了 2 的 53 次方，JavaScript 无法保存额外的精度，就把低位的值都变成了 0。<code>Math.imul</code>方法可以返回正确的值 1。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code>Math<span class="token punctuation">.</span><span class="token function">imul</span><span class="token punctuation">(</span><span class="token number">0x7fffffff</span><span class="token punctuation">,</span> <span class="token number">0x7fffffff</span><span class="token punctuation">)</span> <span class="token comment">// 1</span>
    245. 

  245. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br></div></div><h3 id="math-fround"><a href="#math-fround" class="header-anchor">#</a> Math.fround()</h3> <p><code>Math.fround</code>方法返回一个数的32位单精度浮点数形式。</p> <p>对于32位单精度格式来说，数值精度是24个二进制位（1 位隐藏位与 23 位有效位），所以对于 -2<sup>24</sup> 至 2<sup>24</sup> 之间的整数（不含两个端点），返回结果与参数本身一致。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code>Math<span class="token punctuation">.</span><span class="token function">fround</span><span class="token punctuation">(</span><span class="token number">0</span><span class="token punctuation">)</span>   <span class="token comment">// 0</span>
    246. 

  246. Math<span class="token punctuation">.</span><span class="token function">fround</span><span class="token punctuation">(</span><span class="token number">1</span><span class="token punctuation">)</span>   <span class="token comment">// 1</span>
    247. 

  247. Math<span class="token punctuation">.</span><span class="token function">fround</span><span class="token punctuation">(</span><span class="token number">2</span> <span class="token operator">**</span> <span class="token number">24</span> <span class="token operator">-</span> <span class="token number">1</span><span class="token punctuation">)</span>   <span class="token comment">// 16777215</span>
    248. 

  248. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br></div></div><p>如果参数的绝对值大于 2<sup>24</sup>，返回的结果便开始丢失精度。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code>Math<span class="token punctuation">.</span><span class="token function">fround</span><span class="token punctuation">(</span><span class="token number">2</span> <span class="token operator">**</span> <span class="token number">24</span><span class="token punctuation">)</span>       <span class="token comment">// 16777216</span>
    249. 

  249. Math<span class="token punctuation">.</span><span class="token function">fround</span><span class="token punctuation">(</span><span class="token number">2</span> <span class="token operator">**</span> <span class="token number">24</span> <span class="token operator">+</span> <span class="token number">1</span><span class="token punctuation">)</span>   <span class="token comment">// 16777216</span>
    250. 

  250. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br></div></div><p><code>Math.fround</code>方法的主要作用，是将64位双精度浮点数转为32位单精度浮点数。如果小数的精度超过24个二进制位，返回值就会不同于原值，否则返回值不变（即与64位双精度值一致）。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token comment">// 未丢失有效精度</span>
    251. 

  251. Math<span class="token punctuation">.</span><span class="token function">fround</span><span class="token punctuation">(</span><span class="token number">1.125</span><span class="token punctuation">)</span> <span class="token comment">// 1.125</span>
    252. 

  252. Math<span class="token punctuation">.</span><span class="token function">fround</span><span class="token punctuation">(</span><span class="token number">7.25</span><span class="token punctuation">)</span>  <span class="token comment">// 7.25</span>
    253. 

  253. 

  254. <span class="token comment">// 丢失精度</span>
    255. 

  255. Math<span class="token punctuation">.</span><span class="token function">fround</span><span class="token punctuation">(</span><span class="token number">0.3</span><span class="token punctuation">)</span>   <span class="token comment">// 0.30000001192092896</span>
    256. 

  256. Math<span class="token punctuation">.</span><span class="token function">fround</span><span class="token punctuation">(</span><span class="token number">0.7</span><span class="token punctuation">)</span>   <span class="token comment">// 0.699999988079071</span>
    257. 

  257. Math<span class="token punctuation">.</span><span class="token function">fround</span><span class="token punctuation">(</span><span class="token number">1.0000000123</span><span class="token punctuation">)</span> <span class="token comment">// 1</span>
    258. 

  258. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br><span class="line-number">7</span><br><span class="line-number">8</span><br></div></div><p>对于 <code>NaN</code> 和 <code>Infinity</code>，此方法返回原值。对于其它类型的非数值，<code>Math.fround</code> 方法会先将其转为数值，再返回单精度浮点数。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code>Math<span class="token punctuation">.</span><span class="token function">fround</span><span class="token punctuation">(</span><span class="token number">NaN</span><span class="token punctuation">)</span>      <span class="token comment">// NaN</span>
    259. 

  259. Math<span class="token punctuation">.</span><span class="token function">fround</span><span class="token punctuation">(</span><span class="token number">Infinity</span><span class="token punctuation">)</span> <span class="token comment">// Infinity</span>
    260. 

  260. 

  261. Math<span class="token punctuation">.</span><span class="token function">fround</span><span class="token punctuation">(</span><span class="token string">'5'</span><span class="token punctuation">)</span>      <span class="token comment">// 5</span>
    262. 

  262. Math<span class="token punctuation">.</span><span class="token function">fround</span><span class="token punctuation">(</span><span class="token boolean">true</span><span class="token punctuation">)</span>     <span class="token comment">// 1</span>
    263. 

  263. Math<span class="token punctuation">.</span><span class="token function">fround</span><span class="token punctuation">(</span><span class="token keyword">null</span><span class="token punctuation">)</span>     <span class="token comment">// 0</span>
    264. 

  264. Math<span class="token punctuation">.</span><span class="token function">fround</span><span class="token punctuation">(</span><span class="token punctuation">[</span><span class="token punctuation">]</span><span class="token punctuation">)</span>       <span class="token comment">// 0</span>
    265. 

