Deep flatten an array

deep_flatten.js

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

// ––––– Option 1 –––––

function baseFlatten(array, depth, result) {
  result || (result = []);

  if (array == null) {
    return result;
  }

  for (const value of array) {
    if (depth > 0 && Array.isArray(value)) {
      if (depth > 1) {
        baseFlatten(value, depth - 1, result);
      } else {
        result.push(...value);
      }
    } else {
      result[result.length] = value;
    }
  }
  return result;
}

function deepFlatten(array) {
  const INFINITY = 1 / 0;
  return baseFlatten(array, INFINITY);
}

// ––––– Option 2 –––––

// function deepFlatten(arr) {
//   return arr.reduce(function(prev, curr) {
//     return prev.concat(Array.isArray(curr) ? flatten(curr) : curr);
//   }, []);
// }

console.log(deepFlatten(input));