Flatten Arrays in Vanilla JavaScript with Flat() and FlatMap()

• TypeScript

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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`), 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`. 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`. 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...

• content

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I just filed this issue on the MDN page: https://github.com/mdn/content/issues/11763

That note is misleading.

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