Level 2: Arrays and objects · Exercise 26 of 76 · Free

Flatten to a depth

A JavaScript exercise from Scriptdrill. Solve it with a code editor in your browser that runs your function against example and hidden tests, and it is checked the moment you submit.

The task

Flatten the nested arrays in `arr` by `depth` levels, the way `Array.prototype.flat(depth)` does, but written by hand. `depth` 1 removes one level of nesting, `depth` 0 returns a shallow copy, and `depth` can be `Infinity` to flatten everything. An empty array within the depth contributes nothing; an array beyond the depth (including an empty one) stays as an item. `null` and `undefined` are ordinary items. Do not change `arr`.

What this exercise teaches

Recursion that passes depth - 1 down stops exactly where it should, and works for Infinity because Infinity - 1 is still Infinity. A loop that flattens one level depth times never ends for Infinity, and concat spreads arrays one level deeper than you asked.

A hint if you get stuck

Write it recursively: for each item, if it is an array and `depth > 0`, push the items of `flattenDepth(item, depth - 1)`; otherwise push the item itself. `Array.isArray` tells arrays from everything else.