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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("p",[s._v("上面代码表示，由于两个空对象不相等，所以它们被视为两个值。")]),s._v(" "),t("h3",{attrs:{id:"set-实例的属性和方法"}},[t("a",{staticClass:"header-anchor",attrs:{href:"#set-实例的属性和方法"}},[s._v("#")]),s._v(" Set 实例的属性和方法")]),s._v(" "),t("p",[s._v("Set 结构的实例有以下属性。")]),s._v(" "),t("ul",[t("li",[t("code",[s._v("Set.prototype.constructor")]),s._v("：构造函数，默认就是"),t("code",[s._v("Set")]),s._v("函数。")]),s._v(" "),t("li",[t("code",[s._v("Set.prototype.size")]),s._v("：返回"),t("code",[s._v("Set")]),s._v("实例的成员总数。")])]),s._v(" "),t("p",[s._v("Set 实例的方法分为两大类：操作方法（用于操作数据）和遍历方法（用于遍历成员）。下面先介绍四个操作方法。")]),s._v(" "),t("ul",[t("li",[t("code",[s._v("Set.prototype.add(value)")]),s._v("：添加某个值，返回 Set 结构本身。")]),s._v(" "),t("li",[t("code",[s._v("Set.prototype.delete(value)")]),s._v("：删除某个值，返回一个布尔值，表示删除是否成功。")]),s._v(" "),t("li",[t("code",[s._v("Set.prototype.has(value)")]),s._v("：返回一个布尔值，表示该值是否为"),t("code",[s._v("Set")]),s._v("的成员。")]),s._v(" "),t("li",[t("code",[s._v("Set.prototype.clear()")]),s._v("：清除所有成员，没有返回值。")])]),s._v(" "),t("p",[s._v("上面这些属性和方法的实例如下。")]),s._v(" "),t("div",{staticClass:"language-javascript line-numbers-mode"},[t("pre",{pre:!0,attrs:{class:"language-javascript"}},[t("code",[s._v("s"),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v(".")]),t("span",{pre:!0,attrs:{class:"token function"}},[s._v("add")]),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 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"),t("p",[s._v("扩展运算符（"),t("code",[s._v("...")]),s._v("）内部使用"),t("code",[s._v("for...of")]),s._v("循环，所以也可以用于 Set 结构。")]),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(" set "),t("span",{pre:!0,attrs:{class:"token operator"}},[s._v("=")]),s._v(" "),t("span",{pre:!0,attrs:{class:"token keyword"}},[s._v("new")]),s._v(" "),t("span",{pre:!0,attrs:{class:"token class-name"}},[s._v("Set")]),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 string"}},[s._v("'red'")]),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v(",")]),s._v(" "),t("span",{pre:!0,attrs:{class:"token string"}},[s._v("'green'")]),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v(",")]),s._v(" "),t("span",{pre:!0,attrs:{class:"token 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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("p",[s._v("扩展运算符和 Set 结构相结合，就可以去除数组的重复成员。")]),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(" arr "),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("5")]),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 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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("p",[s._v("因此使用 Set 可以很容易地实现并集（Union）、交集（Intersect）和差集（Difference）。")]),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(" a "),t("span",{pre:!0,attrs:{class:"token operator"}},[s._v("=")]),s._v(" "),t("span",{pre:!0,attrs:{class:"token keyword"}},[s._v("new")]),s._v(" "),t("span",{pre:!0,attrs:{class:"token class-name"}},[s._v("Set")]),t("span",{pre:!0,attrs:{class:"token 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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("\n"),t("span",{pre:!0,attrs:{class:"token comment"}},[s._v("// set {2, 3}")]),s._v("\n\n"),t("span",{pre:!0,attrs:{class:"token comment"}},[s._v("// 差集")]),s._v("\n"),t("span",{pre:!0,attrs:{class:"token keyword"}},[s._v("let")]),s._v(" difference "),t("span",{pre:!0,attrs:{class:"token operator"}},[s._v("=")]),s._v(" "),t("span",{pre:!0,attrs:{class:"token keyword"}},[s._v("new")]),s._v(" "),t("span",{pre:!0,attrs:{class:"token class-name"}},[s._v("Set")]),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("a"),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 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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("span",{staticClass:"line-number"},[s._v("12")]),t("br"),t("span",{staticClass:"line-number"},[s._v("13")]),t("br"),t("span",{staticClass:"line-number"},[s._v("14")]),t("br")])]),t("p",[s._v("如果想在遍历操作中，同步改变原来的 Set 结构，目前没有直接的方法，但有两种变通方法。