[jasonkillian]

In addition to being essentially a combined "filter" and "map", it's also a "better" filter than filter itself in TypeScript in such that it narrows types much more ergonomically[0].

In TypeScript, you might have an array of multiple types (e.g. `Array<A | B>`), and use a `filter` call to only keep the `A`s. However, in many situations TypeScript can't figure this out and the resulting array type is still `Array<A | B>`. However, when you just use `flatMap` to do nothing more than filtering in the same way, TypeScript can determine that the resulting type is just `Array<A>`. It's a bit unfortunate really - `filter` is faster and more readable, but the ergonomics of `flatMap` type-wise are so much nicer! Just some interesting trivia.

[0]: https://github.com/microsoft/TypeScript/issues/16069#issueco...

[amitport]

I wonder if it is possible to add a feature to Typescript to help with this:

You could potentially add a syntax for type guards function types, then add a signature to filter that accepts a type guard and returns an array of the guarded types.

Shouldn't be too much of a stretch given that we have type guards.

The syntax is a bit annoying... should be something like filter<A, B>(cb: A => A is B)

:/

[abrioy]

You can use a type guard[1] as an argument to Array.filter, but the function has to be explicitly typed as such.

I don't know why the type isn't narrowed in Array.filter like it is in if statements without this weird workaround.

  const array: (number | string)[] = [];
  
  const mixedArray = array.filter(value => typeof value === 'string');
  // mixedArray: (number | string)[]
  const arrayOfString = array.filter((value): value is string => typeof value === 'string');
  // arrayOfString: string[]

This example in Typescript playground: https://www.typescriptlang.org/play?#code/MYewdgzgLgBAhgJwXA...

[1]: https://www.typescriptlang.org/docs/handbook/advanced-types....

[amitport]

Oh so there is an overload!

filter<U extends T>(pred: (a: T) => a is U): U[];

Additionally, getting TS better at inferring type guards is an open issue (literally): https://github.com/microsoft/TypeScript/issues/38390