“K extends keyof T"之间的区别还是直接使用“keyof T"? [英] Difference between of "K extends keyof T" vs. directly using "keyof T"?

问题描述

以下打字稿定义之间有什么区别:

Is there any difference between the following typescript definitions:

function prop<T, K extends keyof T>(obj: T, key: K) {
    return obj[key];
}

和

function prop2<T>(obj: T, key: keyof T) {
    return obj[key];
}

我的意思是不，但也许我监督了一些事情.使用第一个版本有什么好处(文档中经常使用)

I mean no, but maybe I have overseen something. Are there any benefits to use the first version (which is often used in the docs)

推荐答案

区别在于第一种情况下返回类型为 T[K] 而第二种情况下返回类型为 <代码>T[keyof T].K 可以是最宽的 keyof T 但它可以是表示键的特定字符串文字类型.这意味着如果 K 是特定属性，则返回值将与该属性具有相同的类型:

The difference is that in the first case the return type will be T[K] while in the second case it will be T[keyof T]. K can at it's widest be keyof T but it can be a specific string literal type representing a key. This means if K is a specific property the return value will be of the same type as the property:

function prop<T, K extends keyof T>(obj: T, key: K) {
    return obj[key];
}
function prop2<T>(obj: T, key: keyof T) {
    return obj[key];
}

let o = {
    p1: 0,
    p2: ''
}

let v = prop(o, 'p1') // is number, K is of type 'p1'
let v2 = prop2(o, 'p1') // is number | string, no extra info is captured

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