一种是利用原 Set 结构映射出一个新的结构，然后赋值给原来的 Set 结构；另一种是利用"),t("code",[s._v("Array.from")]),s._v("方法。")]),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 comment"}},[s._v("// 方法一")]),s._v("\n"),t("span",{pre:!0,attrs:{class:"token keyword"}},[s._v("let")]),s._v(" set "),t("span",{pre:!0,attrs:{class:"token operator"}},[s._v("=")]),s._v(" "),t("span",{pre:!0,attrs:{class:"token keyword"}},[s._v("new")]),s._v(" "),t("span",{pre:!0,attrs:{class:"token class-name"}},[s._v("Set")]),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 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(" "),t("span",{pre:!0,attrs:{class:"token number"}},[s._v("3")]),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("\nset "),t("span",{pre:!0,attrs:{class:"token operator"}},[s._v("=")]),s._v(" "),t("span",{pre:!0,attrs:{class:"token keyword"}},[s._v("new")]),s._v(" "),t("span",{pre:!0,attrs:{class:"token class-name"}},[s._v("Set")]),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("set"),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 function"}},[s._v("map")]),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v("(")]),t("span",{pre:!0,attrs:{class:"token parameter"}},[s._v("val")]),s._v(" "),t("span",{pre:!0,attrs:{class:"token operator"}},[s._v("=>")]),s._v(" val "),t("span",{pre:!0,attrs:{class:"token operator"}},[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(")")]),t("span",{pre:!0,attrs:{class:"token punctuation"}},[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("// set的值是2, 4, 6")]),s._v("\n\n"),t("span",{pre:!0,attrs:{class:"token comment"}},[s._v("// 方法二")]),s._v("\n"),t("span",{pre:!0,attrs:{class:"token keyword"}},[s._v("let")]),s._v(" set "),t("span",{pre:!0,attrs:{class:"token operator"}},[s._v("=")]),s._v(" "),t("span",{pre:!0,attrs:{class:"token keyword"}},[s._v("new")]),s._v(" "),t("span",{pre:!0,attrs:{class:"token class-name"}},[s._v("Set")]),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 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(" "),t("span",{pre:!0,attrs:{class:"token number"}},[s._v("3")]),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("\nset "),t("span",{pre:!0,attrs:{class:"token operator"}},[s._v("=")]),s._v(" "),t("span",{pre:!0,attrs:{class:"token keyword"}},[s._v("new")]),s._v(" "),t("span",{pre:!0,attrs:{class:"token class-name"}},[s._v("Set")]),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v("(")]),s._v("Array"),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v(".")]),t("span",{pre:!0,attrs:{class:"token function"}},[s._v("from")]),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v("(")]),s._v("set"),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v(",")]),s._v(" "),t("span",{pre:!0,attrs:{class:"token parameter"}},[s._v("val")]),s._v(" "),t("span",{pre:!0,attrs:{class:"token operator"}},[s._v("=>")]),s._v(" val "),t("span",{pre:!0,attrs:{class:"token operator"}},[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(")")]),t("span",{pre:!0,attrs:{class:"token punctuation"}},[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("// set的值是2, 4, 6")]),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("上面代码提供了两种方法，直接在遍历操作中改变原来的 Set 结构。")]),s._v(" "),t("h2",{attrs:{id:"weakset"}},[t("a",{staticClass:"header-anchor",attrs:{href:"#weakset"}},[s._v("#")]),s._v(" WeakSet")]),s._v(" "),t("h3",{attrs:{id:"含义"}},[t("a",{staticClass:"header-anchor",attrs:{href:"#含义"}},[s._v("#")]),s._v(" 含义")]),s._v(" "),t("p",[s._v("WeakSet 结构与 Set 类似，也是"),t("strong",[s._v("不重复的值的集合")]),s._v("。但是，它与 Set 有两个区别。")]),s._v(" "),t("p",[s._v("首先，"),t("strong",[s._v("WeakSet 的成员只能是对象")]),s._v("，而不能是其他类型的值。")]),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("const")]),s._v(" ws "),t("span",{pre:!0,attrs:{class:"token operator"}},[s._v("=")]),s._v(" "),t("span",{pre:!0,attrs:{class:"token keyword"}},[s._v("new")]),s._v(" "),t("span",{pre:!0,attrs:{class:"token class-name"}},[s._v("WeakSet")]),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("\nws"),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v(".")]),t("span",{pre:!0,attrs:{class:"token function"}},[s._v("add")]),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("\n"),t("span",{pre:!0,attrs:{class:"token comment"}},[s._v("// TypeError: Invalid value used in weak set")]),s._v("\nws"),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v(".")]),t("span",{pre:!0,attrs:{class:"token function"}},[s._v("add")]),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v("(")]),t("span",{pre:!0,attrs:{class:"token function"}},[s._v("Symbol")]),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("\n"),t("span",{pre:!0,attrs:{class:"token comment"}},[s._v("// TypeError: invalid value used in weak set")]),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("p",[s._v("上面代码试图向 WeakSet 添加一个数值和"),t("code",[s._v("Symbol")]),s._v("值，结果报错，因为 WeakSet 只能放置对象。")]),s._v(" "),t("p",[s._v("其次，"),t("strong",[s._v("WeakSet 中的对象都是弱引用")]),s._v("，即垃圾回收机制不考虑 WeakSet 对该对象的引用，也就是说，如果其他对象都不再引用该对象，那么垃圾回收机制会自动回收该对象所占用的内存，不考虑该对象还存在于 WeakSet 之中。")]),s._v(" "),t("p",[s._v("这是因为垃圾回收机制依赖引用计数，如果一个值的引用次数不为"),t("code",[s._v("0")]),s._v("，垃圾回收机制就不会释放这块内存。