  265. Math<span class="token punctuation">.</span><span class="token function">fround</span><span class="token punctuation">(</span><span class="token punctuation">{</span><span class="token punctuation">}</span><span class="token punctuation">)</span>       <span class="token comment">// NaN</span>
    266. 

  266. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br><span class="line-number">7</span><br><span class="line-number">8</span><br></div></div><p>对于没有部署这个方法的环境，可以用下面的代码模拟。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code>Math<span class="token punctuation">.</span>fround <span class="token operator">=</span> Math<span class="token punctuation">.</span>fround <span class="token operator">||</span> <span class="token keyword">function</span> <span class="token punctuation">(</span><span class="token parameter">x</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
    267. 

  267.   <span class="token keyword">return</span> <span class="token keyword">new</span> <span class="token class-name">Float32Array</span><span class="token punctuation">(</span><span class="token punctuation">[</span>x<span class="token punctuation">]</span><span class="token punctuation">)</span><span class="token punctuation">[</span><span class="token number">0</span><span class="token punctuation">]</span><span class="token punctuation">;</span>
    268. 

  268. <span class="token punctuation">}</span><span class="token punctuation">;</span>
    269. 

  269. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br></div></div><h3 id="math-hypot"><a href="#math-hypot" class="header-anchor">#</a> Math.hypot()</h3> <p><code>Math.hypot</code>方法返回所有参数的平方和的平方根。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code>Math<span class="token punctuation">.</span><span class="token function">hypot</span><span class="token punctuation">(</span><span class="token number">3</span><span class="token punctuation">,</span> <span class="token number">4</span><span class="token punctuation">)</span><span class="token punctuation">;</span>        <span class="token comment">// 5</span>
    270. 

  270. Math<span class="token punctuation">.</span><span class="token function">hypot</span><span class="token punctuation">(</span><span class="token number">3</span><span class="token punctuation">,</span> <span class="token number">4</span><span class="token punctuation">,</span> <span class="token number">5</span><span class="token punctuation">)</span><span class="token punctuation">;</span>     <span class="token comment">// 7.0710678118654755</span>
    271. 

  271. Math<span class="token punctuation">.</span><span class="token function">hypot</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>            <span class="token comment">// 0</span>
    272. 

  272. Math<span class="token punctuation">.</span><span class="token function">hypot</span><span class="token punctuation">(</span><span class="token number">NaN</span><span class="token punctuation">)</span><span class="token punctuation">;</span>         <span class="token comment">// NaN</span>
    273. 

  273. Math<span class="token punctuation">.</span><span class="token function">hypot</span><span class="token punctuation">(</span><span class="token number">3</span><span class="token punctuation">,</span> <span class="token number">4</span><span class="token punctuation">,</span> <span class="token string">'foo'</span><span class="token punctuation">)</span><span class="token punctuation">;</span> <span class="token comment">// NaN</span>
    274. 

  274. Math<span class="token punctuation">.</span><span class="token function">hypot</span><span class="token punctuation">(</span><span class="token number">3</span><span class="token punctuation">,</span> <span class="token number">4</span><span class="token punctuation">,</span> <span class="token string">'5'</span><span class="token punctuation">)</span><span class="token punctuation">;</span>   <span class="token comment">// 7.0710678118654755</span>
    275. 

  275. Math<span class="token punctuation">.</span><span class="token function">hypot</span><span class="token punctuation">(</span><span class="token operator">-</span><span class="token number">3</span><span class="token punctuation">)</span><span class="token punctuation">;</span>          <span class="token comment">// 3</span>
    276. 

  276. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br><span class="line-number">7</span><br></div></div><p>上面代码中，3 的平方加上 4 的平方，等于 5 的平方。</p> <p>如果参数不是数值，<code>Math.hypot</code>方法会将其转为数值。只要有一个参数无法转为数值，就会返回 NaN。</p> <h3 id="对数方法"><a href="#对数方法" class="header-anchor">#</a> 对数方法</h3> <p>ES6 新增了 4 个对数相关方法。</p> <p><strong>（1） Math.expm1()</strong></p> <p><code>Math.expm1(x)</code>返回 e<sup>x</sup> - 1，即<code>Math.exp(x) - 1</code>。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code>Math<span class="token punctuation">.</span><span class="token function">expm1</span><span class="token punctuation">(</span><span class="token operator">-</span><span class="token number">1</span><span class="token punctuation">)</span> <span class="token comment">// -0.6321205588285577</span>
    277. 

  277. Math<span class="token punctuation">.</span><span class="token function">expm1</span><span class="token punctuation">(</span><span class="token number">0</span><span class="token punctuation">)</span>  <span class="token comment">// 0</span>
    278. 

  278. Math<span class="token punctuation">.</span><span class="token function">expm1</span><span class="token punctuation">(</span><span class="token number">1</span><span class="token punctuation">)</span>  <span class="token comment">// 1.718281828459045</span>
    279. 

  279. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br></div></div><p>对于没有部署这个方法的环境，可以用下面的代码模拟。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code>Math<span class="token punctuation">.</span>expm1 <span class="token operator">=</span> Math<span class="token punctuation">.</span>expm1 <span class="token operator">||</span> <span class="token keyword">function</span><span class="token punctuation">(</span><span class="token parameter">x</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
    280. 

  280.   <span class="token keyword">return</span> Math<span class="token punctuation">.</span><span class="token function">exp</span><span class="token punctuation">(</span>x<span class="token punctuation">)</span> <span class="token operator">-</span> <span class="token number">1</span><span class="token punctuation">;</span>
    281. 

  281. <span class="token punctuation">}</span><span class="token punctuation">;</span>
    282. 

