Overload Array.filter() to exclude nully when using Boolean

[danielrentz]

I want to use Array.filter(Boolean) to filter out falsy values, e.g. nulls and empty strings from a string|null array. The result should be a string array. I am using strict mode (strictNullChecks).

The Array interface could be extended with something like this:

filter(callbackfn: typeof Boolean, thisArg?: any): (NonNullable<T>)[];

TypeScript Version: 3.4.5

Search Terms: array filter boolean narrow

Code

const a1 = ["a", null, "b", ""];
const a2 = a1.filter(Boolean);

Expected behavior:
Type of a2 should be string[].

Actual behavior:
Type of a2 is (string|null)[].

Playground Link: https://www.typescriptlang.org/play/index.html#src=const%20a1%20%3D%20%5B%22a%22%2C%20null%2C%20%22b%22%2C%20%22%22%5D%3B%0D%0Aconst%20a2%20%3D%20a1.filter(Boolean)%3B%0D%0Aconst%20s%20%3D%20a2%5B0%5D.toUpperCase()%3B%0D%0A

[dragomirtitian]

Duplicate of #16655 I believe. Which was included and then reverted due to it causing breaking changes. Comment

This was reverted due to it causing a breaking change in 3.5. The simple repro looks like this:

declare const Bullean: {
    new(value?: any): Boolean;
    <T>(value?: T): value is Exclude<T, false | null | undefined | '' | 0>;
};

declare const arr: any[];
// Was any[], now (3.5) unknown[]
const arr2 = arr.filter(Bullean);

There might be something wrong in generic inference, but it's not yet clear. We'll need this scenario to work before adding it back in.

[typescript-bot]

This issue has been marked as a 'Duplicate' and has seen no recent activity. It has been automatically closed for house-keeping purposes.