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- (window.webpackJsonp=window.webpackJsonp||[]).push([[165],{493:function(s,t,a){"use strict";a.r(t);var n=a(3),e=Object(n.a)({},(function(){var s=this,t=s._self._c;return t("ContentSlotsDistributor",{attrs:{"slot-key":s.$parent.slotKey}},[t("h1",{attrs:{id:"变量声明"}},[t("a",{staticClass:"header-anchor",attrs:{href:"#变量声明"}},[s._v("#")]),s._v(" 变量声明")]),s._v(" "),t("p",[t("code",[s._v("let")]),s._v(" 和 "),t("code",[s._v("const")]),s._v(" 是 JavaScript 里相对较新的变量声明方式。"),t("code",[s._v("let")]),s._v(" 在很多方面与 "),t("code",[s._v("var")]),s._v(" 是相似的，但是可以帮助大家避免在 JavaScript 里常见一些问题。"),t("code",[s._v("const")]),s._v(" 是对 "),t("code",[s._v("let")]),s._v(" 的一个增强，它能阻止对一个变量再次赋值。")]),s._v(" "),t("p",[s._v("因为 TypeScript 是 JavaScript 的超集，所以它本身就支持 "),t("code",[s._v("let")]),s._v(" 和 "),t("code",[s._v("const")]),s._v("。 下面我们会详细说明这些新的声明方式以及为什么推荐使用它们来代替 "),t("code",[s._v("var")]),s._v("。")]),s._v(" "),t("p",[s._v("如果你已经对 "),t("code",[s._v("var")]),s._v(" 声明的怪异之处了如指掌，那么你可以轻松地略过这节。")]),s._v(" 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"),t("div",{staticClass:"line-numbers-wrapper"},[t("span",{staticClass:"line-number"},[s._v("1")]),t("br"),t("span",{staticClass:"line-number"},[s._v("2")]),t("br"),t("span",{staticClass:"line-number"},[s._v("3")]),t("br"),t("span",{staticClass:"line-number"},[s._v("4")]),t("br"),t("span",{staticClass:"line-number"},[s._v("5")]),t("br"),t("span",{staticClass:"line-number"},[s._v("6")]),t("br"),t("span",{staticClass:"line-number"},[s._v("7")]),t("br"),t("span",{staticClass:"line-number"},[s._v("8")]),t("br"),t("span",{staticClass:"line-number"},[s._v("9")]),t("br"),t("span",{staticClass:"line-number"},[s._v("10")]),t("br")])]),t("p",[s._v("上面的例子是一个典型的闭包场景，"),t("code",[s._v("g")]),s._v(" 可以获取到 "),t("code",[s._v("f")]),s._v(" 函数里定义的 "),t("code",[s._v("a")]),s._v(" 变量。 每当 "),t("code",[s._v("g")]),s._v(" 被调用时，它都可以访问到 "),t("code",[s._v("f")]),s._v(" 里的 "),t("code",[s._v("a")]),s._v(" 变量。 即使当 "),t("code",[s._v("g")]),s._v(" 在 "),t("code",[s._v("f")]),s._v(" 已经执行完后才被调用，它仍然可以访问 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'10'")]),s._v("\n"),t("span",{pre:!0,attrs:{class:"token function"}},[s._v("f")]),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v("(")]),t("span",{pre:!0,attrs:{class:"token boolean"}},[s._v("false")]),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v(")")]),s._v(" "),t("span",{pre:!0,attrs:{class:"token comment"}},[s._v("// returns 'undefined'")]),s._v("\n")])]),s._v(" "),t("div",{staticClass:"line-numbers-wrapper"},[t("span",{staticClass:"line-number"},[s._v("1")]),t("br"),t("span",{staticClass:"line-number"},[s._v("2")]),t("br"),t("span",{staticClass:"line-number"},[s._v("3")]),t("br"),t("span",{staticClass:"line-number"},[s._v("4")]),t("br"),t("span",{staticClass:"line-number"},[s._v("5")]),t("br"),t("span",{staticClass:"line-number"},[s._v("6")]),t("br"),t("span",{staticClass:"line-number"},[s._v("7")]),t("br"),t("span",{staticClass:"line-number"},[s._v("8")]),t("br"),t("span",{staticClass:"line-number"},[s._v("9")]),t("br"),t("span",{staticClass:"line-number"},[s._v("10")]),t("br")])]),t("p",[s._v("有些同学可能要多看几遍这个例子。 变量 "),t("code",[s._v("x")]),s._v(" 是定义在 "),t("code",[s._v("if")]),s._v(" 语句里面，但是我们却可以在语句的外面访问它。 