Add filterNonNullish

[david-shortman]

It's common to filter non-nullish values. Today, in Typescript, that would be written as:

// Accessing the type
filter((value): value is ValueType => value !== null && value !== undefined)

// Avoiding accessing the type
filter((value): value is NonNullable<typeof value> => value !== null && value !== undefined)

Instead, it would be preferable to have an operator like:

filterNonNullish()

Existing alternatives include

Defining a custom function:

const nonNullish = <T>(t: T | null | undefined): t is T => t !== null && t !== undefined;
filter(nonNullish)

Using Boolean (but this checks for falsy-ness, which is less strict/ more error prone):

filter(Boolean)

[benlesh]

I don't think we'd add that directly to RxJS, especially when you can have a relatively straightforward utility function that is more generally useful:

function isNonNull<T>(value: T): value is NonNullable<T> {
  return value != null;
}

declare const source$: Observable<false | 0 | '' | null | undefined>;

const result$ = source$.filter(isNonNull); // Observable<false | 0 | ''>;

Protip, if you don't like seeing NonNullable<whatever> in your IDE Tooltips, you can just use what it's shorthand for: T & {}.

And if you wanted to have your own custom operator, you could do this... (I've changed the name to filterOutNonNullish because filter allows values when the predicate passes, and filterNonNullish is ambiguous IMO.

export const filterOutNonNullish = filter(isNonNull);

// usage

source$.pipe(
  filterOutNonNullish
)