30 seconds of code

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Flattens an array up to the specified depth.

Use recursion, decrementing depth by 1 for each level of depth. Use Array.prototype.reduce() and Array.prototype.concat() to merge elements or arrays. Base case, for depth equal to 1 stops recursion. Omit the second argument, depth to flatten only to a depth of 1 (single flatten).

const flatten = (arr, depth = 1) =>
  arr.reduce((a, v) => a.concat(depth > 1 && Array.isArray(v) ? flatten(v, depth - 1) : v), []);