  282. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br></div></div><p><strong>（2）Math.log1p()</strong></p> <p><code>Math.log1p(x)</code>方法返回<code>1 + x</code>的自然对数，即<code>Math.log(1 + x)</code>。如果<code>x</code>小于-1，返回<code>NaN</code>。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code>Math<span class="token punctuation">.</span><span class="token function">log1p</span><span class="token punctuation">(</span><span class="token number">1</span><span class="token punctuation">)</span>  <span class="token comment">// 0.6931471805599453</span>
    283. 

  283. Math<span class="token punctuation">.</span><span class="token function">log1p</span><span class="token punctuation">(</span><span class="token number">0</span><span class="token punctuation">)</span>  <span class="token comment">// 0</span>
    284. 

  284. Math<span class="token punctuation">.</span><span class="token function">log1p</span><span class="token punctuation">(</span><span class="token operator">-</span><span class="token number">1</span><span class="token punctuation">)</span> <span class="token comment">// -Infinity</span>
    285. 

  285. Math<span class="token punctuation">.</span><span class="token function">log1p</span><span class="token punctuation">(</span><span class="token operator">-</span><span class="token number">2</span><span class="token punctuation">)</span> <span class="token comment">// NaN</span>
    286. 

  286. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br></div></div><p>对于没有部署这个方法的环境，可以用下面的代码模拟。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code>Math<span class="token punctuation">.</span>log1p <span class="token operator">=</span> Math<span class="token punctuation">.</span>log1p <span class="token operator">||</span> <span class="token keyword">function</span><span class="token punctuation">(</span><span class="token parameter">x</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
    287. 

  287.   <span class="token keyword">return</span> Math<span class="token punctuation">.</span><span class="token function">log</span><span class="token punctuation">(</span><span class="token number">1</span> <span class="token operator">+</span> x<span class="token punctuation">)</span><span class="token punctuation">;</span>
    288. 

  288. <span class="token punctuation">}</span><span class="token punctuation">;</span>
    289. 

  289. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br></div></div><p><strong>（3）Math.log10()</strong></p> <p><code>Math.log10(x)</code>返回以 10 为底的<code>x</code>的对数。如果<code>x</code>小于 0，则返回 NaN。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code>Math<span class="token punctuation">.</span><span class="token function">log10</span><span class="token punctuation">(</span><span class="token number">2</span><span class="token punctuation">)</span>      <span class="token comment">// 0.3010299956639812</span>
    290. 

  290. Math<span class="token punctuation">.</span><span class="token function">log10</span><span class="token punctuation">(</span><span class="token number">1</span><span class="token punctuation">)</span>      <span class="token comment">// 0</span>
    291. 

  291. Math<span class="token punctuation">.</span><span class="token function">log10</span><span class="token punctuation">(</span><span class="token number">0</span><span class="token punctuation">)</span>      <span class="token comment">// -Infinity</span>
    292. 

  292. Math<span class="token punctuation">.</span><span class="token function">log10</span><span class="token punctuation">(</span><span class="token operator">-</span><span class="token number">2</span><span class="token punctuation">)</span>     <span class="token comment">// NaN</span>
    293. 

  293. Math<span class="token punctuation">.</span><span class="token function">log10</span><span class="token punctuation">(</span><span class="token number">100000</span><span class="token punctuation">)</span> <span class="token comment">// 5</span>
    294. 

  294. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br></div></div><p>对于没有部署这个方法的环境，可以用下面的代码模拟。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code>Math<span class="token punctuation">.</span>log10 <span class="token operator">=</span> Math<span class="token punctuation">.</span>log10 <span class="token operator">||</span> <span class="token keyword">function</span><span class="token punctuation">(</span><span class="token parameter">x</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
    295. 

  295.   <span class="token keyword">return</span> Math<span class="token punctuation">.</span><span class="token function">log</span><span class="token punctuation">(</span>x<span class="token punctuation">)</span> <span class="token operator">/</span> Math<span class="token punctuation">.</span><span class="token constant">LN10</span><span class="token punctuation">;</span>
    296. 

  296. <span class="token punctuation">}</span><span class="token punctuation">;</span>
    297. 

  297. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br></div></div><p><strong>（4）Math.log2()</strong></p> <p><code>Math.log2(x)</code>返回以 2 为底的<code>x</code>的对数。如果<code>x</code>小于 0，则返回 NaN。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code>Math<span class="token punctuation">.</span><span class="token function">log2</span><span class="token punctuation">(</span><span class="token number">3</span><span class="token punctuation">)</span>       <span class="token comment">// 1.584962500721156</span>
    298. 

  298. Math<span class="token punctuation">.</span><span class="token function">log2</span><span class="token punctuation">(</span><span class="token number">2</span><span class="token punctuation">)</span>       <span class="token comment">// 1</span>
    299. 

  299. Math<span class="token punctuation">.</span><span class="token function">log2</span><span class="token punctuation">(</span><span class="token number">1</span><span class="token punctuation">)</span>       <span class="token comment">// 0</span>
    300. 

  300. Math<span class="token punctuation">.</span><span class="token function">log2</span><span class="token punctuation">(</span><span class="token number">0</span><span class="token punctuation">)</span>       <span class="token comment">// -Infinity</span>
    301. 

  301. Math<span class="token punctuation">.</span><span class="token function">log2</span><span class="token punctuation">(</span><span class="token operator">-</span><span class="token number">2</span><span class="token punctuation">)</span>      <span class="token comment">// NaN</span>
    302. 

  302. Math<span class="token punctuation">.</span><span class="token function">log2</span><span class="token punctuation">(</span><span class="token number">1024</span><span class="token punctuation">)</span>    <span class="token comment">// 10</span>
    303. 

  303. Math<span class="token punctuation">.</span><span class="token function">log2</span><span class="token punctuation">(</span><span class="token number">1</span> <span class="token operator">&lt;&lt;</span> <span class="token number">29</span><span class="token punctuation">)</span> <span class="token comment">// 29</span>
    304. 

