Filter narrowing down its output type without type guards

[TomasHubelbauer]

I have come across issues such as #7657, #20812 and #20218 as well as SO questions such as this one, but they all deal with type guards.

Could it be made so that the following code:

const items: (number | null)[] = [1,2,3,null];
const numbers = items.filter(i => i !== null);

…just works - infers that numbers is number[] with --strictNullChecks on? I think this should be out of the box behavior with TypeScript and lib.d.ts typings. Is this possible?

[jack-williams]

Duplicate of #16069

[TomasHubelbauer]

Indeed, thank you. Hopefully that issue gets more attention soon.

[jmarceli]

I was having a similar problem, did you try:

const items: (number | null)[] = [1, 2, 3, null];
const numbers = items.filter((i): i is number => i !== null);

Here is a working example:
https://typescript-play.js.org/#code/MYewdgzgLgBAllApgWwgLhgCjAV2QI0QCcYAfGXAG0oEoBtAXRgF4Y6BGAGhgCZuBmblUoMA3ACgA9JNCRYuAsQgt4SVADoAZnEpIimOCwB88GAEJmrYTXXIAhgAcDx0wCpeNCbOgU8hIsqsCCgQWjp6mAY0GIZwygr+LoYWVjjUNvZOhswmhu48nuLi3iCUiOqUIADm2H5KhUA

[TomasHubelbauer]

I did end to doing that. I raised this issue to see if there is a future where we won't have to spell this out to the compiler. Looks like the mentioned issue is tracking the progress towards that.