Typescript Generic Array Type and Push Item

Question

I'm trying to create a generic function which handles varies arrays of various types (that only share a few properties in common) and perform simple operations such as delete or add an item. How should the typing's look for such a function?

interface A {
    value: number;
    aasdsad?: number;
    abc: string;
}

interface B {
    value: number;
    asdsad1?: string;
}

export class Main {

    public getOneOfTheArrays(): A[] | B[] {
        return []; // return A[] or B[] depending on certain conditions
    }

    public getValue(index: number) {
        const arr: A[] | B[] = this.getOneOfTheArrays();
        const a = this.copy(arr[index]);
        a.value = 1234;
        arr.push(a); // <- typing's error here
    }

    public copy(obj: A | B): A | B {
        return Object.assign({}, obj);
    }

}

DEMO1 DEMO2

Error: arr.push(a) -> Argument of type 'A | B' is not assignable to parameter of type 'A & B'. Type 'B' is not assignable to type 'A & B'. Property 'abc' is missing in type 'B' but required in type 'A'.

Solution so far: (arr as typeof a[]).push(a);

Answer 1

You can not push A | B to A[] | B[] only when a is typeof B and arr typeof A[] because type B is missing abc attribute.

You ar able to push it when:

arr.push(a as A);
(arr as B[]).push(a as B);
(arr as B[]).push(a as A);

Sou you have to check i some way that a is B and arr is A[], and it that case apply some different logic.

Playground