  304. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br><span class="line-number">7</span><br></div></div><p>对于没有部署这个方法的环境，可以用下面的代码模拟。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code>Math<span class="token punctuation">.</span>log2 <span class="token operator">=</span> Math<span class="token punctuation">.</span>log2 <span class="token operator">||</span> <span class="token keyword">function</span><span class="token punctuation">(</span><span class="token parameter">x</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
    305. 

  305.   <span class="token keyword">return</span> Math<span class="token punctuation">.</span><span class="token function">log</span><span class="token punctuation">(</span>x<span class="token punctuation">)</span> <span class="token operator">/</span> Math<span class="token punctuation">.</span><span class="token constant">LN2</span><span class="token punctuation">;</span>
    306. 

  306. <span class="token punctuation">}</span><span class="token punctuation">;</span>
    307. 

  307. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br></div></div><h3 id="双曲函数方法"><a href="#双曲函数方法" class="header-anchor">#</a> 双曲函数方法</h3> <p>ES6 新增了 6 个双曲函数方法。</p> <ul><li><code>Math.sinh(x)</code> 返回<code>x</code>的双曲正弦（hyperbolic sine）</li> <li><code>Math.cosh(x)</code> 返回<code>x</code>的双曲余弦（hyperbolic cosine）</li> <li><code>Math.tanh(x)</code> 返回<code>x</code>的双曲正切（hyperbolic tangent）</li> <li><code>Math.asinh(x)</code> 返回<code>x</code>的反双曲正弦（inverse hyperbolic sine）</li> <li><code>Math.acosh(x)</code> 返回<code>x</code>的反双曲余弦（inverse hyperbolic cosine）</li> <li><code>Math.atanh(x)</code> 返回<code>x</code>的反双曲正切（inverse hyperbolic tangent）</li></ul> <h2 id="指数运算符"><a href="#指数运算符" class="header-anchor">#</a> 指数运算符</h2> <p>ES2016 新增了一个指数运算符（<code>**</code>）。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token number">2</span> <span class="token operator">**</span> <span class="token number">2</span> <span class="token comment">// 4</span>
    308. 

  308. <span class="token number">2</span> <span class="token operator">**</span> <span class="token number">3</span> <span class="token comment">// 8</span>
    309. 

  309. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br></div></div><p>这个运算符的一个特点是右结合，而不是常见的左结合。多个指数运算符连用时，是从最右边开始计算的。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token comment">// 相当于 2 ** (3 ** 2)</span>
    310. 

  310. <span class="token number">2</span> <span class="token operator">**</span> <span class="token number">3</span> <span class="token operator">**</span> <span class="token number">2</span>
    311. 

  311. <span class="token comment">// 512</span>
    312. 

  312. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br></div></div><p>上面代码中，首先计算的是第二个指数运算符，而不是第一个。</p> <p>指数运算符可以与等号结合，形成一个新的赋值运算符（<code>**=</code>）。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token keyword">let</span> a <span class="token operator">=</span> <span class="token number">1.5</span><span class="token punctuation">;</span>
    313. 

  313. a <span class="token operator">**=</span> <span class="token number">2</span><span class="token punctuation">;</span>
    314. 

  314. <span class="token comment">// 等同于 a = a * a;</span>
    315. 

  315. 

  316. <span class="token keyword">let</span> b <span class="token operator">=</span> <span class="token number">4</span><span class="token punctuation">;</span>
    317. 

  317. b <span class="token operator">**=</span> <span class="token number">3</span><span class="token punctuation">;</span>
    318. 

  318. <span class="token comment">// 等同于 b = b * b * b;</span>
    319. 

  319. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br><span class="line-number">7</span><br></div></div><p>注意，V8 引擎的指数运算符与<code>Math.pow</code>的实现不相同，对于特别大的运算结果，两者会有细微的差异。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code>Math<span class="token punctuation">.</span><span class="token function">pow</span><span class="token punctuation">(</span><span class="token number">99</span><span class="token punctuation">,</span> <span class="token number">99</span><span class="token punctuation">)</span>
    320. 

  320. <span class="token comment">// 3.697296376497263e+197</span>
    321. 

  321. 

  322. <span class="token number">99</span> <span class="token operator">**</span> <span class="token number">99</span>
    323. 

  323. <span class="token comment">// 3.697296376497268e+197</span>
    324. 

  324. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br></div></div><p>上面代码中，两个运算结果的最后一位有效数字是有差异的。</p> <h2 id="bigint-数据类型"><a href="#bigint-数据类型" class="header-anchor">#</a> BigInt 数据类型</h2> <h3 id="简介"><a href="#简介" class="header-anchor">#</a> 简介</h3> <p>JavaScript 所有数字都保存成 64 位浮点数，这给数值的表示带来了两大限制。一是数值的精度只能到 53 个二进制位（相当于 16 个十进制位），大于这个范围的整数，JavaScript 是无法精确表示的，这使得 JavaScript 不适合进行科学和金融方面的精确计算。二是大于或等于2的1024次方的数值，JavaScript 无法表示，会返回<code>Infinity</code>。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token comment">// 超过 53 个二进制位的数值，无法保持精度</span>
    325. 

  325. Math<span class="token punctuation">.</span><span class="token function">pow</span><span class="token punctuation">(</span><span class="token number">2</span><span class="token punctuation">,</span> <span class="token number">53</span><span class="token punctuation">)</span> <span class="token operator">===</span> Math<span class="token punctuation">.</span><span class="token function">pow</span><span class="token punctuation">(</span><span class="token number">2</span><span class="token punctuation">,</span> <span class="token number">53</span><span class="token punctuation">)</span> <span class="token operator">+</span> <span class="token number">1</span> <span class="token comment">// true</span>
    326. 

