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可以为 null 的类型")]),s._v(" "),t("p",[s._v("TypeScript 具有两种特殊的类型，"),t("code",[s._v("null")]),s._v(" 和 "),t("code",[s._v("undefined")]),s._v("，它们分别具有值 "),t("code",[s._v("null")]),s._v(" 和 "),t("code",[s._v("undefined")]),s._v("。我们在"),t("a",{attrs:{href:"/chapter2/type"}},[s._v("基础类型")]),s._v("一节里已经做过简要说明。 默认情况下，类型检查器认为 "),t("code",[s._v("null")]),s._v(" 与 "),t("code",[s._v("undefined")]),s._v(" 可以赋值给任何类型。 "),t("code",[s._v("null")]),s._v(" 与 "),t("code",[s._v("undefined")]),s._v(" 是所有其它类型的一个有效值。 这也意味着，你阻止不了将它们赋值给其它类型，就算是你想要阻止这种情况也不行。"),t("code",[s._v("null")]),s._v("的发明者，Tony Hoare，称它为"),t("a",{attrs:{href:"https://en.wikipedia.org/wiki/Null_pointer#History",target:"_blank",rel:"noopener noreferrer"}},[s._v("价值亿万美金的错误"),t("OutboundLink")],1),s._v("。")]),s._v(" "),t("p",[t("code",[s._v("--strictNullChecks")]),s._v(" 标记可以解决此错误：当你声明一个变量时，它不会自动地包含 "),t("code",[s._v("null")]),s._v(" 或 "),t("code",[s._v("undefined")]),s._v("。 你可以使用联合类型明确的包含它们：")]),s._v(" 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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("p",[s._v("可选属性也会有同样的处理：")]),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("class")]),s._v(" "),t("span",{pre:!0,attrs:{class:"token class-name"}},[t("span",{pre:!0,attrs:{class:"token constant"}},[s._v("C")])]),s._v(" "),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v("{")]),s._v("\n  a"),t("span",{pre:!0,attrs:{class:"token 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operator"}},[s._v("=")]),s._v(" "),t("span",{pre:!0,attrs:{class:"token number"}},[s._v("12")]),s._v("\nc"),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 keyword"}},[s._v("undefined")]),s._v(" "),t("span",{pre:!0,attrs:{class:"token comment"}},[s._v("// error, 'undefined' 不能赋值给 'number'")]),s._v("\nc"),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("13")]),s._v("\nc"),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 keyword"}},[s._v("undefined")]),s._v(" "),t("span",{pre:!0,attrs:{class:"token comment"}},[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("h3",{attrs:{id:"类型保护和类型断言"}},[t("a",{staticClass:"header-anchor",attrs:{href:"#类型保护和类型断言"}},[s._v("#")]),s._v(" 类型保护和类型断言")]),s._v(" "),t("p",[s._v("由于可以为 "),t("code",[s._v("null")]),s._v(" 的类型能和其它类型定义为联合类型，那么你需要使用类型保护来去除 "),t("code",[s._v("null")]),s._v("。幸运地是这与在 "),t("code",[s._v("JavaScript")]),s._v(" 里写的代码一致：")]),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("sn"),t("span",{pre:!0,attrs:{class:"token operator"}},[s._v(":")]),s._v(" "),t("span",{pre:!0,attrs:{class:"token builtin"}},[s._v("string")]),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(")")]),t("span",{pre:!0,attrs:{class:"token operator"}},[s._v(":")]),s._v(" "),t("span",{pre:!0,attrs:{class:"token builtin"}},[s._v("string")]),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("if")]),s._v(" "),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v("(")]),s._v("sn "),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(" "),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v("{")]),s._v("\n    "),t("span",{pre:!0,attrs:{class:"token keyword"}},[s._v("return")]),s._v(" "),t("span",{pre:!0,attrs:{class:"token string"}},[s._v("'default'")]),s._v("\n  "),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v("}")]),s._v(" "),t("span",{pre:!0,attrs:{class:"token keyword"}},[s._v("else")]),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("return")]),s._v(" sn\n  "),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")])]),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("这里很明显地去除了 "),t("code",[s._v("null")]),s._v("，你也可以使用短路运算符：")]),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("sn"),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("如果编译器不能够去除 "),t("code",[s._v("null")]),s._v(" 或 "),t("code",[s._v("undefined")]),s._v("，你可以使用类型断言手动去除。