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.
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...