  326. 

  327. <span class="token comment">// 超过 2 的 1024 次方的数值，无法表示</span>
    328. 

  328. Math<span class="token punctuation">.</span><span class="token function">pow</span><span class="token punctuation">(</span><span class="token number">2</span><span class="token punctuation">,</span> <span class="token number">1024</span><span class="token punctuation">)</span> <span class="token comment">// Infinity</span>
    329. 

  329. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br></div></div><p><a href="https://github.com/tc39/proposal-bigint" target="_blank" rel="noopener noreferrer">ES2020<span><svg xmlns="http://www.w3.org/2000/svg" aria-hidden="true" focusable="false" x="0px" y="0px" viewBox="0 0 100 100" width="15" height="15" class="icon outbound"><path fill="currentColor" d="M18.8,85.1h56l0,0c2.2,0,4-1.8,4-4v-32h-8v28h-48v-48h28v-8h-32l0,0c-2.2,0-4,1.8-4,4v56C14.8,83.3,16.6,85.1,18.8,85.1z"></path> <polygon fill="currentColor" points="45.7,48.7 51.3,54.3 77.2,28.5 77.2,37.2 85.2,37.2 85.2,14.9 62.8,14.9 62.8,22.9 71.5,22.9"></polygon></svg> <span class="sr-only">(opens new window)</span></span></a> 引入了一种新的数据类型 BigInt（大整数），来解决这个问题。BigInt 只用来表示整数，没有位数的限制，任何位数的整数都可以精确表示。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token keyword">const</span> a <span class="token operator">=</span> <span class="token number">2172141653n</span><span class="token punctuation">;</span>
    330. 

  330. <span class="token keyword">const</span> b <span class="token operator">=</span> <span class="token number">15346349309n</span><span class="token punctuation">;</span>
    331. 

  331. 

  332. <span class="token comment">// BigInt 可以保持精度</span>
    333. 

  333. a <span class="token operator">*</span> b <span class="token comment">// 33334444555566667777n</span>
    334. 

  334. 

  335. <span class="token comment">// 普通整数无法保持精度</span>
    336. 

  336. <span class="token function">Number</span><span class="token punctuation">(</span>a<span class="token punctuation">)</span> <span class="token operator">*</span> <span class="token function">Number</span><span class="token punctuation">(</span>b<span class="token punctuation">)</span> <span class="token comment">// 33334444555566670000</span>
    337. 

  337. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br><span class="line-number">7</span><br><span class="line-number">8</span><br></div></div><p>为了与 Number 类型区别，BigInt 类型的数据必须添加后缀<code>n</code>。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token number">1234</span> <span class="token comment">// 普通整数</span>
    338. 

  338. <span class="token number">1234n</span> <span class="token comment">// BigInt</span>
    339. 

  339. 

  340. <span class="token comment">// BigInt 的运算</span>
    341. 

  341. <span class="token number">1n</span> <span class="token operator">+</span> <span class="token number">2n</span> <span class="token comment">// 3n</span>
    342. 

  342. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br></div></div><p>BigInt 同样可以使用各种进制表示，都要加上后缀<code>n</code>。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token number">0b1101n</span> <span class="token comment">// 二进制</span>
    343. 

  343. <span class="token number">0o777n</span> <span class="token comment">// 八进制</span>
    344. 

  344. <span class="token number">0xFFn</span> <span class="token comment">// 十六进制</span>
    345. 

  345. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br></div></div><p>BigInt 与普通整数是两种值，它们之间并不相等。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token number">42n</span> <span class="token operator">===</span> <span class="token number">42</span> <span class="token comment">// false</span>
    346. 

  346. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br></div></div><p><code>typeof</code>运算符对于 BigInt 类型的数据返回<code>bigint</code>。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token keyword">typeof</span> <span class="token number">123n</span> <span class="token comment">// 'bigint'</span>
    347. 

  347. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br></div></div><p>BigInt 可以使用负号（<code>-</code>），但是不能使用正号（<code>+</code>），因为会与 asm.js 冲突。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token operator">-</span><span class="token number">42n</span> <span class="token comment">// 正确</span>
    348. 

  348. <span class="token operator">+</span><span class="token number">42n</span> <span class="token comment">// 报错</span>
    349. 

  349. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br></div></div><p>JavaScript 以前不能计算70的阶乘（即<code>70!</code>），因为超出了可以表示的精度。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token keyword">let</span> p <span class="token operator">=</span> <span class="token number">1</span><span class="token punctuation">;</span>
    350. 

  350. <span class="token keyword">for</span> <span class="token punctuation">(</span><span class="token keyword">let</span> i <span class="token operator">=</span> <span class="token number">1</span><span class="token punctuation">;</span> i <span class="token operator">&lt;=</span> <span class="token number">70</span><span class="token punctuation">;</span> i<span class="token operator">++</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
    351. 

  351.   p <span class="token operator">*=</span> i<span class="token punctuation">;</span>
    352. 

  352. <span class="token punctuation">}</span>
    353. 

  353. console<span class="token punctuation">.</span><span class="token function">log</span><span class="token punctuation">(</span>p<span class="token punctuation">)</span><span class="token punctuation">;</span> <span class="token comment">// 1.197857166996989e+100</span>
    354. 

  354. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br></div></div><p>现在支持大整数了，就可以算了，浏览器的开发者工具运行下面代码，就OK。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token keyword">let</span> p <span class="token operator">=</span> <span class="token number">1n</span><span class="token punctuation">;</span>
    355. 

  355. <span class="token keyword">for</span> <span class="token punctuation">(</span><span class="token keyword">let</span> i <span class="token operator">=</span> <span class="token number">1n</span><span class="token punctuation">;</span> i <span class="token operator">&lt;=</span> <span class="token number">70n</span><span class="token punctuation">;</span> i<span class="token operator">++</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
    356. 

