Implement Array.prototype.flat

Question

Implement `flat(arr, depth)` that flattens a nested array to the given depth. Also implement `Array.prototype.flat` on the prototype.

Accepted Answer

// Recursive implementation
function flat(arr, depth = 1) {
  if (depth === 0) return arr.slice()

return arr.reduce((result, item) => {
    if (Array.isArray(item) && depth > 0) {
      result.push(...flat(item, depth - 1))
    } else {
      result.push(item)
    }
    return result
  }, [])
}

// Polyfill on Array.prototype
Array.prototype.myFlat = function(depth = 1) {
  return flat(this, depth)
}

// Tests
flat([1, [2, [3, [4]]]])          // [1, 2, [3, [4]]]
flat([1, [2, [3, [4]]]], 2)       // [1, 2, 3, [4]]
flat([1, [2, [3, [4]]]], Infinity) // [1, 2, 3, 4]

// One-liner alternatives
const flatAlt = (arr, d = 1) =>
  d > 0
    ? arr.reduce((a, v) => a.concat(Array.isArray(v) ? flatAlt(v, d - 1) : v), [])
    : arr.slice()

Implement Array.prototype.flat

Answer