Return “type” of generic type

Question

I have a immutable clone function:

import { isObject, toPairs } from 'lodash';

export function cloneDeepWithoutUndefinedKeys<T>(o: T): any {
    if (Array.isArray(o)) {
        return o.map((el) => cloneDeepWithoutUndefinedKeys(el));
    } else if (isObject(o)) {
        const c: { [key: string]: any } = {};
        for (const [key, value] of toPairs(o as { [key: string]: any })) {
            if (value === undefined) {
                continue;
            }
            c[key] = cloneDeepWithoutUndefinedKeys(value);
        }
        return c;
    } else {
        return o;
    }
}

The return of the function I had to make it any but I want it to be sameTypeOf(T) . Is this possible?

Answer 1

I'd use this.

export function cloneDeepWithoutUndefinedKeys<T extends any>(o: T): T {
    if (Array.isArray(o)) {
        return (o.map((el: any) => cloneDeepWithoutUndefinedKeys(el))) as T; // ADDED
    } else if (isObject(o)) {
        const c: { [key: string]: any } = {};
        for (const [key, value] of toPairs(o)) {
            if (value === undefined) {
                continue;
            }
            c[key] = cloneDeepWithoutUndefinedKeys(value);
        }
        return c as T; // ADDED
    } else {
      return o
    }
}

Without the modifications I made, Typescript doubts that the function really returns the same type ( T ), but it is not smart enough to infer that it's really of the same type, basically. So we have to calm it down by manually affirming the type.