这是因为 "),t("code",[s._v("var")]),s._v(" 声明的作用域是函数作用域，函数参数也使用函数作用域。")]),s._v(" "),t("p",[s._v("这些作用域规则可能会引发一些错误。 其中之一就是，多次声明同一个变量并不会报错：")]),s._v(" "),t("div",{staticClass:"language-javascript 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"),t("div",{staticClass:"line-numbers-wrapper"},[t("span",{staticClass:"line-number"},[s._v("1")]),t("br"),t("span",{staticClass:"line-number"},[s._v("2")]),t("br"),t("span",{staticClass:"line-number"},[s._v("3")]),t("br"),t("span",{staticClass:"line-number"},[s._v("4")]),t("br"),t("span",{staticClass:"line-number"},[s._v("5")]),t("br"),t("span",{staticClass:"line-number"},[s._v("6")]),t("br"),t("span",{staticClass:"line-number"},[s._v("7")]),t("br"),t("span",{staticClass:"line-number"},[s._v("8")]),t("br"),t("span",{staticClass:"line-number"},[s._v("9")]),t("br"),t("span",{staticClass:"line-number"},[s._v("10")]),t("br"),t("span",{staticClass:"line-number"},[s._v("11")]),t("br")])]),t("p",[s._v("这里很容易看出一些问题，里层的 "),t("code",[s._v("for")]),s._v(" 循环会覆盖变量 "),t("code",[s._v("i")]),s._v("，因为所有 "),t("code",[s._v("i")]),s._v(" 都引用相同的函数作用域内的变量。 有经验的开发者们很清楚，这些问题可能在代码审查时漏掉，引发无穷的麻烦。")]),s._v(" "),t("h3",{attrs:{id:"捕获变量怪异之处"}},[t("a",{staticClass:"header-anchor",attrs:{href:"#捕获变量怪异之处"}},[s._v("#")]),s._v(" 捕获变量怪异之处")]),s._v(" "),t("p",[s._v("猜一下下面的代码会返回什么：")]),s._v(" "),t("div",{staticClass:"language-javascript line-numbers-mode"},[t("pre",{pre:!0,attrs:{class:"language-javascript"}},[t("code",[t("span",{pre:!0,attrs:{class:"token keyword"}},[s._v("for")]),s._v(" "),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v("(")]),t("span",{pre:!0,attrs:{class:"token keyword"}},[s._v("var")]),s._v(" i "),t("span",{pre:!0,attrs:{class:"token operator"}},[s._v("=")]),s._v(" "),t("span",{pre:!0,attrs:{class:"token number"}},[s._v("0")]),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v(";")]),s._v(" i "),t("span",{pre:!0,attrs:{class:"token operator"}},[s._v("<")]),s._v(" "),t("span",{pre:!0,attrs:{class:"token number"}},[s._v("10")]),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v(";")]),s._v(" i"),t("span",{pre:!0,attrs:{class:"token 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存在一些问题，这恰好说明了为什么用 "),t("code",[s._v("let")]),s._v(" 语句来声明变量。 除了名字不同外， "),t("code",[s._v("let")]),s._v(" 与 "),t("code",[s._v("var")]),s._v(" 的写法一致：")]),s._v(" "),t("div",{staticClass:"language-javascript line-numbers-mode"},[t("pre",{pre:!0,attrs:{class:"language-javascript"}},[t("code",[t("span",{pre:!0,attrs:{class:"token keyword"}},[s._v("let")]),s._v(" hello "),t("span",{pre:!0,attrs:{class:"token operator"}},[s._v("=")]),s._v(" "),t("span",{pre:!0,attrs:{class:"token string"}},[s._v("'Hello!'")]),s._v("\n")])]),s._v(" "),t("div",{staticClass:"line-numbers-wrapper"},[t("span",{staticClass:"line-number"},[s._v("1")]),t("br")])]),t("p",[s._v("主要的区别不在语法上，而是语义，我们接下来会深入研究。")]),s._v(" "),t("h3",{attrs:{id:"块作用域"}},[t("a",{staticClass:"header-anchor",attrs:{href:"#块作用域"}},[s._v("#")]),s._v(" 块作用域")]),s._v(" "),t("p",[s._v("当用 "),t("code",[s._v("let")]),s._v(" 声明一个变量，它使用的是块作用域。 