Array/Observable filter does not infer chained type

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'.

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 😕

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.

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

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

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.)

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.

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Array/Observable filter does not infer chained type #19640

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Array/Observable filter does not infer chained type #19640

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topaxi commented on Nov 1, 2017

topaxi commented on Nov 1, 2017

typescript-bot commented on Nov 17, 2017

wclr commented on Jan 17, 2018

mhegazy commented on Jan 17, 2018

wclr commented on Jan 17, 2018

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