Difference between of “K extends keyof T” vs. directly using “keyof T”?

Question

Is there any difference between the following typescript definitions:

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


        

Answer 1

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