结束使用该值之后，有时会忘记取消引用，导致内存无法释放，进而可能会引发内存泄漏。WeakSet 里面的引用，都不计入垃圾回收机制，所以就不存在这个问题。因此，"),t("strong",[s._v("WeakSet 适合临时存放一组对象，以及存放跟对象绑定的信息")]),s._v("。只要这些对象在外部消失，它在 WeakSet 里面的引用就会自动消失。")]),s._v(" "),t("p",[s._v("由于上面这个特点，"),t("strong",[s._v("WeakSet 的成员是不适合引用的，因为它会随时消失")]),s._v("。另外，由于 WeakSet 内部有多少个成员，取决于垃圾回收机制有没有运行，运行前后很可能成员个数是不一样的，而垃圾回收机制何时运行是不可预测的，因此 "),t("strong",[s._v("ES6 规定 WeakSet 不可遍历")]),s._v("。")]),s._v(" "),t("p",[s._v("这些特点同样适用于本章后面要介绍的 WeakMap 结构。")]),s._v(" "),t("h3",{attrs:{id:"语法"}},[t("a",{staticClass:"header-anchor",attrs:{href:"#语法"}},[s._v("#")]),s._v(" 语法")]),s._v(" "),t("p",[s._v("WeakSet 是一个构造函数，可以使用"),t("code",[s._v("new")]),s._v("命令，创建 WeakSet 数据结构。")]),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("const")]),s._v(" ws "),t("span",{pre:!0,attrs:{class:"token operator"}},[s._v("=")]),s._v(" "),t("span",{pre:!0,attrs:{class:"token keyword"}},[s._v("new")]),s._v(" "),t("span",{pre:!0,attrs:{class:"token class-name"}},[s._v("WeakSet")]),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("\n")])]),s._v(" "),t("div",{staticClass:"line-numbers-wrapper"},[t("span",{staticClass:"line-number"},[s._v("1")]),t("br")])]),t("p",[s._v("作为构造函数，"),t("strong",[s._v("WeakSet 可以接受一个数组或类似数组的对象作为参数")]),s._v("。（实际上，任何具有 Iterable 接口的对象，都可以作为 WeakSet 的参数。）该数组的所有成员，都会自动成为 WeakSet 实例对象的成员。")]),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("const")]),s._v(" a "),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 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("]")]),t("span",{pre:!0,attrs:{class:"token punctuation"}},[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("]")]),t("span",{pre:!0,attrs:{class:"token punctuation"}},[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("const")]),s._v(" ws "),t("span",{pre:!0,attrs:{class:"token operator"}},[s._v("=")]),s._v(" "),t("span",{pre:!0,attrs:{class:"token keyword"}},[s._v("new")]),s._v(" "),t("span",{pre:!0,attrs:{class:"token class-name"}},[s._v("WeakSet")]),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v("(")]),s._v("a"),t("span",{pre:!0,attrs:{class:"token punctuation"}},[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("// WeakSet {[1, 2], [3, 4]}")]),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("上面代码中，"),t("code",[s._v("a")]),s._v("是一个数组，它有两个成员，也都是数组。将"),t("code",[s._v("a")]),s._v("作为 WeakSet 构造函数的参数，"),t("code",[s._v("a")]),s._v("的成员会自动成为 WeakSet 的成员。")]),s._v(" "),t("p",[t("strong",[s._v("注意，是"),t("code",[s._v("a")]),s._v("数组的成员成为 WeakSet 的成员，而不是"),t("code",[s._v("a")]),s._v("数组本身。这意味着，数组的成员只能是对象。")])]),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("const")]),s._v(" b "),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("]")]),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v(";")]),s._v("\n"),t("span",{pre:!0,attrs:{class:"token keyword"}},[s._v("const")]),s._v(" ws "),t("span",{pre:!0,attrs:{class:"token operator"}},[s._v("=")]),s._v(" "),t("span",{pre:!0,attrs:{class:"token keyword"}},[s._v("new")]),s._v(" "),t("span",{pre:!0,attrs:{class:"token class-name"}},[s._v("WeakSet")]),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 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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("p",[s._v("另一种情况是，Map 的键名有非字符串，这时可以选择转为数组 JSON。")]),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("function")]),s._v(" "),t("span",{pre:!0,attrs:{class:"token function"}},[s._v("mapToArrayJson")]),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v("(")]),t("span",{pre:!0,attrs:{class:"token parameter"}},[s._v("map")]),t("span",{pre:!0,attrs:{class:"token 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方法添加成员")]),s._v("\n"),t("span",{pre:!0,attrs:{class:"token keyword"}},[s._v("const")]),s._v(" wm1 "),t("span",{pre:!0,attrs:{class:"token operator"}},[s._v("=")]),s._v(" "),t("span",{pre:!0,attrs:{class:"token keyword"}},[s._v("new")]),s._v(" "),t("span",{pre:!0,attrs:{class:"token class-name"}},[s._v("WeakMap")]),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("\n"),t("span",{pre:!0,attrs:{class:"token keyword"}},[s._v("const")]),s._v(" key "),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 literal-property property"}},[s._v("foo")]),t("span",{pre:!0,attrs:{class:"token operator"}},[s._v(":")]),s._v(" "),t("span",{pre:!0,attrs:{class:"token number"}},[s._v("1")]),t("span",{pre:!0,attrs:{class:"token 