不同于使用 "),t("code",[s._v("var")]),s._v(" 声明的变量那样可以在包含它们的函数外访问，块作用域变量在包含它们的块或 "),t("code",[s._v("for")]),s._v(" 循环之外是不能访问的。")]),s._v(" "),t("div",{staticClass:"language-typescript line-numbers-mode"},[t("pre",{pre:!0,attrs:{class:"language-typescript"}},[t("code",[t("span",{pre:!0,attrs:{class:"token keyword"}},[s._v("function")]),s._v(" "),t("span",{pre:!0,attrs:{class:"token function"}},[s._v("f")]),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v("(")]),s._v("input"),t("span",{pre:!0,attrs:{class:"token operator"}},[s._v(":")]),s._v(" "),t("span",{pre:!0,attrs:{class:"token builtin"}},[s._v("boolean")]),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v(")")]),s._v(" "),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v("{")]),s._v("\n "),t("span",{pre:!0,attrs:{class:"token keyword"}},[s._v("let")]),s._v(" a "),t("span",{pre:!0,attrs:{class:"token operator"}},[s._v("=")]),s._v(" "),t("span",{pre:!0,attrs:{class:"token number"}},[s._v("100")]),s._v("\n\n "),t("span",{pre:!0,attrs:{class:"token keyword"}},[s._v("if")]),s._v(" "),t("span",{pre:!0,attrs:{class:"token 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语句块里。")]),s._v(" "),t("p",[s._v("在 "),t("code",[s._v("catch")]),s._v(" 语句里声明的变量也具有同样的作用域规则。")]),s._v(" "),t("div",{staticClass:"language-typescript line-numbers-mode"},[t("pre",{pre:!0,attrs:{class:"language-typescript"}},[t("code",[t("span",{pre:!0,attrs:{class:"token keyword"}},[s._v("try")]),s._v(" "),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v("{")]),s._v("\n "),t("span",{pre:!0,attrs:{class:"token keyword"}},[s._v("throw")]),s._v(" "),t("span",{pre:!0,attrs:{class:"token string"}},[s._v("'Oh no!'")]),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v(";")]),s._v("\n"),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v("}")]),s._v("\n"),t("span",{pre:!0,attrs:{class:"token keyword"}},[s._v("catch")]),s._v(" "),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v("(")]),s._v("e"),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v(")")]),s._v(" "),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v("{")]),s._v("\n "),t("span",{pre:!0,attrs:{class:"token builtin"}},[s._v("console")]),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v(".")]),t("span",{pre:!0,attrs:{class:"token function"}},[s._v("log")]),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v("(")]),t("span",{pre:!0,attrs:{class:"token string"}},[s._v("'Catch it.'")]),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v(")")]),s._v("\n"),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v("}")]),s._v("\n\n"),t("span",{pre:!0,attrs:{class:"token comment"}},[s._v("// Error: 'e' 在这里不存在")]),s._v("\n"),t("span",{pre:!0,attrs:{class:"token builtin"}},[s._v("console")]),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v(".")]),t("span",{pre:!0,attrs:{class:"token function"}},[s._v("log")]),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v("(")]),s._v("e"),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v(")")]),s._v("\n")])]),s._v(" "),t("div",{staticClass:"line-numbers-wrapper"},[t("span",{staticClass:"line-number"},[s._v("1")]),t("br"),t("span",{staticClass:"line-number"},[s._v("2")]),t("br"),t("span",{staticClass:"line-number"},[s._v("3")]),t("br"),t("span",{staticClass:"line-number"},[s._v("4")]),t("br"),t("span",{staticClass:"line-number"},[s._v("5")]),t("br"),t("span",{staticClass:"line-number"},[s._v("6")]),t("br"),t("span",{staticClass:"line-number"},[s._v("7")]),t("br"),t("span",{staticClass:"line-number"},[s._v("8")]),t("br"),t("span",{staticClass:"line-number"},[s._v("9")]),t("br")])]),t("p",[s._v("拥有块级作用域的变量的另一个特点是，它们不能在被声明之前读或写。 