语法是添加 "),t("code",[s._v("!")]),s._v(" 后缀： "),t("code",[s._v("identifier!")]),s._v(" 从 "),t("code",[s._v("identifier")]),s._v(" 的类型里去除了 "),t("code",[s._v("null")]),s._v(" 和 "),t("code",[s._v("undefined")]),s._v("：")]),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("broken")]),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v("(")]),s._v("name"),t("span",{pre:!0,attrs:{class:"token operator"}},[s._v(":")]),s._v(" "),t("span",{pre:!0,attrs:{class:"token builtin"}},[s._v("string")]),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(")")]),t("span",{pre:!0,attrs:{class:"token operator"}},[s._v(":")]),s._v(" "),t("span",{pre:!0,attrs:{class:"token builtin"}},[s._v("string")]),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("postfix")]),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v("(")]),s._v("epithet"),t("span",{pre:!0,attrs:{class:"token operator"}},[s._v(":")]),s._v(" "),t("span",{pre:!0,attrs:{class:"token builtin"}},[s._v("string")]),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v(")")]),s._v(" "),t("span",{pre:!0,attrs:{class:"token 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operator"}},[s._v("!")]),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v(".")]),t("span",{pre:!0,attrs:{class:"token function"}},[s._v("charAt")]),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 string"}},[s._v("'.  the '")]),s._v(" "),t("span",{pre:!0,attrs:{class:"token operator"}},[s._v("+")]),s._v(" epithet "),t("span",{pre:!0,attrs:{class:"token comment"}},[s._v("// ok")]),s._v("\n  "),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v("}")]),s._v("\n  name "),t("span",{pre:!0,attrs:{class:"token operator"}},[s._v("=")]),s._v(" name "),t("span",{pre:!0,attrs:{class:"token operator"}},[s._v("||")]),s._v(" "),t("span",{pre:!0,attrs:{class:"token string"}},[s._v("'Bob'")]),s._v("\n  "),t("span",{pre:!0,attrs:{class:"token keyword"}},[s._v("return")]),s._v(" "),t("span",{pre:!0,attrs:{class:"token function"}},[s._v("postfix")]),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v("(")]),t("span",{pre:!0,attrs:{class:"token string"}},[s._v("'great'")]),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 function"}},[s._v("broken")]),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("\n\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("span",{staticClass:"line-number"},[s._v("18")]),t("br")])]),t("p",[s._v("本例使用了嵌套函数，因为编译器无法去除嵌套函数的 "),t("code",[s._v("null")]),s._v("（除非是立即调用的函数表达式）。因为它无法跟踪所有对嵌套函数的调用，尤其是你将内层函数做为外层函数的返回值。如果无法知道函数在哪里被调用，就无法知道调用时 "),t("code",[s._v("name")]),s._v(" 的类型。")]),s._v(" "),t("h2",{attrs:{id:"字符串字面量类型"}},[t("a",{staticClass:"header-anchor",attrs:{href:"#字符串字面量类型"}},[s._v("#")]),s._v(" 字符串字面量类型")]),s._v(" "),t("p",[s._v("字符串字面量类型允许你指定字符串必须具有的确切值。在实际应用中，字符串字面量类型可以与联合类型，类型保护很好的配合。通过结合使用这些特性，你可以实现类似枚举类型的字符串。")]),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("type")]),s._v(" "),t("span",{pre:!0,attrs:{class:"token class-name"}},[s._v("Easing")]),s._v(" "),t("span",{pre:!0,attrs:{class:"token operator"}},[s._v("=")]),s._v(" "),t("span",{pre:!0,attrs:{class:"token string"}},[s._v("'ease-in'")]),s._v(" "),t("span",{pre:!0,attrs:{class:"token operator"}},[s._v("|")]),s._v(" "),t("span",{pre:!0,attrs:{class:"token string"}},[s._v("'ease-out'")]),s._v(" 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TypeScript。相信很多同学学习到这里已经迫不及待想要大展身手了，那么下面我们就开始把理论转换为实践，一起来用 TypeScript 重构 axios 吧！")])])}),[],!1,null,null,null);t.default=e.exports}}]);