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comment"}},[s._v("// WeakMap 也可以接受一个数组，")]),s._v("\n"),t("span",{pre:!0,attrs:{class:"token comment"}},[s._v("// 作为构造函数的参数")]),s._v("\n"),t("span",{pre:!0,attrs:{class:"token keyword"}},[s._v("const")]),s._v(" k1 "),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(" "),t("span",{pre:!0,attrs:{class:"token number"}},[s._v("3")]),t("span",{pre:!0,attrs:{class:"token punctuation"}},[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("const")]),s._v(" k2 "),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("4")]),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(" "),t("span",{pre:!0,attrs:{class:"token number"}},[s._v("6")]),t("span",{pre:!0,attrs:{class:"token punctuation"}},[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("const")]),s._v(" wm2 "),t("span",{pre:!0,attrs:{class:"token operator"}},[s._v("=")]),s._v(" "),t("span",{pre:!0,attrs:{class:"token keyword"}},[s._v("new")]),s._v(" "),t("span",{pre:!0,attrs:{class:"token class-name"}},[s._v("WeakMap")]),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("k1"),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v(",")]),s._v(" "),t("span",{pre:!0,attrs:{class:"token string"}},[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(" "),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v("[")]),s._v("k2"),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v(",")]),s._v(" "),t("span",{pre:!0,attrs:{class:"token string"}},[s._v("'bar'")]),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("\nwm2"),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v(".")]),t("span",{pre:!0,attrs:{class:"token function"}},[s._v("get")]),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v("(")]),s._v("k2"),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v(")")]),s._v(" "),t("span",{pre:!0,attrs:{class:"token comment"}},[s._v('// "bar"')]),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("span",{staticClass:"line-number"},[s._v("11")]),t("br"),t("span",{staticClass:"line-number"},[s._v("12")]),t("br")])]),t("p",[t("code",[s._v("WeakMap")]),s._v("与"),t("code",[s._v("Map")]),s._v("的区别有两点。")]),s._v(" "),t("p",[s._v("首先，"),t("code",[s._v("WeakMap")]),s._v("只接受对象作为键名（"),t("code",[s._v("null")]),s._v("除外），不接受其他类型的值作为键名。")]),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("const")]),s._v(" map "),t("span",{pre:!0,attrs:{class:"token operator"}},[s._v("=")]),s._v(" "),t("span",{pre:!0,attrs:{class:"token keyword"}},[s._v("new")]),s._v(" "),t("span",{pre:!0,attrs:{class:"token class-name"}},[s._v("WeakMap")]),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("\nmap"),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v(".")]),t("span",{pre:!0,attrs:{class:"token function"}},[s._v("set")]),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 comment"}},[s._v("// TypeError: 1 is not an object!")]),s._v("\nmap"),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v(".")]),t("span",{pre:!0,attrs:{class:"token function"}},[s._v("set")]),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v("(")]),t("span",{pre:!0,attrs:{class:"token function"}},[s._v("Symbol")]),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 number"}},[s._v("2")]),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v(")")]),s._v("\n"),t("span",{pre:!0,attrs:{class:"token comment"}},[s._v("// TypeError: Invalid value used as weak map key")]),s._v("\nmap"),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v(".")]),t("span",{pre:!0,attrs:{class:"token function"}},[s._v("set")]),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v("(")]),t("span",{pre:!0,attrs:{class:"token keyword"}},[s._v("null")]),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 comment"}},[s._v("// TypeError: Invalid value used as weak map key")]),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("p",[s._v("上面代码中，如果将数值"),t("code",[s._v("1")]),s._v("和"),t("code",[s._v("Symbol")]),s._v("值作为 WeakMap 的键名，都会报错。")]),s._v(" "),t("p",[s._v("其次，"),t("code",[s._v("WeakMap")]),s._v("的键名所指向的对象，不计入垃圾回收机制。")]),s._v(" "),t("p",[t("code",[s._v("WeakMap")]),s._v("的设计目的在于，有时我们想在某个对象上面存放一些数据，但是这会形成对于这个对象的引用。