虽然这些变量始终“存在”于它们的作用域里，但在直到声明它的代码之前的区域都属于"),t("em",[s._v("暂时性死区")]),s._v("。 它只是用来说明我们不能在 "),t("code",[s._v("let")]),s._v(" 语句之前访问它们，幸运的是 "),t("code",[s._v("TypeScript")]),s._v(" 可以告诉我们这些信息。")]),s._v(" "),t("div",{staticClass:"language-typescript line-numbers-mode"},[t("pre",{pre:!0,attrs:{class:"language-typescript"}},[t("code",[s._v("a"),t("span",{pre:!0,attrs:{class:"token operator"}},[s._v("++")]),s._v(" "),t("span",{pre:!0,attrs:{class:"token comment"}},[s._v("// TS2448: Block-scoped variable 'a' used before its declaration.")]),s._v("\n"),t("span",{pre:!0,attrs:{class:"token keyword"}},[s._v("let")]),s._v(" a\n")])]),s._v(" "),t("div",{staticClass:"line-numbers-wrapper"},[t("span",{staticClass:"line-number"},[s._v("1")]),t("br"),t("span",{staticClass:"line-number"},[s._v("2")]),t("br")])]),t("p",[s._v("注意一点，我们仍然可以在一个拥有块作用域变量被声明前获取它。 只是我们不能在变量声明前去调用那个函数。 如果生成代码目标为 ES2015，现代的运行时会抛出一个错误；然而，现今 TypeScript 是不会报错的。")]),s._v(" "),t("div",{staticClass:"language-typescript line-numbers-mode"},[t("pre",{pre:!0,attrs:{class:"language-typescript"}},[t("code",[t("span",{pre:!0,attrs:{class:"token keyword"}},[s._v("function")]),s._v(" "),t("span",{pre:!0,attrs:{class:"token function"}},[s._v("foo")]),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v("(")]),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v(")")]),s._v(" 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变量的内部状态是可修改的。 幸运的是，"),t("code",[s._v("TypeScript")]),s._v(" 允许你将对象的成员设置成只读的。接口一章有详细说明。")]),s._v(" "),t("h2",{attrs:{id:"let-vs-const"}},[t("a",{staticClass:"header-anchor",attrs:{href:"#let-vs-const"}},[s._v("#")]),s._v(" let vs. const")]),s._v(" "),t("p",[s._v("现在我们有两种作用域相似的声明方式，我们自然会问到底应该使用哪个。与大多数泛泛的问题一样，答案是：依情况而定。")]),s._v(" "),t("p",[s._v("使用最小特权原则，所有变量除了你计划去修改的都应该使用 "),t("code",[s._v("const")]),s._v("。 基本原则就是如果一个变量不需要对它写入，那么其它使用这些代码的人也不能够写入它们，并且要思考为什么会需要对这些变量重新赋值。使用 "),t("code",[s._v("const")]),s._v(" 也可以让我们更容易的推测数据的流动。")]),s._v(" "),t("h2",{attrs:{id:"解构"}},[t("a",{staticClass:"header-anchor",attrs:{href:"#解构"}},[s._v("#")]),s._v(" 解构")]),s._v(" "),t("h3",{attrs:{id:"解构数组"}},[t("a",{staticClass:"header-anchor",attrs:{href:"#解构数组"}},[s._v("#")]),s._v(" 解构数组")]),s._v(" "),t("p",[s._v("最简单的解构莫过于数组的解构赋值了：")]),s._v(" "),t("div",{staticClass:"language-typescript line-numbers-mode"},[t("pre",{pre:!0,attrs:{class:"language-typescript"}},[t("code",[t("span",{pre:!0,attrs:{class:"token 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"),t("span",{pre:!0,attrs:{class:"token builtin"}},[s._v("number")]),s._v(" "),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v("}")]),s._v("\n"),t("span",{pre:!0,attrs:{class:"token keyword"}},[s._v("function")]),s._v(" "),t("span",{pre:!0,attrs:{class:"token function"}},[s._v("f")]),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v("(")]),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v("{")]),s._v(" a"),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v(",")]),s._v(" b "),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v("}")]),t("span",{pre:!0,attrs:{class:"token operator"}},[s._v(":")]),s._v(" "),t("span",{pre:!0,attrs:{class:"token constant"}},[s._v("C")]),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v(")")]),t("span",{pre:!0,attrs:{class:"token operator"}},[s._v(":")]),s._v(" "),t("span",{pre:!0,attrs:{class:"token keyword"}},[s._v("void")]),s._v(" "),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v("{")]),s._v("\n "),t("span",{pre:!0,attrs:{class:"token comment"}},[s._v("// ...")]),s._v("\n"),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v("}")]),s._v("\n")])]),s._v(" "),t("div",{staticClass:"line-numbers-wrapper"},[t("span",{staticClass:"line-number"},[s._v("1")]),t("br"),t("span",{staticClass:"line-number"},[s._v("2")]),t("br"),t("span",{staticClass:"line-number"},[s._v("3")]),t("br"),t("span",{staticClass:"line-number"},[s._v("4")]),t("br")])]),t("p",[s._v("但是，通常情况下更多的是指定默认值，解构默认值有些棘手。 首先，你需要在默认值之前设置其格式。")]),s._v(" "),t("div",{staticClass:"language-typescript line-numbers-mode"},[t("pre",{pre:!0,attrs:{class:"language-typescript"}},[t("code",[t("span",{pre:!0,attrs:{class:"token keyword"}},[s._v("function")]),s._v(" "),t("span",{pre:!0,attrs:{class:"token function"}},[s._v("f")]),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v("(")]),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v("{")]),s._v(" a "),t("span",{pre:!0,attrs:{class:"token operator"}},[s._v("=")]),s._v(" "),t("span",{pre:!0,attrs:{class:"token string"}},[s._v("''")]),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v(",")]),s._v(" b "),t("span",{pre:!0,attrs:{class:"token operator"}},[s._v("=")]),s._v(" "),t("span",{pre:!0,attrs:{class:"token number"}},[s._v("0")]),s._v(" "),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v("}")]),s._v(" "),t("span",{pre:!0,attrs:{class:"token operator"}},[s._v("=")]),s._v(" "),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v("{")]),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v("}")]),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v(")")]),t("span",{pre:!0,attrs:{class:"token operator"}},[s._v(":")]),s._v(" "),t("span",{pre:!0,attrs:{class:"token keyword"}},[s._v("void")]),s._v(" "),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v("{")]),s._v("\n "),t("span",{pre:!0,attrs:{class:"token comment"}},[s._v("// ...")]),s._v("\n"),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v("}")]),s._v("\n"),t("span",{pre:!0,attrs:{class:"token function"}},[s._v("f")]),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v("(")]),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v(")")]),s._v("\n")])]),s._v(" "),t("div",{staticClass:"line-numbers-wrapper"},[t("span",{staticClass:"line-number"},[s._v("1")]),t("br"),t("span",{staticClass:"line-number"},[s._v("2")]),t("br"),t("span",{staticClass:"line-number"},[s._v("3")]),t("br"),t("span",{staticClass:"line-number"},[s._v("4")]),t("br")])]),t("blockquote",[t("p",[s._v("上面的代码是一个类型推断的例子，将在后续章节介绍。")])]),s._v(" "),t("p",[s._v("其次，你需要知道在解构属性上给予一个默认或可选的属性用来替换主初始化列表。 