TypeScript:如何编写具有条件返回类型的函数 [英] TypeScript: How to write a function with conditional return type

问题描述

export type Name = { name: string }
export type Id = { id: number }
export type Value<T> = T extends string ? Name : Id

export function create<T extends string | number>(value: T): Value<T> {
    if (typeof value === "string") return { name: value }

    return { id: value }
}

我正在使用 TypeScript 中的条件类型.我想写一个有条件返回类型的函数.如果函数被传递一个字符串，它返回一个名称，否则返回一个 Id.

I am playing around with the conditional types in TypeScript. i would like to write a function with a conditional return type. If the function gets passed a string it returns a Name otherwise it returns an Id.

我的返回语句出现以下错误:

I get the following error on my return statements:

Type '{ name: T & string; }' is not assignable to type 'Value<T>'.

我错过了什么?谢谢！

该示例直接取自于 Build 2018 的 Anders Hejlsberg 演讲:https://youtu.be/hDACN-BGvI8?t=2241

The example taken directly from Anders Hejlsberg talk at Build 2018: https://youtu.be/hDACN-BGvI8?t=2241

他甚至说我们不必再写函数重载了……"

He even states "we do not have to write function overloads anymore..."

如果我将代码更改为仅声明，编译错误就会消失:

If I change the code to just a declaration, the compile errors go away:

export type Name = { name: string }
export type Id = { id: number }

export type Value<T> = T extends string ? Name : Id

declare function create<T extends string | number>(value: T): Value<T>

const a = create("Bob")     // a : Name
const b = create(5)         // b : Id

这样我们就可以声明函数签名了.我想我的问题变成了，我们将如何实际实现该功能?

So we can declare the function signature. I guess my question then becomes, how would we actually implement the function?

推荐答案

问题是函数内部 T 不知道，所以你不能真正给 Value<赋值.一种选择是使用类型断言.更类型安全的选项是使用单独的实现签名，这样输入和输出类型会更宽松:

The problem is inside the function T is not known, so you can't really assign a value to Value<T>. One option would be to use a type assertion. A more type safe option would be to use a separate implementation signature, that would be more relaxed about the input and output type:

export function create<T extends string | number>(value: T): Value<T> // public signature
export function create(value: string | number): Name | Id { // more relaxed private implementation signature 
    if (typeof value === "string") return { name: value }

    return { id: value }
}

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