Implement Array.prototype.flat() — flatten with depth

Question

Accepted Answer

**Approach:** Recursive DFS, decrement depth; `depth = Infinity` fully flattens.

```javascript
function flat(arr, depth = 1) {
  const res = [];
  (function helper(a, d) {
    for (const v of a) {
      if (Array.isArray(v) && d > 0) helper(v, d - 1);
      else res.push(v);
    }
  })(arr, depth);
  return res;
}
function flatMap(arr, fn) { return flat(arr.map(fn), 1); }
function flatIter(arr, depth = 1) {
  const stack = [...arr.map(v => [v, depth])].reverse();
  const out = [];
  while (stack.length) {
    const [val, d] = stack.pop();
    if (Array.isArray(val) && d > 0) {
      for (let i = val.length - 1; i >= 0; i--) stack.push([val[i], d - 1]);
    } else out.push(val);
  }
  return out;
}

// Demo
console.log(flat([1,[2,[3,[4]]]], 1)); // [1,2,[3,[4]]]
console.log(flat([1,[2,[3,[4]]]], 2)); // [1,2,3,[4]]
console.log(flat([1,[2,[3,[4]]]], Infinity)); // [1,2,3,4]
console.log(flatMap([1,2,3], x => [x, x*2])); // [1,2,2,4,3,6]
console.log(flatIter([1,[2,[3]]], 1)); // [1,2,[3]]

Implement Array.prototype.flat() — flatten with depth

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