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"),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(" sentence"),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 string"}},[s._v("'Hello, my name is '")]),s._v(" "),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("'.\\n\\n'")]),s._v(" "),t("span",{pre:!0,attrs:{class:"token operator"}},[s._v("+")]),s._v("\n "),t("span",{pre:!0,attrs:{class:"token string"}},[s._v("'I\\'ll be '")]),s._v(" "),t("span",{pre:!0,attrs:{class:"token operator"}},[s._v("+")]),s._v(" 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punctuation"}},[s._v("(")]),s._v("x"),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("]")]),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v(".")]),t("span",{pre:!0,attrs:{class:"token function"}},[s._v("substr")]),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(")")]),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v(")")]),s._v(" "),t("span",{pre:!0,attrs:{class:"token comment"}},[s._v("// Error, 'number' 不存在 'substr' 方法")]),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("当访问一个越界的元素，会使用联合类型替代：")]),s._v(" "),t("div",{staticClass:"language-typescript line-numbers-mode"},[t("pre",{pre:!0,attrs:{class:"language-typescript"}},[t("code",[s._v("x"),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 operator"}},[s._v("=")]),s._v(" "),t("span",{pre:!0,attrs:{class:"token string"}},[s._v("'world'")]),s._v(" "),t("span",{pre:!0,attrs:{class:"token comment"}},[s._v("// OK, 字符串可以赋值给(string | number)类型")]),s._v("\n\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("x"),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v("[")]),t("span",{pre:!0,attrs:{class:"token number"}},[s._v("5")]),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("toString")]),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("// OK, 'string' 和 'number' 都有 toString")]),s._v("\n\nx"),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v("[")]),t("span",{pre:!0,attrs:{class:"token number"}},[s._v("6")]),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 boolean"}},[s._v("true")]),s._v(" "),t("span",{pre:!0,attrs:{class:"token comment"}},[s._v("// Error, 布尔不是(string | number)类型")]),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("联合类型是高级主题，我们会在以后的章节里讨论它。")]),s._v(" "),t("p",[t("strong",[s._v("注意")]),s._v("：自从 TyeScript 3.1 版本之后，访问越界元素会报错，我们不应该再使用该特性。")]),s._v(" "),t("h2",{attrs:{id:"枚举"}},[t("a",{staticClass:"header-anchor",attrs:{href:"#枚举"}},[s._v("#")]),s._v(" 枚举")]),s._v(" "),t("p",[t("code",[s._v("enum")]),s._v(" 类型是对 JavaScript 标准数据类型的一个补充。 像 C# 等其它语言一样，使用枚举类型可以为一组数值赋予友好的名字。")]),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("enum")]),s._v(" Color "),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v("{")]),s._v("Red"),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v(",")]),s._v(" Green"),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v(",")]),s._v(" Blue"),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(" c"),t("span",{pre:!0,attrs:{class:"token operator"}},[s._v(":")]),s._v(" Color "),t("span",{pre:!0,attrs:{class:"token operator"}},[s._v("=")]),s._v(" Color"),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v(".")]),s._v("Green\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("默认情况下，从 "),t("code",[s._v("0")]),s._v(" 开始为元素编号。 你也可以手动的指定成员的数值。 例如，我们将上面的例子改成从 "),t("code",[s._v("1")]),s._v(" 开始编号：")]),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("enum")]),s._v(" Color "),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v("{")]),s._v("Red "),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(",")]),s._v(" Green"),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v(",")]),s._v(" Blue"),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(" c"),t("span",{pre:!0,attrs:{class:"token operator"}},[s._v(":")]),s._v(" Color "),t("span",{pre:!0,attrs:{class:"token operator"}},[s._v("=")]),s._v(" Color"),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v(".")]),s._v("Green\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("或者，全部都采用手动赋值：")]),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("enum")]),s._v(" Color "),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v("{")]),s._v("Red "),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(",")]),s._v(" Green "),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(",")]),s._v(" Blue "),t("span",{pre:!0,attrs:{class:"token operator"}},[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(" c"),t("span",{pre:!0,attrs:{class:"token operator"}},[s._v(":")]),s._v(" Color "),t("span",{pre:!0,attrs:{class:"token operator"}},[s._v("=")]),s._v(" Color"),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v(".")]),s._v("Green\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("枚举类型提供的一个便利是你可以由枚举的值得到它的名字。 