打字稿函数/回调类型:使用“错误"分配函数时没有错误；返回值 [英] Typescript function/callback Type: no error when assigning function with "wrong" return value

问题描述

在下面的代码中，我将函数 doSomething 的类型定义为这种类型:(value: User) =>用户

In the following code I defined the type of a function doSomething to be this type: (value: User) => User

奇怪的是，当为 doSomething 分配一个返回类型错误的函数时，Typescript 不会抱怨.这是为什么?

Strangely when assigning a function with a wrong return type to doSomething then Typescript will not complain. Why is that?

interface User {
    userName: string;
}

const test: User = {
    userName: 'test',
    bla: '123' // OK:  TS complains
}

let doSomething: (value: User) => User;

doSomething = () => { // NOK: TS is not complaining about the missing parameter
  return {
    userName: 'test',
    bla: '123' // NOK: Why does TS not complain?
  }
};

在定义回调函数的类型然后传递返回错误"值的回调时会发生同样的事情:

Same thing happens when defining the type of a callback function and then pass a callback which returns a "wrong" value:

interface User {
    userName: string;
}

class Test {
    doSomething(callback: (value: User) => User): void {}
}

const test = new Test();

test.doSomething(() => { // NOK: TS is not complaining about the missing parameter
  return {
    userName: 'test',
    bla: '123' // NOK: Why does TS not complain?
  }
})

参见 stackblitz 示例:https://stackblitz.com/edit/typescript-callback-2

See example on stackblitz: https://stackblitz.com/edit/typescript-callback-2

推荐答案

你可以安全地使用一个回调，它的参数比定义的类型少，但不能不同或更多:

You can safely use a callback that has less parameters then the defined type, but not different or more:

test.doSomething(() => {...}                  // allowed
test.doSomething((user) => {...}              // allowed
test.doSomething((user1, user2) => {...}      // error
test.doSomething((username:string) => {...}   // error

通过这种方式，您可以提供一个可能不需要调用它的所有值的回调.

This way you can provide a callback that probably doesn't need all values that it is called with.

然后您可以安全地向返回的对象添加更多属性，但您不能跳过必需的属性或更改它们的类型:

Then you can safely add more properties to the returned object, but you can't skip mandatory ones or change their types:

return { userName: 'test', bla: '123'}   // ok, just an extra parameter
return { }                               // not ok, userName is required
return { userName: 42}                   // not ok, userName is required to be a string

没关系，因为结果的接收者可以忽略所有额外的属性，不会造成任何伤害.

It's ok because the receiver of the result can just ignore all extra properties, it causes no harm.

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