请看下面的例子。")]),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("const")]),s._v(" e1 "),t("span",{pre:!0,attrs:{class:"token operator"}},[s._v("=")]),s._v(" document"),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v(".")]),t("span",{pre:!0,attrs:{class:"token function"}},[s._v("getElementById")]),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v("(")]),t("span",{pre:!0,attrs:{class:"token string"}},[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("\n"),t("span",{pre:!0,attrs:{class:"token keyword"}},[s._v("const")]),s._v(" e2 "),t("span",{pre:!0,attrs:{class:"token operator"}},[s._v("=")]),s._v(" document"),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v(".")]),t("span",{pre:!0,attrs:{class:"token function"}},[s._v("getElementById")]),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v("(")]),t("span",{pre:!0,attrs:{class:"token string"}},[s._v("'bar'")]),t("span",{pre:!0,attrs:{class:"token punctuation"}},[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("const")]),s._v(" arr "),t("span",{pre:!0,attrs:{class:"token operator"}},[s._v("=")]),s._v(" "),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("e1"),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v(",")]),s._v(" "),t("span",{pre:!0,attrs:{class:"token string"}},[s._v("'foo 元素'")]),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v("]")]),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("e2"),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v(",")]),s._v(" "),t("span",{pre:!0,attrs:{class:"token string"}},[s._v("'bar 元素'")]),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v("]")]),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v(",")]),s._v("\n"),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("span",{staticClass:"line-number"},[s._v("5")]),t("br"),t("span",{staticClass:"line-number"},[s._v("6")]),t("br")])]),t("p",[s._v("上面代码中，"),t("code",[s._v("e1")]),s._v("和"),t("code",[s._v("e2")]),s._v("是两个对象，我们通过"),t("code",[s._v("arr")]),s._v("数组对这两个对象添加一些文字说明。这就形成了"),t("code",[s._v("arr")]),s._v("对"),t("code",[s._v("e1")]),s._v("和"),t("code",[s._v("e2")]),s._v("的引用。")]),s._v(" "),t("p",[s._v("一旦不再需要这两个对象，我们就必须手动删除这个引用，否则垃圾回收机制就不会释放"),t("code",[s._v("e1")]),s._v("和"),t("code",[s._v("e2")]),s._v("占用的内存。")]),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 comment"}},[s._v("// 不需要 e1 和 e2 的时候")]),s._v("\n"),t("span",{pre:!0,attrs:{class:"token comment"}},[s._v("// 必须手动删除引用")]),s._v("\narr "),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(" "),t("span",{pre:!0,attrs:{class:"token keyword"}},[s._v("null")]),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v(";")]),s._v("\narr "),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 operator"}},[s._v("=")]),s._v(" "),t("span",{pre:!0,attrs:{class:"token keyword"}},[s._v("null")]),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("上面这样的写法显然很不方便。一旦忘了写，就会造成内存泄露。")]),s._v(" "),t("p",[s._v("WeakMap 就是为了解决这个问题而诞生的，它的键名所引用的对象都是弱引用，即垃圾回收机制不将该引用考虑在内。因此，只要所引用的对象的其他引用都被清除，垃圾回收机制就会释放该对象所占用的内存。也就是说，一旦不再需要，WeakMap 里面的键名对象和所对应的键值对会自动消失，不用手动删除引用。")]),s._v(" "),t("p",[s._v("基本上，如果你要往对象上添加数据，又不想干扰垃圾回收机制，就可以使用 WeakMap。一个典型应用场景是，在网页的 DOM 元素上添加数据，就可以使用"),t("code",[s._v("WeakMap")]),s._v("结构。当该 DOM 元素被清除，其所对应的"),t("code",[s._v("WeakMap")]),s._v("记录就会自动被移除。")]),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("const")]),s._v(" wm "),t("span",{pre:!0,attrs:{class:"token operator"}},[s._v("=")]),s._v(" "),t("span",{pre:!0,attrs:{class:"token keyword"}},[s._v("new")]),s._v(" "),t("span",{pre:!0,attrs:{class:"token class-name"}},[s._v("WeakMap")]),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("\n\n"),t("span",{pre:!0,attrs:{class:"token keyword"}},[s._v("const")]),s._v(" element "),t("span",{pre:!0,attrs:{class:"token operator"}},[s._v("=")]),s._v(" document"),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v(".")]),t("span",{pre:!0,attrs:{class:"token function"}},[s._v("getElementById")]),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v("(")]),t("span",{pre:!0,attrs:{class:"token string"}},[s._v("'example'")]),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v(")")]),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v(";")]),s._v("\n\nwm"),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v(".")]),t("span",{pre:!0,attrs:{class:"token function"}},[s._v("set")]),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v("(")]),s._v("element"),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v(",")]),s._v(" "),t("span",{pre:!0,attrs:{class:"token string"}},[s._v("'some information'")]),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v(")")]),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v(";")]),s._v("\nwm"),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v(".")]),t("span",{pre:!0,attrs:{class:"token function"}},[s._v("get")]),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v("(")]),s._v("element"),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v(")")]),s._v(" "),t("span",{pre:!0,attrs:{class:"token comment"}},[s._v('// "some information"')]),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("上面代码中，先新建一个 Weakmap 实例。