  356.   p <span class="token operator">*=</span> i<span class="token punctuation">;</span>
    357. 

  357. <span class="token punctuation">}</span>
    358. 

  358. console<span class="token punctuation">.</span><span class="token function">log</span><span class="token punctuation">(</span>p<span class="token punctuation">)</span><span class="token punctuation">;</span> <span class="token comment">// 11978571...00000000n</span>
    359. 

  359. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br></div></div><h3 id="bigint-对象"><a href="#bigint-对象" class="header-anchor">#</a> BigInt 对象</h3> <p>JavaScript 原生提供<code>BigInt</code>对象，可以用作构造函数生成 BigInt 类型的数值。转换规则基本与<code>Number()</code>一致，将其他类型的值转为 BigInt。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token function">BigInt</span><span class="token punctuation">(</span><span class="token number">123</span><span class="token punctuation">)</span> <span class="token comment">// 123n</span>
    360. 

  360. <span class="token function">BigInt</span><span class="token punctuation">(</span><span class="token string">'123'</span><span class="token punctuation">)</span> <span class="token comment">// 123n</span>
    361. 

  361. <span class="token function">BigInt</span><span class="token punctuation">(</span><span class="token boolean">false</span><span class="token punctuation">)</span> <span class="token comment">// 0n</span>
    362. 

  362. <span class="token function">BigInt</span><span class="token punctuation">(</span><span class="token boolean">true</span><span class="token punctuation">)</span> <span class="token comment">// 1n</span>
    363. 

  363. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br></div></div><p><code>BigInt()</code>构造函数必须有参数，而且参数必须可以正常转为数值，下面的用法都会报错。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token keyword">new</span> <span class="token class-name">BigInt</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token comment">// TypeError</span>
    364. 

  364. <span class="token function">BigInt</span><span class="token punctuation">(</span><span class="token keyword">undefined</span><span class="token punctuation">)</span> <span class="token comment">//TypeError</span>
    365. 

  365. <span class="token function">BigInt</span><span class="token punctuation">(</span><span class="token keyword">null</span><span class="token punctuation">)</span> <span class="token comment">// TypeError</span>
    366. 

  366. <span class="token function">BigInt</span><span class="token punctuation">(</span><span class="token string">'123n'</span><span class="token punctuation">)</span> <span class="token comment">// SyntaxError</span>
    367. 

  367. <span class="token function">BigInt</span><span class="token punctuation">(</span><span class="token string">'abc'</span><span class="token punctuation">)</span> <span class="token comment">// SyntaxError</span>
    368. 

  368. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br></div></div><p>上面代码中，尤其值得注意字符串<code>123n</code>无法解析成 Number 类型，所以会报错。</p> <p>参数如果是小数，也会报错。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token function">BigInt</span><span class="token punctuation">(</span><span class="token number">1.5</span><span class="token punctuation">)</span> <span class="token comment">// RangeError</span>
    369. 

  369. <span class="token function">BigInt</span><span class="token punctuation">(</span><span class="token string">'1.5'</span><span class="token punctuation">)</span> <span class="token comment">// SyntaxError</span>
    370. 

  370. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br></div></div><p>BigInt 对象继承了 Object 对象的两个实例方法。</p> <ul><li><code>BigInt.prototype.toString()</code></li> <li><code>BigInt.prototype.valueOf()</code></li></ul> <p>它还继承了 Number 对象的一个实例方法。</p> <ul><li><code>BigInt.prototype.toLocaleString()</code></li></ul> <p>此外，还提供了三个静态方法。</p> <ul><li><code>BigInt.asUintN(width, BigInt)</code>： 给定的 BigInt 转为 0 到 2<sup>width</sup> - 1 之间对应的值。</li> <li><code>BigInt.asIntN(width, BigInt)</code>：给定的 BigInt 转为 -2<sup>width - 1</sup> 到 2<sup>width - 1</sup> - 1 之间对应的值。</li> <li><code>BigInt.parseInt(string[, radix])</code>：近似于<code>Number.parseInt()</code>，将一个字符串转换成指定进制的 BigInt。</li></ul> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token keyword">const</span> max <span class="token operator">=</span> <span class="token number">2n</span> <span class="token operator">**</span> <span class="token punctuation">(</span><span class="token number">64n</span> <span class="token operator">-</span> <span class="token number">1n</span><span class="token punctuation">)</span> <span class="token operator">-</span> <span class="token number">1n</span><span class="token punctuation">;</span>
    371. 

  371. 

  372. BigInt<span class="token punctuation">.</span><span class="token function">asIntN</span><span class="token punctuation">(</span><span class="token number">64</span><span class="token punctuation">,</span> max<span class="token punctuation">)</span>
    373. 

  373. <span class="token comment">// 9223372036854775807n</span>
    374. 

  374. BigInt<span class="token punctuation">.</span><span class="token function">asIntN</span><span class="token punctuation">(</span><span class="token number">64</span><span class="token punctuation">,</span> max <span class="token operator">+</span> <span class="token number">1n</span><span class="token punctuation">)</span>
    375. 

  375. <span class="token comment">// -9223372036854775808n</span>
    376. 

  376. BigInt<span class="token punctuation">.</span><span class="token function">asUintN</span><span class="token punctuation">(</span><span class="token number">64</span><span class="token punctuation">,</span> max <span class="token operator">+</span> <span class="token number">1n</span><span class="token punctuation">)</span>
    377. 

  377. <span class="token comment">// 9223372036854775808n</span>
    378. 