要知道 C 的定义有一个 b 可选属性：")]),s._v(" "),t("div",{staticClass:"language-typescript line-numbers-mode"},[t("pre",{pre:!0,attrs:{class:"language-typescript"}},[t("code",[t("span",{pre:!0,attrs:{class:"token keyword"}},[s._v("function")]),s._v(" "),t("span",{pre:!0,attrs:{class:"token function"}},[s._v("f")]),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v("(")]),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v("{")]),s._v(" a"),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v(",")]),s._v(" b "),t("span",{pre:!0,attrs:{class:"token operator"}},[s._v("=")]),s._v(" "),t("span",{pre:!0,attrs:{class:"token number"}},[s._v("0")]),s._v(" "),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v("}")]),s._v(" "),t("span",{pre:!0,attrs:{class:"token operator"}},[s._v("=")]),s._v(" "),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v("{")]),s._v(" a"),t("span",{pre:!0,attrs:{class:"token operator"}},[s._v(":")]),s._v(" "),t("span",{pre:!0,attrs:{class:"token string"}},[s._v("''")]),s._v(" "),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v("}")]),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v(")")]),t("span",{pre:!0,attrs:{class:"token operator"}},[s._v(":")]),s._v(" "),t("span",{pre:!0,attrs:{class:"token keyword"}},[s._v("void")]),s._v(" "),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v("{")]),s._v("\n "),t("span",{pre:!0,attrs:{class:"token comment"}},[s._v("// ...")]),s._v("\n"),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v("}")]),s._v("\n"),t("span",{pre:!0,attrs:{class:"token function"}},[s._v("f")]),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v("(")]),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v("{")]),s._v(" a"),t("span",{pre:!0,attrs:{class:"token operator"}},[s._v(":")]),s._v(" "),t("span",{pre:!0,attrs:{class:"token string"}},[s._v("'yes'")]),s._v(" "),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v("}")]),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v(")")]),s._v(" "),t("span",{pre:!0,attrs:{class:"token comment"}},[s._v("// OK, 默认 b = 0")]),s._v("\n"),t("span",{pre:!0,attrs:{class:"token function"}},[s._v("f")]),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v("(")]),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v(")")]),s._v(" "),t("span",{pre:!0,attrs:{class:"token comment"}},[s._v("// OK, 默认 a: '', b = 0")]),s._v("\n"),t("span",{pre:!0,attrs:{class:"token function"}},[s._v("f")]),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v("(")]),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v("{")]),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v("}")]),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v(")")]),s._v(" "),t("span",{pre:!0,attrs:{class:"token comment"}},[s._v("// Error, 一旦传入参数则 a 是必须的")]),s._v("\n")])]),s._v(" "),t("div",{staticClass:"line-numbers-wrapper"},[t("span",{staticClass:"line-number"},[s._v("1")]),t("br"),t("span",{staticClass:"line-number"},[s._v("2")]),t("br"),t("span",{staticClass:"line-number"},[s._v("3")]),t("br"),t("span",{staticClass:"line-number"},[s._v("4")]),t("br"),t("span",{staticClass:"line-number"},[s._v("5")]),t("br"),t("span",{staticClass:"line-number"},[s._v("6")]),t("br")])]),t("p",[s._v("要小心使用解构。 从前面的例子可以看出，就算是最简单的解构表达式也是难以理解的。 尤其当存在深层嵌套解构的时候，就算这时没有堆叠在一起的重命名，默认值和类型注解，也是令人难以理解的。 