Can't filter a type out of an array of unions

[ghost]

try flow

const foo: Array<string | number> = [];
const bar = foo.filter(n => typeof n === 'string');

bar is still Array<string | number>

I also can't annotate bar to Array<string> without casting it through any first

[TrySound]

It's known problem. You can find a lot of issues about this.

As a solution you can use reduce or flatMap which can be refined with actual values instead of magical booleans.

[ghost]

@TrySound - I don't see how either reduce or flatMap is a replacement for filter. Can you give an example?

[TrySound]

Sorry, "or".

reduce from array prototype

const a = b.reduce((acc, d) => {
  if (typeof d === 'number') {
    acc.push(d);
  }
  return acc;
}, [])

or flatMap from any library or handwritten or from the future

const a = flatMap(b, d => typeof d === 'number' ? [d] : []);

they are both easy to type.

[TrySound]

Performance impact with flatMap doesn't matter if you don't work with large arrays.

[ghost]

Performance impact with flatMap doesn't matter if you don't work with large arrays.

Like canvas pixel arrays? :) I think I'll just cast through any for now, as much as I hate doing that

[TrySound]

@markkahn Then reduce as more common version of filter will solve your problem. Just don't use filter if you need refinement.

[ghost]

What's your reasoning for pushing towards something like reduce vs. casting through any?

const bar: Array<string> = (foo.filter(n => typeof n === 'string'): any);

.filter has a clear meaning, I feel like reduce is being used way outside of it's normal intention here.

[ghost]

Dupe of #1414

[TrySound]

any is not safe.

[ghost]

of course not, one of its purposes is to bypass the typechecker in exactly situations like this where it fails. Perhaps it's just philosophical, but I'd rather keep the intent of my code correct than jump through weird coding patterns to make flow happy.

Though I see your point, and I'd argue for something like this instead:

const filteredAry = [];
ary.forEach(v => (typeof v === 'string') && filteredAry.push(v));