  378. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br><span class="line-number">7</span><br><span class="line-number">8</span><br></div></div><p>上面代码中，<code>max</code>是64位带符号的 BigInt 所能表示的最大值。如果对这个值加<code>1n</code>，<code>BigInt.asIntN()</code>将会返回一个负值，因为这时新增的一位将被解释为符号位。而<code>BigInt.asUintN()</code>方法由于不存在符号位，所以可以正确返回结果。</p> <p>如果<code>BigInt.asIntN()</code>和<code>BigInt.asUintN()</code>指定的位数，小于数值本身的位数，那么头部的位将被舍弃。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token keyword">const</span> max <span class="token operator">=</span> <span class="token number">2n</span> <span class="token operator">**</span> <span class="token punctuation">(</span><span class="token number">64n</span> <span class="token operator">-</span> <span class="token number">1n</span><span class="token punctuation">)</span> <span class="token operator">-</span> <span class="token number">1n</span><span class="token punctuation">;</span>
    379. 

  379. 

  380. BigInt<span class="token punctuation">.</span><span class="token function">asIntN</span><span class="token punctuation">(</span><span class="token number">32</span><span class="token punctuation">,</span> max<span class="token punctuation">)</span> <span class="token comment">// -1n</span>
    381. 

  381. BigInt<span class="token punctuation">.</span><span class="token function">asUintN</span><span class="token punctuation">(</span><span class="token number">32</span><span class="token punctuation">,</span> max<span class="token punctuation">)</span> <span class="token comment">// 4294967295n</span>
    382. 

  382. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br></div></div><p>上面代码中，<code>max</code>是一个64位的 BigInt，如果转为32位，前面的32位都会被舍弃。</p> <p>下面是<code>BigInt.parseInt()</code>的例子。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token comment">// Number.parseInt() 与 BigInt.parseInt() 的对比</span>
    383. 

  383. Number<span class="token punctuation">.</span><span class="token function">parseInt</span><span class="token punctuation">(</span><span class="token string">'9007199254740993'</span><span class="token punctuation">,</span> <span class="token number">10</span><span class="token punctuation">)</span>
    384. 

  384. <span class="token comment">// 9007199254740992</span>
    385. 

  385. BigInt<span class="token punctuation">.</span><span class="token function">parseInt</span><span class="token punctuation">(</span><span class="token string">'9007199254740993'</span><span class="token punctuation">,</span> <span class="token number">10</span><span class="token punctuation">)</span>
    386. 

  386. <span class="token comment">// 9007199254740993n</span>
    387. 

  387. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br></div></div><p>上面代码中，由于有效数字超出了最大限度，<code>Number.parseInt</code>方法返回的结果是不精确的，而<code>BigInt.parseInt</code>方法正确返回了对应的 BigInt。</p> <p>对于二进制数组，BigInt 新增了两个类型<code>BigUint64Array</code>和<code>BigInt64Array</code>，这两种数据类型返回的都是64位 BigInt。<code>DataView</code>对象的实例方法<code>DataView.prototype.getBigInt64()</code>和<code>DataView.prototype.getBigUint64()</code>，返回的也是 BigInt。</p> <h3 id="转换规则"><a href="#转换规则" class="header-anchor">#</a> 转换规则</h3> <p>可以使用<code>Boolean()</code>、<code>Number()</code>和<code>String()</code>这三个方法，将 BigInt 可以转为布尔值、数值和字符串类型。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token function">Boolean</span><span class="token punctuation">(</span><span class="token number">0n</span><span class="token punctuation">)</span> <span class="token comment">// false</span>
    388. 

  388. <span class="token function">Boolean</span><span class="token punctuation">(</span><span class="token number">1n</span><span class="token punctuation">)</span> <span class="token comment">// true</span>
    389. 

  389. <span class="token function">Number</span><span class="token punctuation">(</span><span class="token number">1n</span><span class="token punctuation">)</span>  <span class="token comment">// 1</span>
    390. 

  390. <span class="token function">String</span><span class="token punctuation">(</span><span class="token number">1n</span><span class="token punctuation">)</span>  <span class="token comment">// &quot;1&quot;</span>
    391. 

  391. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br></div></div><p>上面代码中，注意最后一个例子，转为字符串时后缀<code>n</code>会消失。</p> <p>另外，取反运算符（<code>!</code>）也可以将 BigInt 转为布尔值。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token operator">!</span><span class="token number">0n</span> <span class="token comment">// true</span>
    392. 

  392. <span class="token operator">!</span><span class="token number">1n</span> <span class="token comment">// false</span>
    393. 

  393. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br></div></div><h3 id="数学运算"><a href="#数学运算" class="header-anchor">#</a> 数学运算</h3> <p>数学运算方面，BigInt 类型的<code>+</code>、<code>-</code>、<code>*</code>和<code>**</code>这四个二元运算符，与 Number 类型的行为一致。除法运算<code>/</code>会舍去小数部分，返回一个整数。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token number">9n</span> <span class="token operator">/</span> <span class="token number">5n</span>
    394. 

  394. <span class="token comment">// 1n</span>
    395. 

  395. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br></div></div><p>几乎所有的数值运算符都可以用在 BigInt，但是有两个例外。</p> <ul><li>不带符号的右移位运算符<code>&gt;&gt;&gt;</code></li> <li>一元的求正运算符<code>+</code></li></ul> <p>上面两个运算符用在 BigInt 会报错。前者是因为<code>&gt;&gt;&gt;</code>运算符是不带符号的，但是 BigInt 总是带有符号的，导致该运算无意义，完全等同于右移运算符<code>&gt;&gt;</code>。后者是因为一元运算符<code>+</code>在 asm.js 里面总是返回 Number 类型，为了不破坏 asm.js 就规定<code>+1n</code>会报错。</p> <p>BigInt 不能与普通数值进行混合运算。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token number">1n</span> <span class="token operator">+</span> <span class="token number">1.3</span> <span class="token comment">// 报错</span>
    396. 