解构表达式要尽量保持小而简单。")]),s._v(" "),t("h2",{attrs:{id:"展开"}},[t("a",{staticClass:"header-anchor",attrs:{href:"#展开"}},[s._v("#")]),s._v(" 展开")]),s._v(" "),t("div",{staticClass:"language-typescript line-numbers-mode"},[t("pre",{pre:!0,attrs:{class:"language-typescript"}},[t("code",[t("span",{pre:!0,attrs:{class:"token keyword"}},[s._v("let")]),s._v(" first "),t("span",{pre:!0,attrs:{class:"token operator"}},[s._v("=")]),s._v(" "),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v("[")]),t("span",{pre:!0,attrs:{class:"token number"}},[s._v("1")]),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v(",")]),s._v(" "),t("span",{pre:!0,attrs:{class:"token number"}},[s._v("2")]),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v("]")]),s._v("\n"),t("span",{pre:!0,attrs:{class:"token keyword"}},[s._v("let")]),s._v(" second "),t("span",{pre:!0,attrs:{class:"token operator"}},[s._v("=")]),s._v(" "),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v("[")]),t("span",{pre:!0,attrs:{class:"token number"}},[s._v("3")]),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v(",")]),s._v(" "),t("span",{pre:!0,attrs:{class:"token number"}},[s._v("4")]),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v("]")]),s._v("\n"),t("span",{pre:!0,attrs:{class:"token keyword"}},[s._v("let")]),s._v(" bothPlus "),t("span",{pre:!0,attrs:{class:"token operator"}},[s._v("=")]),s._v(" "),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v("[")]),t("span",{pre:!0,attrs:{class:"token number"}},[s._v("0")]),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v(",")]),s._v(" "),t("span",{pre:!0,attrs:{class:"token operator"}},[s._v("...")]),s._v("first"),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v(",")]),s._v(" "),t("span",{pre:!0,attrs:{class:"token operator"}},[s._v("...")]),s._v("second"),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v(",")]),s._v(" "),t("span",{pre:!0,attrs:{class:"token number"}},[s._v("5")]),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v("]")]),s._v("\n")])]),s._v(" "),t("div",{staticClass:"line-numbers-wrapper"},[t("span",{staticClass:"line-number"},[s._v("1")]),t("br"),t("span",{staticClass:"line-number"},[s._v("2")]),t("br"),t("span",{staticClass:"line-number"},[s._v("3")]),t("br")])]),t("p",[s._v("这会令 "),t("code",[s._v("bothPlus")]),s._v(" 的值为 "),t("code",[s._v("[0, 1, 2, 3, 4, 5]")]),s._v("。 展开操作创建了 "),t("code",[s._v("first")]),s._v(" 和 "),t("code",[s._v("second的")]),s._v(" 一份浅拷贝。 它们不会被展开操作所改变。")]),s._v(" "),t("p",[s._v("你还可以展开对象：")]),s._v(" "),t("div",{staticClass:"language-typescript line-numbers-mode"},[t("pre",{pre:!0,attrs:{class:"language-typescript"}},[t("code",[t("span",{pre:!0,attrs:{class:"token keyword"}},[s._v("let")]),s._v(" defaults "),t("span",{pre:!0,attrs:{class:"token operator"}},[s._v("=")]),s._v(" "),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v("{")]),s._v(" food"),t("span",{pre:!0,attrs:{class:"token operator"}},[s._v(":")]),s._v(" "),t("span",{pre:!0,attrs:{class:"token string"}},[s._v("'spicy'")]),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v(",")]),s._v(" price"),t("span",{pre:!0,attrs:{class:"token operator"}},[s._v(":")]),s._v(" "),t("span",{pre:!0,attrs:{class:"token string"}},[s._v("'$10'")]),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v(",")]),s._v(" ambiance"),t("span",{pre:!0,attrs:{class:"token