Typesafe get(obj, [keys...]

get.ts

interface Example {
  a: {
    b: {
      aNumber: number;
    }
  }
}

interface HasB {
    b: {
        aNumber: number;
    }
}

const example: Example = { a: { b: { aNumber: 1 } } }


function get<A extends Object, K extends string>(obj: A, key: K): K extends keyof A
    ? A[K] : never;
function get<A extends Object, K1 extends string>(obj: A, keys: [K1]): K1 extends keyof A
    ? A[K1] : never;
function get<A extends Object, K1 extends string, K2 extends string>(obj: A, keys: [K1, K2]): K1 extends keyof A
    ? K2 extends keyof A[K1]
        ? A[K1][K2]
        : never
    : never;
function get<A extends Object, K1 extends string, K2 extends string, K3 extends string>(obj: A, keys: [K1, K2, K3]): K1 extends keyof A
    ? K2 extends keyof A[K1]
        ? K3 extends keyof A[K1][K2]
            ? A[K1][K2][K3]
            : never
        : never
    : never;
function get(obj: any, keys: any) {
    if (typeof keys === 'string') {
        return obj[keys];
    }
    //else
    return keys.reduce((obj: any, key: any) => {
        return obj && obj[key] ? obj[key] : null;
    }, obj);
}

const isObjWithB = get(example, 'a');
const isObjWithB2 = get(example, ['a']);
const isObjWithANumber = get(example, ['a', 'b']);
const isNumber = get(example, ['a', 'b', 'aNumber']);