How to use array methods with union type elements?

Question

I have the following code:

class A {}

class B {}

const arr = [new A(), new B()]

function findB(): B {
    return arr.find<B>(el => el instanceof B)
}

I got an error

Type 'A | B' is not assignable to type 'B'. Property 'b' is missing in type 'A' but required in type 'B'.

The same with "filter" and other methods.

interface Array<T> {
    find<S extends T>(predicate: (this: void, value: T, index: number, obj: T[]) => value is S, thisArg?: any): S | undefined;
    find(predicate: (value: T, index: number, obj: T[]) => boolean, thisArg?: any): T | undefined;
}

Based on "Array" definition the only way to use those methods with union types is by overriding interface? Like this for example

interface Array<T> {
    find<S>(predicate: (this: void, value: T, index: number, obj: T[]) => boolean, thisArg?: any): S | undefined;
}

Or there are other ways to solve this problem?

Nail Achmedzhanov · Accepted Answer · 2019-03-09 19:03:45Z

1

Define guard "el is B". It works (even in strict mode)

class A {}

class B {}

const arr = [new A(), new B()]

function findB(): B | undefined {
    return arr.find((el): el is B => el instanceof B);
}

function filterB(): B[] {
    return arr.filter((el): el is B  => el instanceof B);
}

answered Mar 9, 2019 at 19:03

Nail Achmedzhanov

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