How do I add an index signature for a mapped type?

Question

Suppose I have interface

interface X {
   a: string;
   b: number;
   c: boolean;
}

and a function

function values(x: X) {
            


        

Answer 1

You can:

interface X {
    a: string;
    b: number;
    c: boolean;
    [key: string]: X[keyof X];
}

The result of X[keyof X] will now be (string | number | boolean), which works even better than any because the return of your function will be (string | number | boolean)[].

Example

Another way that should work with both examples is:

function values(x: X) {
    const keys = Object.keys(x) as (keyof X)[];
    return keys.map(s => x[s]);
}

Not pretty, but at least more typed than (x as any).

Of course it can be made generic too:

function values(x: T) {
    const keys = Object.keys(x) as (keyof T)[];
    return keys.map(s => x[s]);
}