使用字符串类型参数访问枚举时出现 TypeScript TS7015 错误 [英] TypeScript TS7015 error when accessing an enum using a string type parameter

问题描述

我是 TypeScript 的新手，我不明白我需要做什么来修复产生 TS7015 错误的行(使用字符串变量引用枚举成员)，因为紧随其后的行不会出错(引用一个使用字符串文字的枚举成员):

I am new to TypeScript and I don't understand what I need to do to fix the line that generates the TS7015 error (referencing an enum member using a string variable) because the line immediately following that does not error (referencing an enum member using a string literal):

enum State {
    Happy = 0,
    Sad = 1,
    Drunk = 2
}

function Emote(enumKey:string) {
    console.log(State[enumKey]); // error TS7015: Element implicitly has an 'any' type because index expression is not of type 'number'.
    console.log(State["Happy"]); // no error
}

"noImplicitAny": true 在项目的tsconfig.json 中设置，检测到错误

"noImplicitAny": true is set in the project's tsconfig.json the error is detected

"noImplictAny": false 在项目的tsconfig.json 中设置，未检测到错误

"noImplictAny": false is set in the project's tsconfig.json no error is detected

我正在使用 ntypescript"进行编译:^1.201603060104.1"

我现在正在使用 "tsc":"1.8.10"

C:>npm install -g typescript

`-- typescript@1.8.10

验证安装:

C:\>tsc --version

Version 1.8.10

这是我的 tsconfig.json 文件:

{
  "compileOnSave": true,
  "compilerOptions": {
    "target": "ES5",
    "module": "System",
    "moduleResolution": "node",
    "emitDecoratorMetadata": true,
    "experimentalDecorators": true,
    "removeComments": true,
    "noImplicitAny": true,
    "sourceMap": true,
    "mapRoot": "map/",
    "diagnostics": true
  },
  "exclude": [
    "node_modules",
    "typings"
  ]
}

这是编译器输出:

C:\>tsc

test.ts(8,17): error TS7015: Element implicitly has an 'any' type because index expression is not of type 'number'.

推荐答案

如果您使用的是 TypeScript 2.1+，您可以将 enumKey 的类型更改为 keyof typeof State，像这样:

If you're using TypeScript 2.1+, you can change enumKey's type to keyof typeof State, like this:

function Emote(enumKey: keyof typeof State) {...}

或者，如果函数的输入需要是 string，则:

or, if the function's input is required to be a string, this:

var state : State = State[enumKey as keyof typeof State];

说明:

因为enumKey 是一个任意的string，TypeScript 不知道enumKey 是否是State<的一个成员的名字/code>，所以它会产生一个错误.TypeScript 2.1 引入了 keyof 运算符，它返回类型的已知公共属性名称的联合.使用 keyof 允许我们断言该属性确实在目标对象中.

Because enumKey is an arbitrary string, TypeScript doesn't know whether enumKey is the name of a member of State, so it generates an error. TypeScript 2.1 introduced the keyof operator which returns a union of the known, public property names of a type. Using keyof allows us to assert that the property is indeed in the target object.

但是，当您创建枚举时，TypeScript 实际上会同时生成 type(通常是 number 的子类型)和 value(您可以在表达式中引用的枚举对象).当您编写keyof State 时，您实际上将获得number 的文字属性名称的联合.要获取枚举对象的属性名称，您可以使用 keyof typeof State.

However, when you create an enum, TypeScript actually produces both a type (which is typically a subtype of number) and a value (the enum object that you can reference in expressions). When you write keyof State, you're actually going to get a union of the literal property names of number. To instead get the property names of the enum object, you can use keyof typeof State.

来源:

https://github.com/Microsoft/TypeScript/issues/13775#issuecomment-276381229https://www.typescriptlang.org/docs/handbook/enums.html#enums-at-compile-time

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