feature: add flattenArray() operator

[Laurens-makel]

I'd like to propose the flattenArray() array operator, which is great when working with two-dimensional arrays.

It should take an optional parameter depth, which defaults to 1, just like the .flat() method does.

https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Array/flat

Currently

of([ [1,2], [3,4] ]).pipe(
  map(array => array.flat())
);
// [1, 2, 3, 4]

Operator

import { map } from 'rxjs';

export const flattenArray = <T>(depth=1) =>
    map((array: T[]) => array.flat(depth));

With operator:

of([ [1,2], [3,4] ]).pipe(
  flattenArray()
);
// [1, 2, 3, 4]

[endlacer]

Considering that 'filterArray,' 'mapArray,' 'reduceArray,' etc., already exist, this would be suitable. However, I believe these one-line replacement methods could unnecessarily complicate understanding for your coworkers.

[LcsGa]

I think that creating such operators is not a very good idea. map is made to transform the previous observable into a new one, with a transform function.

• operators are hard to understand for a lot of new devs.
• The RxJS team themselves are trying to remove as many operators as possible, to simplify the library.

In my opinion, what we should aim for is composition. It is more flexible as it is not tied to RxJS and it aligns with the RxJS core team's way of doing. So here, instead of ̀flattenArray, we could do this:

function flatten<T>(depth?: number) {
  return (arr: T[]) => arr.flat(depth);
}

const arr = [[1, 2], [3, [4]]];

of(arr)
  .pipe(map(flatten(2)))
  .subscribe(console.log); // [1, 2, 3, 4]

console.log(flatten()(arr)); // [1, 2, 3, [4]]