operator"}},[s._v(":")]),s._v(" "),t("span",{pre:!0,attrs:{class:"token string"}},[s._v("'noisy'")]),s._v(" "),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v("}")]),s._v("\n"),t("span",{pre:!0,attrs:{class:"token keyword"}},[s._v("let")]),s._v(" search "),t("span",{pre:!0,attrs:{class:"token operator"}},[s._v("=")]),s._v(" "),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v("{")]),s._v(" "),t("span",{pre:!0,attrs:{class:"token operator"}},[s._v("...")]),s._v("defaults"),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v(",")]),s._v(" food"),t("span",{pre:!0,attrs:{class:"token operator"}},[s._v(":")]),s._v(" "),t("span",{pre:!0,attrs:{class:"token string"}},[s._v("'rich'")]),s._v(" "),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v("}")]),s._v("\n")])]),s._v(" "),t("div",{staticClass:"line-numbers-wrapper"},[t("span",{staticClass:"line-number"},[s._v("1")]),t("br"),t("span",{staticClass:"line-number"},[s._v("2")]),t("br")])]),t("p",[s._v("search的值为 "),t("code",[s._v("{ food: 'rich', price: '$10', ambiance: 'noisy' }")]),s._v("。 对象的展开比数组的展开要复杂的多。像数组展开一样，它是从左至右进行处理，但结果仍为对象。这就意味着出现在展开对象后面的属性会覆盖前面的属性。因此，如果我们修改上面的例子，在结尾处进行展开的话：")]),s._v(" "),t("div",{staticClass:"language-typescript line-numbers-mode"},[t("pre",{pre:!0,attrs:{class:"language-typescript"}},[t("code",[t("span",{pre:!0,attrs:{class:"token keyword"}},[s._v("let")]),s._v(" defaults "),t("span",{pre:!0,attrs:{class:"token operator"}},[s._v("=")]),s._v(" "),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v("{")]),s._v(" food"),t("span",{pre:!0,attrs:{class:"token operator"}},[s._v(":")]),s._v(" "),t("span",{pre:!0,attrs:{class:"token string"}},[s._v("'spicy'")]),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v(",")]),s._v(" price"),t("span",{pre:!0,attrs:{class:"token operator"}},[s._v(":")]),s._v(" "),t("span",{pre:!0,attrs:{class:"token string"}},[s._v("'$10'")]),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v(",")]),s._v(" ambiance"),t("span",{pre:!0,attrs:{class:"token operator"}},[s._v(":")]),s._v(" "),t("span",{pre:!0,attrs:{class:"token string"}},[s._v("'noisy'")]),s._v(" "),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v("}")]),s._v("\n"),t("span",{pre:!0,attrs:{class:"token keyword"}},[s._v("let")]),s._v(" search "),t("span",{pre:!0,attrs:{class:"token operator"}},[s._v("=")]),s._v(" "),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v("{")]),s._v(" food"),t("span",{pre:!0,attrs:{class:"token operator"}},[s._v(":")]),s._v(" "),t("span",{pre:!0,attrs:{class:"token string"}},[s._v("'rich'")]),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v(",")]),s._v(" "),t("span",{pre:!0,attrs:{class:"token operator"}},[s._v("...")]),s._v("defaults "),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v("}")]),s._v("\n")])]),s._v(" "),t("div",{staticClass:"line-numbers-wrapper"},[t("span",{staticClass:"line-number"},[s._v("1")]),t("br"),t("span",{staticClass:"line-number"},[s._v("2")]),t("br")])]),t("p",[s._v("那么，"),t("code",[s._v("defaults")]),s._v(" 里的 "),t("code",[s._v("food")]),s._v(" 属性会重写 "),t("code",[s._v("food: 'rich'")]),s._v("，在这里这并不是我们想要的结果。")])])}),[],!1,null,null,null);t.default=e.exports}}]);
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