然后，将一个 DOM 节点作为键名存入该实例，并将一些附加信息作为键值，一起存放在 WeakMap 里面。这时，WeakMap 里面对"),t("code",[s._v("element")]),s._v("的引用就是弱引用，不会被计入垃圾回收机制。")]),s._v(" "),t("p",[s._v("也就是说，上面的 DOM 节点对象的引用计数是"),t("code",[s._v("1")]),s._v("，而不是"),t("code",[s._v("2")]),s._v("。这时，一旦消除对该节点的引用，它占用的内存就会被垃圾回收机制释放。Weakmap 保存的这个键值对，也会自动消失。")]),s._v(" "),t("p",[s._v("总之，"),t("code",[s._v("WeakMap")]),s._v("的专用场合就是，它的键所对应的对象，可能会在将来消失。"),t("code",[s._v("WeakMap")]),s._v("结构有助于防止内存泄漏。")]),s._v(" "),t("p",[s._v("注意，WeakMap 弱引用的只是键名，而不是键值。键值依然是正常引用。")]),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("const")]),s._v(" wm "),t("span",{pre:!0,attrs:{class:"token operator"}},[s._v("=")]),s._v(" "),t("span",{pre:!0,attrs:{class:"token keyword"}},[s._v("new")]),s._v(" "),t("span",{pre:!0,attrs:{class:"token class-name"}},[s._v("WeakMap")]),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("\n"),t("span",{pre:!0,attrs:{class:"token keyword"}},[s._v("let")]),s._v(" key "),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(";")]),s._v("\n"),t("span",{pre:!0,attrs:{class:"token keyword"}},[s._v("let")]),s._v(" obj "),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 literal-property property"}},[s._v("foo")]),t("span",{pre:!0,attrs:{class:"token operator"}},[s._v(":")]),s._v(" "),t("span",{pre:!0,attrs:{class:"token number"}},[s._v("1")]),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v("}")]),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v(";")]),s._v("\n\nwm"),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v(".")]),t("span",{pre:!0,attrs:{class:"token function"}},[s._v("set")]),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v("(")]),s._v("key"),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v(",")]),s._v(" obj"),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v(")")]),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v(";")]),s._v("\nobj "),t("span",{pre:!0,attrs:{class:"token operator"}},[s._v("=")]),s._v(" "),t("span",{pre:!0,attrs:{class:"token keyword"}},[s._v("null")]),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v(";")]),s._v("\nwm"),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v(".")]),t("span",{pre:!0,attrs:{class:"token function"}},[s._v("get")]),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v("(")]),s._v("key"),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v(")")]),s._v("\n"),t("span",{pre:!0,attrs:{class:"token comment"}},[s._v("// Object {foo: 1}")]),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("p",[s._v("上面代码中，键值"),t("code",[s._v("obj")]),s._v("是正常引用。所以，即使在 WeakMap 外部消除了"),t("code",[s._v("obj")]),s._v("的引用，WeakMap 内部的引用依然存在。")]),s._v(" "),t("h3",{attrs:{id:"weakmap-的语法"}},[t("a",{staticClass:"header-anchor",attrs:{href:"#weakmap-的语法"}},[s._v("#")]),s._v(" WeakMap 的语法")]),s._v(" "),t("p",[s._v("WeakMap 与 Map 在 API 上的区别主要是两个，一是没有遍历操作（即没有"),t("code",[s._v("keys()")]),s._v("、"),t("code",[s._v("values()")]),s._v("和"),t("code",[s._v("entries()")]),s._v("方法），也没有"),t("code",[s._v("size")]),s._v("属性。因为没有办法列出所有键名，某个键名是否存在完全不可预测，跟垃圾回收机制是否运行相关。这一刻可以取到键名，下一刻垃圾回收机制突然运行了，这个键名就没了，为了防止出现不确定性，就统一规定不能取到键名。二是无法清空，即不支持"),t("code",[s._v("clear")]),s._v("方法。因此，"),t("code",[s._v("WeakMap")]),s._v("只有四个方法可用："),t("code",[s._v("get()")]),s._v("、"),t("code",[s._v("set()")]),s._v("、"),t("code",[s._v("has()")]),s._v("、"),t("code",[s._v("delete()")]),s._v("。")]),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("const")]),s._v(" wm "),t("span",{pre:!0,attrs:{class:"token operator"}},[s._v("=")]),s._v(" "),t("span",{pre:!0,attrs:{class:"token keyword"}},[s._v("new")]),s._v(" "),t("span",{pre:!0,attrs:{class:"token class-name"}},[s._v("WeakMap")]),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("\n\n"),t("span",{pre:!0,attrs:{class:"token comment"}},[s._v("// size、forEach、clear 方法都不存在")]),s._v("\nwm"),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v(".")]),s._v("size "),t("span",{pre:!0,attrs:{class:"token comment"}},[s._v("// undefined")]),s._v("\nwm"),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v(".")]),s._v("forEach "),t("span",{pre:!0,attrs:{class:"token comment"}},[s._v("// undefined")]),s._v("\nwm"),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v(".")]),s._v("clear "),t("span",{pre:!0,attrs:{class:"token comment"}},[s._v("// 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("h3",{attrs:{id:"weakmap-的示例"}},[t("a",{staticClass:"header-anchor",attrs:{href:"#weakmap-的示例"}},[s._v("#")]),s._v(" WeakMap 的示例")]),s._v(" "),t("p",[s._v("WeakMap 的例子很难演示，因为无法观察它里面的引用会自动消失。