| 1 |
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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(" sym "),t("span",{pre:!0,attrs:{class:"token operator"}},[s._v("=")]),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 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\n"),t("span",{pre:!0,attrs:{class:"token function"}},[s._v("String")]),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v("(")]),s._v("sym"),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v(")")]),s._v(" "),t("span",{pre:!0,attrs:{class:"token comment"}},[s._v('// "Symbol(foo)"')]),s._v("\nsym"),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v(".")]),t("span",{pre:!0,attrs:{class:"token function"}},[s._v("toString")]),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('// "Symbol(foo)"')]),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("上面的用法不是很方便。"),t("a",{attrs:{href:"https://github.com/tc39/proposal-Symbol-description",target:"_blank",rel:"noopener noreferrer"}},[s._v("ES2019"),t("OutboundLink")],1),s._v(" 提供了一个实例属性"),t("code",[s._v("description")]),s._v("，直接返回 Symbol 的描述。")]),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(" sym "),t("span",{pre:!0,attrs:{class:"token operator"}},[s._v("=")]),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 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\nsym"),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v(".")]),s._v("description "),t("span",{pre:!0,attrs:{class:"token comment"}},[s._v('// "foo"')]),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("h2",{attrs:{id:"作为属性名的-symbol"}},[t("a",{staticClass:"header-anchor",attrs:{href:"#作为属性名的-symbol"}},[s._v("#")]),s._v(" 作为属性名的 Symbol")]),s._v(" "),t("p",[s._v("由于每一个 Symbol 值都是不相等的，这意味着 Symbol 值可以作为标识符，用于对象的属性名，就能保证不会出现同名的属性。这对于一个对象由多个模块构成的情况非常有用，能防止某一个键被不小心改写或覆盖。")]),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(" mySymbol "),t("span",{pre:!0,attrs:{class:"token operator"}},[s._v("=")]),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\n"),t("span",{pre:!0,attrs:{class:"token comment"}},[s._v("// 第一种写法")]),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 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("\na"),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v("[")]),s._v("mySymbol"),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 string"}},[s._v("'Hello!'")]),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v(";")]),s._v("\n\n"),t("span",{pre:!0,attrs:{class:"token comment"}},[s._v("// 第二种写法")]),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 punctuation"}},[s._v("{")]),s._v("\n "),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v("[")]),s._v("mySymbol"),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 string"}},[s._v("'Hello!'")]),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\n"),t("span",{pre:!0,attrs:{class:"token comment"}},[s._v("// 第三种写法")]),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 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("\nObject"),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v(".")]),t("span",{pre:!0,attrs:{class:"token function"}},[s._v("defineProperty")]),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(" mySymbol"),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v(",")]),s._v(" "),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v("{")]),s._v(" "),t("span",{pre:!0,attrs:{class:"token literal-property property"}},[s._v("value")]),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(" "),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("// 以上写法都得到同样结果")]),s._v("\na"),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v("[")]),s._v("mySymbol"),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v("]")]),s._v(" "),t("span",{pre:!0,attrs:{class:"token comment"}},[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("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("span",{staticClass:"line-number"},[s._v("15")]),t("br"),t("span",{staticClass:"line-number"},[s._v("16")]),t("br"),t("span",{staticClass:"line-number"},[s._v("17")]),t("br")])]),t("p",[s._v("上面代码"),t("strong",[s._v("通过方括号结构和"),t("code",[s._v("Object.defineProperty")]),s._v("，将对象的属性名指定为一个 Symbol 值")]),s._v("。")]),s._v(" "),t("p",[s._v("注意，Symbol 值作为对象属性名时，不能用点运算符。")]),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(" mySymbol "),t("span",{pre:!0,attrs:{class:"token operator"}},[s._v("=")]),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 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 punctuation"}},[s._v(";")]),s._v("\n\na"),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v(".")]),s._v("mySymbol "),t("span",{pre:!0,attrs:{class:"token operator"}},[s._v("=")]),s._v(" "),t("span",{pre:!0,attrs:{class:"token string"}},[s._v("'Hello!'")]),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v(";")]),s._v("\na"),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v("[")]),s._v("mySymbol"),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v("]")]),s._v(" "),t("span",{pre:!0,attrs:{class:"token comment"}},[s._v("// undefined")]),s._v("\na"),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v("[")]),t("span",{pre:!0,attrs:{class:"token string"}},[s._v("'mySymbol'")]),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v("]")]),s._v(" "),t("span",{pre:!0,attrs:{class:"token comment"}},[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("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，因为点运算符后面总是字符串，所以不会读取"),t("code",[s._v("mySymbol")]),s._v("作为标识名所指代的那个值，导致"),t("code",[s._v("a")]),s._v("的属性名实际上是一个字符串，而不是一个 Symbol 值。")]),s._v(" "),t("p",[s._v("同理，在对象的内部，使用 Symbol 值定义属性时，Symbol 值必须放在方括号之中。")]),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(" s "),t("span",{pre:!0,attrs:{class:"token operator"}},[s._v("=")]),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\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("{")]),s._v("\n "),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v("[")]),s._v("s"),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("function")]),s._v(" "),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v("(")]),t("span",{pre:!0,attrs:{class:"token parameter"}},[s._v("arg")]),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("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"),t("code",[s._v("s")]),s._v("不放在方括号中，该属性的键名就是字符串"),t("code",[s._v("s")]),s._v("，而不是"),t("code",[s._v("s")]),s._v("所代表的那个 Symbol 值。")]),s._v(" "),t("p",[s._v("采用增强的对象写法，上面代码的"),t("code",[s._v("obj")]),s._v("对象可以写得更简洁一些。")]),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(" obj "),t("span",{pre:!0,attrs:{class:"token operator"}},[s._v("=")]),s._v(" 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