Array/Observable filter does not infer chained type

[topaxi]

TypeScript Version: 2.6.1 using strict: true

Code

[true, true, false, null]
  .filter(thing => thing !== null)
  .map(thing => thing.toString())

Expected behavior:

Compiles

Actual behavior:

3 col 17 error| 2531[QF available]: Object is possibly 'null'.

Looks like a duplicate of #5101.
A workaround would be to write .filter<boolean>((thing): thing is boolean => thing !== null) that tells us that the expression is a type guard.
Another workaround that I've found useful is to write a mapDefined helper:

function mapDefined<T, U>(iter: Iterable<T>, mapFn: (x: T) => U | undefined): U[] {
    const out: U[] = [];
    for (const x of iter) {
        const res = mapFn(x);
        if (res) {
            out.push(res);
        }
    }
    return out;
}

mapDefined([true, true, false, null], thing => thing === null ? undefined : thing.toString());

EDIT: The .filter won't work without an explicit type argument. This is fixed by #17600.

[topaxi]

I noticed this by a different case actually, probably should have posted that one.

['foo', 'bar', null]
  .filter(Boolean)
  .map(s => s.toUpperCase());

This worked with TS ~2.3 and is not working anymore with TS ~2.6. I thought the above would be a simpler reproduction but it seems it is something completely different :) Thanks for your hint though, this helps but still required quite a few new guards in our codebase which worked in a previous TS version 😕

I just tried that with typescript@2.3 and it doesn't seem to work? I don't think Boolean has ever been considered a type predicate.

[topaxi]

You're right doesn't work on arrays, we do have some rxjs observables where this works though, no idea what exactly is different there or if it's project specific.. I will report back later this week.

[typescript-bot]

Automatically closing this issue for housekeeping purposes. The issue labels indicate that it is unactionable at the moment or has already been addressed.

[wclr]

Not sure why this not working:

const list: (string | null)[] = ['A', null, 'C']

list.filter<string>((_) => _ !== null).map(x => x.)

Tried latest dev 2.7 still not working and not inferring

[mhegazy]

A user-defined type guard is not inferred automatically, a type annotation is expected. i.e.:

const list: (string | null)[] = ['A', null, 'C']

function isNotNull<T>(a: T | null): a is T { return a !== null; }

list.filter(isNotNull).map(x => x.)

[wclr]

A user-defined type guard is not inferred automatically, a type annotation is expected. i.e.:

It is good to hear, but still complicated, not clear whey it not possible to have auto inference here.