此时，其他引用都解除了，已经没有引用指向 WeakMap 的键名了，导致无法证实那个键名是不是存在。")]),s._v(" "),t("p",[s._v("贺师俊老师"),t("a",{attrs:{href:"https://github.com/ruanyf/es6tutorial/issues/362#issuecomment-292109104",target:"_blank",rel:"noopener noreferrer"}},[s._v("提示"),t("OutboundLink")],1),s._v("，如果引用所指向的值占用特别多的内存，就可以通过 Node 的"),t("code",[s._v("process.memoryUsage")]),s._v("方法看出来。根据这个思路，网友"),t("a",{attrs:{href:"https://github.com/ruanyf/es6tutorial/issues/362#issuecomment-292451925",target:"_blank",rel:"noopener noreferrer"}},[s._v("vtxf"),t("OutboundLink")],1),s._v("补充了下面的例子。")]),s._v(" "),t("p",[s._v("首先，打开 Node 命令行。")]),s._v(" "),t("div",{staticClass:"language-bash line-numbers-mode"},[t("pre",{pre:!0,attrs:{class:"language-bash"}},[t("code",[s._v("$ "),t("span",{pre:!0,attrs:{class:"token function"}},[s._v("node")]),s._v(" --expose-gc\n")])]),s._v(" "),t("div",{staticClass:"line-numbers-wrapper"},[t("span",{staticClass:"line-number"},[s._v("1")]),t("br")])]),t("p",[s._v("上面代码中，"),t("code",[s._v("--expose-gc")]),s._v("参数表示允许手动执行垃圾回收机制。")]),s._v(" "),t("p",[s._v("然后，执行下面的代码。")]),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 comment"}},[s._v("// 手动执行一次垃圾回收，保证获取的内存使用状态准确")]),s._v("\n"),t("span",{pre:!0,attrs:{class:"token operator"}},[s._v(">")]),s._v(" global"),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v(".")]),t("span",{pre:!0,attrs:{class:"token function"}},[s._v("gc")]),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("\n"),t("span",{pre:!0,attrs:{class:"token keyword"}},[s._v("undefined")]),s._v("\n\n"),t("span",{pre:!0,attrs:{class:"token comment"}},[s._v("// 查看内存占用的初始状态，heapUsed 为 4M 左右")]),s._v("\n"),t("span",{pre:!0,attrs:{class:"token operator"}},[s._v(">")]),s._v(" process"),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v(".")]),t("span",{pre:!0,attrs:{class:"token function"}},[s._v("memoryUsage")]),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("\n"),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v("{")]),s._v(" "),t("span",{pre:!0,attrs:{class:"token literal-property property"}},[s._v("rss")]),t("span",{pre:!0,attrs:{class:"token operator"}},[s._v(":")]),s._v(" "),t("span",{pre:!0,attrs:{class:"token number"}},[s._v("21106688")]),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v(",")]),s._v("\n "),t("span",{pre:!0,attrs:{class:"token literal-property property"}},[s._v("heapTotal")]),t("span",{pre:!0,attrs:{class:"token operator"}},[s._v(":")]),s._v(" "),t("span",{pre:!0,attrs:{class:"token number"}},[s._v("7376896")]),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v(",")]),s._v("\n "),t("span",{pre:!0,attrs:{class:"token literal-property property"}},[s._v("heapUsed")]),t("span",{pre:!0,attrs:{class:"token operator"}},[s._v(":")]),s._v(" "),t("span",{pre:!0,attrs:{class:"token number"}},[s._v("4153936")]),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v(",")]),s._v("\n "),t("span",{pre:!0,attrs:{class:"token literal-property property"}},[s._v("external")]),t("span",{pre:!0,attrs:{class:"token operator"}},[s._v(":")]),s._v(" "),t("span",{pre:!0,attrs:{class:"token number"}},[s._v("9059")]),s._v(" "),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v("}")]),s._v("\n\n"),t("span",{pre:!0,attrs:{class:"token operator"}},[s._v(">")]),s._v(" "),t("span",{pre:!0,attrs:{class:"token keyword"}},[s._v("let")]),s._v(" wm "),t("span",{pre:!0,attrs:{class:"token operator"}},[s._v("=")]),s._v(" "),t("span",{pre:!0,attrs:{class:"token keyword"}},[s._v("new")]),s._v(" "),t("span",{pre:!0,attrs:{class:"token class-name"}},[s._v("WeakMap")]),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("\n"),t("span",{pre:!0,attrs:{class:"token keyword"}},[s._v("undefined")]),s._v("\n\n"),t("span",{pre:!0,attrs:{class:"token comment"}},[s._v("// 新建一个变量 key，指向一个 5*1024*1024 的数组")]),s._v("\n"),t("span",{pre:!0,attrs:{class:"token operator"}},[s._v(">")]),s._v(" "),t("span",{pre:!0,attrs:{class:"token keyword"}},[s._v("let")]),s._v(" key "),t("span",{pre:!0,attrs:{class:"token operator"}},[s._v("=")]),s._v(" "),t("span",{pre:!0,attrs:{class:"token keyword"}},[s._v("new")]),s._v(" "),t("span",{pre:!0,attrs:{class:"token class-name"}},[s._v("Array")]),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v("(")]),t("span",{pre:!0,attrs:{class:"token number"}},[s._v("5")]),s._v(" "),t("span",{pre:!0,attrs:{class:"token operator"}},[s._v("*")]),s._v(" "),t("span",{pre:!0,attrs:{class:"token