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);

Przemyslaw Jan Beigert · Accepted Answer · 2019-02-11 10:13:56Z

3

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

answered Feb 11, 2019 at 10:13

Przemyslaw Jan Beigert

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9 Comments

Stevanicus Over a year ago

The closest we got so far: (arr as typeof a[]).push(a);

Przemyslaw Jan Beigert Over a year ago

You will turn off compilation error, but what about case when you're trying to add B to A[]. You will have array of A where one of element has no attribute abc. So arr[42].abc will be undefined when TS will tell you thats string

Stevanicus Over a year ago

That's true. But technically it can only be one type at a given time. I assume what I'm trying to achieve isn't possible?

Przemyslaw Jan Beigert Over a year ago

As I understand you: its not possible only in case Array<A>.push(B) other 3 cases are fine.

Stevanicus Over a year ago

What about with the use of generics and advanced types such as declare type OneOf<T> = T extends A ? A : B so it's clear to the developer that the function can only be called with one specific type at a time and that arr cannot be (A|B)[] at a given moment in time.

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