Omitting keys of a particular type-CodePudding

Here is what I would like:

const a = getNumber({a: 3, b: "three"}, "a"); // a is type of number (or even 3)
const b = getNumber({a: 3, b: "three"}, "b"); // does not compile

The closest I have gotten is

type NumberOnly<T> = {
    [k in keyof T]: T[k] extends number ? number : never;
}
const getNumber = <T extends Record<string, any>, 
                   K extends  keyof NumberOnly<T>>(t: T, k: K):NumberOnly<T>[K] 
     => t[k];

a is type number and b is ... never.

Close but no cigar.

CodePudding user response：

This should fix the problem:

type NumberOnly<T> = {
    [K in keyof T as T[K] extends number ? K : never]: T[K]
}

We use the as operator inside the index signature. When T[K] is not number the key will be omitted.

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