number"}},[s._v("1024")]),s._v(" "),t("span",{pre:!0,attrs:{class:"token operator"}},[s._v("*")]),s._v(" "),t("span",{pre:!0,attrs:{class:"token number"}},[s._v("1024")]),t("span",{pre:!0,attrs:{class:"token punctuation"}},[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("undefined")]),s._v("\n\n"),t("span",{pre:!0,attrs:{class:"token comment"}},[s._v("// 设置 WeakMap 实例的键名，也指向 key 数组")]),s._v("\n"),t("span",{pre:!0,attrs:{class:"token comment"}},[s._v("// 这时，key 数组实际被引用了两次，")]),s._v("\n"),t("span",{pre:!0,attrs:{class:"token comment"}},[s._v("// 变量 key 引用一次，WeakMap 的键名引用了第二次")]),s._v("\n"),t("span",{pre:!0,attrs:{class:"token comment"}},[s._v("// 但是，WeakMap 是弱引用，对于引擎来说，引用计数还是1")]),s._v("\n"),t("span",{pre:!0,attrs:{class:"token operator"}},[s._v(">")]),s._v(" wm"),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v(".")]),t("span",{pre:!0,attrs:{class:"token function"}},[s._v("set")]),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v("(")]),s._v("key"),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v(",")]),s._v(" "),t("span",{pre:!0,attrs:{class:"token number"}},[s._v("1")]),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v(")")]),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v(";")]),s._v("\nWeakMap "),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v("{")]),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v("}")]),s._v("\n\n"),t("span",{pre:!0,attrs:{class:"token operator"}},[s._v(">")]),s._v(" global"),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v(".")]),t("span",{pre:!0,attrs:{class:"token function"}},[s._v("gc")]),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("\n"),t("span",{pre:!0,attrs:{class:"token keyword"}},[s._v("undefined")]),s._v("\n\n"),t("span",{pre:!0,attrs:{class:"token comment"}},[s._v("// 这时内存占用 heapUsed 增加到 45M 了")]),s._v("\n"),t("span",{pre:!0,attrs:{class:"token operator"}},[s._v(">")]),s._v(" process"),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v(".")]),t("span",{pre:!0,attrs:{class:"token function"}},[s._v("memoryUsage")]),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("\n"),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v("{")]),s._v(" "),t("span",{pre:!0,attrs:{class:"token literal-property property"}},[s._v("rss")]),t("span",{pre:!0,attrs:{class:"token operator"}},[s._v(":")]),s._v(" "),t("span",{pre:!0,attrs:{class:"token number"}},[s._v("67538944")]),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v(",")]),s._v("\n "),t("span",{pre:!0,attrs:{class:"token literal-property property"}},[s._v("heapTotal")]),t("span",{pre:!0,attrs:{class:"token operator"}},[s._v(":")]),s._v(" "),t("span",{pre:!0,attrs:{class:"token number"}},[s._v("7376896")]),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v(",")]),s._v("\n "),t("span",{pre:!0,attrs:{class:"token literal-property property"}},[s._v("heapUsed")]),t("span",{pre:!0,attrs:{class:"token operator"}},[s._v(":")]),s._v(" "),t("span",{pre:!0,attrs:{class:"token number"}},[s._v("45782816")]),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v(",")]),s._v("\n "),t("span",{pre:!0,attrs:{class:"token literal-property property"}},[s._v("external")]),t("span",{pre:!0,attrs:{class:"token operator"}},[s._v(":")]),s._v(" "),t("span",{pre:!0,attrs:{class:"token number"}},[s._v("8945")]),s._v(" "),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v("}")]),s._v("\n\n"),t("span",{pre:!0,attrs:{class:"token comment"}},[s._v("// 清除变量 key 对数组的引用，")]),s._v("\n"),t("span",{pre:!0,attrs:{class:"token comment"}},[s._v("// 但没有手动清除 WeakMap 实例的键名对数组的引用")]),s._v("\n"),t("span",{pre:!0,attrs:{class:"token operator"}},[s._v(">")]),s._v(" key 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内部的引用，就会自动被垃圾回收清除。由此可见，有了 WeakMap 的帮助，解决内存泄漏就会简单很多。")]),s._v(" "),t("h3",{attrs:{id:"weakmap-的用途"}},[t("a",{staticClass:"header-anchor",attrs:{href:"#weakmap-的用途"}},[s._v("#")]),s._v(" WeakMap 的用途")]),s._v(" "),t("p",[s._v("前文说过，WeakMap 应用的典型场合就是 DOM 节点作为键名。下面是一个例子。")]),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(" myElement "),t("span",{pre:!0,attrs:{class:"token operator"}},[s._v("=")]),s._v(" document"),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v(".")]),t("span",{pre:!0,attrs:{class:"token function"}},[s._v("getElementById")]),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v("(")]),t("span",{pre:!0,attrs:{class:"token string"}},[s._v("'logo'")]),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v(")")]),t("span",{pre:!0,attrs:{class:"token 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