例如，我们知道数值为 2，但是不确定它映射到 Color 里的哪个名字，我们可以查找相应的名字：")]),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("enum")]),s._v(" Color "),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v("{")]),s._v("Red "),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(",")]),s._v(" Green"),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v(",")]),s._v(" Blue"),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(" colorName"),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(" Color"),t("span",{pre:!0,attrs:{class:"token punctuation"}},[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\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("colorName"),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v(")")]),s._v(" "),t("span",{pre:!0,attrs:{class:"token comment"}},[s._v("// 显示'Green'因为上面代码里它的值是2")]),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("h2",{attrs:{id:"any"}},[t("a",{staticClass:"header-anchor",attrs:{href:"#any"}},[s._v("#")]),s._v(" any")]),s._v(" "),t("p",[s._v("有时候，我们会想要为那些在编程阶段还不清楚类型的变量指定一个类型。 这些值可能来自于动态的内容，比如来自用户输入或第三方代码库。 这种情况下，我们不希望类型检查器对这些值进行检查而是直接让它们通过编译阶段的检查。 那么我们可以使用 "),t("code",[s._v("any")]),s._v(" 类型来标记这些变量：")]),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(" notSure"),t("span",{pre:!0,attrs:{class:"token operator"}},[s._v(":")]),s._v(" "),t("span",{pre:!0,attrs:{class:"token builtin"}},[s._v("any")]),s._v(" "),t("span",{pre:!0,attrs:{class:"token operator"}},[s._v("=")]),s._v(" "),t("span",{pre:!0,attrs:{class:"token number"}},[s._v("4")]),s._v("\nnotSure "),t("span",{pre:!0,attrs:{class:"token operator"}},[s._v("=")]),s._v(" "),t("span",{pre:!0,attrs:{class:"token string"}},[s._v("'maybe a string instead'")]),s._v("\nnotSure "),t("span",{pre:!0,attrs:{class:"token operator"}},[s._v("=")]),s._v(" "),t("span",{pre:!0,attrs:{class:"token boolean"}},[s._v("false")]),s._v(" "),t("span",{pre:!0,attrs:{class:"token comment"}},[s._v("// 也可以是个 boolean")]),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("在对现有代码进行改写的时候，"),t("code",[s._v("any")]),s._v(" 类型是十分有用的，它允许你在编译时可选择地包含或移除类型检查。并且当你只知道一部分数据的类型时，"),t("code",[s._v("any")]),s._v(" 类型也是有用的。 比如，你有一个数组，它包含了不同的类型的数据：")]),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(" list"),t("span",{pre:!0,attrs:{class:"token operator"}},[s._v(":")]),s._v(" "),t("span",{pre:!0,attrs:{class:"token builtin"}},[s._v("any")]),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 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 boolean"}},[s._v("true")]),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v(",")]),s._v(" "),t("span",{pre:!0,attrs:{class:"token string"}},[s._v("'free'")]),t("span",{pre:!0,attrs:{class:"token punctuation"}},[s._v("]")]),s._v("\n\nlist"),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 number"}},[s._v("100")]),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:"void"}},[t("a",{staticClass:"header-anchor",attrs:{href:"#void"}},[s._v("#")]),s._v(" void")]),s._v(" "),t("p",[s._v("某种程度上来说，"),t("code",[s._v("void")]),s._v(" 类型像是与 "),t("code",[s._v("any")]),s._v(" 类型相反，它表示没有任何类型。 当一个函数没有返回值时，你通常会见到其返回值类型是 "),t("code",[s._v("void")]),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("warnUser")]),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 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("'This is my warning message'")]),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")])]),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("声明一个 "),t("code",[s._v("void")]),s._v(" 类型的变量没有什么大用，因为你只能为它赋予 "),t("code",[s._v("undefined")]),s._v(" 和 "),t("code",[s._v("null")]),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("let")]),s._v(" unusable"),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 operator"}},[s._v("=")]),s._v(" "),t("span",{pre:!0,attrs:{class:"token keyword"}},[s._v("undefined")]),s._v("\n")])]),s._v(" 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