  396. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br></div></div><p>上面代码报错是因为无论返回的是 BigInt 或 Number，都会导致丢失精度信息。比如<code>(2n**53n + 1n) + 0.5</code>这个表达式，如果返回 BigInt 类型，<code>0.5</code>这个小数部分会丢失；如果返回 Number 类型，有效精度只能保持 53 位，导致精度下降。</p> <p>同样的原因，如果一个标准库函数的参数预期是 Number 类型，但是得到的是一个 BigInt，就会报错。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token comment">// 错误的写法</span>
    397. 

  397. Math<span class="token punctuation">.</span><span class="token function">sqrt</span><span class="token punctuation">(</span><span class="token number">4n</span><span class="token punctuation">)</span> <span class="token comment">// 报错</span>
    398. 

  398. 

  399. <span class="token comment">// 正确的写法</span>
    400. 

  400. Math<span class="token punctuation">.</span><span class="token function">sqrt</span><span class="token punctuation">(</span><span class="token function">Number</span><span class="token punctuation">(</span><span class="token number">4n</span><span class="token punctuation">)</span><span class="token punctuation">)</span> <span class="token comment">// 2</span>
    401. 

  401. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br></div></div><p>上面代码中，<code>Math.sqrt</code>的参数预期是 Number 类型，如果是 BigInt 就会报错，必须先用<code>Number</code>方法转一下类型，才能进行计算。</p> <p>asm.js 里面，<code>|0</code>跟在一个数值的后面会返回一个32位整数。根据不能与 Number 类型混合运算的规则，BigInt 如果与<code>|0</code>进行运算会报错。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token number">1n</span> <span class="token operator">|</span> <span class="token number">0</span> <span class="token comment">// 报错</span>
    402. 

  402. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br></div></div><h3 id="其他运算"><a href="#其他运算" class="header-anchor">#</a> 其他运算</h3> <p>BigInt 对应的布尔值，与 Number 类型一致，即<code>0n</code>会转为<code>false</code>，其他值转为<code>true</code>。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token keyword">if</span> <span class="token punctuation">(</span><span class="token number">0n</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
    403. 

  403.   console<span class="token punctuation">.</span><span class="token function">log</span><span class="token punctuation">(</span><span class="token string">'if'</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    404. 

  404. <span class="token punctuation">}</span> <span class="token keyword">else</span> <span class="token punctuation">{</span>
    405. 

  405.   console<span class="token punctuation">.</span><span class="token function">log</span><span class="token punctuation">(</span><span class="token string">'else'</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    406. 

  406. <span class="token punctuation">}</span>
    407. 

  407. <span class="token comment">// else</span>
    408. 

  408. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br></div></div><p>上面代码中，<code>0n</code>对应<code>false</code>，所以会进入<code>else</code>子句。</p> <p>比较运算符（比如<code>&gt;</code>）和相等运算符（<code>==</code>）允许 BigInt 与其他类型的值混合计算，因为这样做不会损失精度。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token number">0n</span> <span class="token operator">&lt;</span> <span class="token number">1</span> <span class="token comment">// true</span>
    409. 

  409. <span class="token number">0n</span> <span class="token operator">&lt;</span> <span class="token boolean">true</span> <span class="token comment">// true</span>
    410. 

  410. <span class="token number">0n</span> <span class="token operator">==</span> <span class="token number">0</span> <span class="token comment">// true</span>
    411. 

  411. <span class="token number">0n</span> <span class="token operator">==</span> <span class="token boolean">false</span> <span class="token comment">// true</span>
    412. 

  412. <span class="token number">0n</span> <span class="token operator">===</span> <span class="token number">0</span> <span class="token comment">// false</span>
    413. 

  413. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br></div></div><p>BigInt 与字符串混合运算时，会先转为字符串，再进行运算。</p> <div class="language-javascript line-numbers-mode"><pre class="language-javascript"><code><span class="token string">''</span> <span class="token operator">+</span> <span class="token number">123n</span> <span class="token comment">// &quot;123&quot;</span>
    414. 

  414. </code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br></div></div></div></div>  <div class="page-edit"><div class="edit-link"><a href="https://github.com/heBody/blog/edit/master/docs/《ES6 教程》笔记/07.数值的扩展.md" target="_blank" rel="noopener noreferrer">编辑</a> <span><svg xmlns="http://www.w3.org/2000/svg" aria-hidden="true" focusable="false" x="0px" y="0px" viewBox="0 0 100 100" width="15" height="15" class="icon outbound"><path fill="currentColor" d="M18.8,85.1h56l0,0c2.2,0,4-1.8,4-4v-32h-8v28h-48v-48h28v-8h-32l0,0c-2.2,0-4,1.8-4,4v56C14.8,83.3,16.6,85.1,18.8,85.1z"></path> <polygon fill="currentColor" points="45.7,48.7 51.3,54.3 77.2,28.5 77.2,37.2 85.2,37.2 85.2,14.9 62.8,14.9 62.8,22.9 71.5,22.9"></polygon></svg> <span class="sr-only">(opens new window)</span></span></div> <div class="tags"><a href="/blog/tags/?tag=ES6" title="标签">#ES6</a></div> <div class="last-updated"><span class="prefix">上次更新:</span> <span class="time">2022/12/14, 19:36:42</span></div></div> <div class="page-nav-wapper"><div class="page-nav-centre-wrap"><a href="/blog/pages/0473261a6ab0ee8c/" class="page-nav-centre page-nav-centre-prev"><div class="tooltip">正则的扩展</div></a> <a href="/blog/pages/8ed309d668b20264/" class="page-nav-centre page-nav-centre-next"><div class="tooltip">函数的扩展</div></a></div> <div class="page-nav"><p class="inner"><span class="prev">
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