[TS] `filter(Boolean)` loses type information, TypeScript 3.5 makes it worse

[rkirov]

Bug Report

More of a heads up, than a bug report, but this has been a major PITA for upgrading Google internal to TS 3.5, so I wanted to have a discussion here too.

Current Behavior
With TS3.4 pipe(filter(Boolean)) loses type information, everything after it is of type any. See stackblitz below:

const source = of('World').pipe(
  map(x => `Hello ${x}!`),
  filter(Boolean),
  map(y => y.madeup)  // no type error
);

The type of Boolean in lib.d.ts is

interface BooleanConstructor {
    new (value?: any): Boolean;
    (value?: any): boolean;
    readonly prototype: Boolean;
}

With TS3.5 Boolean is typed as

interface BooleanConstructor {
    new (value?: any): Boolean;
    <T>(value?: T): boolean;
    readonly prototype: Boolean;
}

https://github.com/microsoft/TypeScript/blob/master/lib/lib.es5.d.ts#L536

However, in most cases it appears <T> is not inferrable (I don't blame TS, this is really round-about), so TS3.5 pick the new default unknown, which in turn makes the type for the next operation unknown from previously any causing many compilation failures.

Reproduction
https://stackblitz.com/edit/rxjs-erah82

Possible Solution
I have been recommending replacing filter(Boolean) with the following more explicit workaround filter((x): x is OutputType => !!x) where OutputType is explicitly spelled out the expected type in the next operation in the chain.

Or maybe once microsoft/TypeScript#16655 (comment) lands filter(Boolean) will just work for Rxjs.

[cartant]

Thanks. This is worthy of a linting rule that includes a fixer, IMO.

[cartant]

Seems to be solved by adding the following overload signature (before the others):

export function filter<T>(predicate: BooleanConstructor): OperatorFunction<T, NonNullable<T>>;

ATM, I cannot see any downsides to this. The problem also affects find, first and last, AFAICT, so they would need an additional, initial overload, too.

IIRC, NonNullable was introduced in TS 2.8, so the additional signature should be okay in v6.

[rkirov]

Hmm, I did a small test with this change over our codebase and while some issues go away, there is another problem with type inference. In the simplest form this looks like:

interface I {
  a: string | null;
}

let i$: Observable<I> = of();
let s$: Observable<string> = i$.pipe(
  map(i => i.a),  // type-error here
  filter(Boolean) 
);

see
https://codesandbox.io/s/typescript-playground-lchy6?fontsize=14

It appears that TS's inferencer works "backwards" from the declared type - Observable<string>
thus picking T = string instead of T = string | null for the type parameter of filter and the return type of map, causing a type error. This might make this change hard to land and add some new confusion :(

[rkirov]

@alxhub came up with a fix for this situtation, by explicitly adding the |null|undefined types after the inferencer has picked some T.

function filterB<T>(
  predicate: BooleanConstructor
): OperatorFunction<T|null|undefined, NonNullable<T>> {
  return filter(predicate);
}

I will do another test to see if this fixes most breakages.

[benlesh]

@rkirov i'm afraid that what you've outlined above doesn't really have a fix that I can see. Were you able to work around it on your end?

[rkirov]

I tested the change outlined above in g3 and it passes, so we should be good. Also we upgraded to TS3.5, by removing all offending filter(Boolean)s, so it's all good.

My only hesitation is that with the change, one will always be allowed to do a filter boolean on a non-nullable type:

of(1,2,3).pipe(filter(Boolean)); // it would nice if that errs.

There is an argument for wanting that to be statically disallowed, but one can't have it all. Not having hidden 'any's any longer is a major win.

[cartant]

of(1,2,3).pipe(filter(Boolean)) is valid, though, as zeroes will be filtered. Similarly, with strings, empty strings will be filtered. The joy of falsy values.

[rkirov]

Right, I should have said "of({a: ''}, {a: 'foo'}).pipe(filter(Boolean)) should be an error", but yeah, that's too much to ask of the type system. I think this is a good change, thanks for fixing it quickly.