js 数组扁平化

function flatten(arr) {
  let result = [];
  for (let i = 0; i < arr.length; i++) {
    if (Array.isArray(arr[i])) {
      result = result.concat(flatten(arr[i]));
    } else {
      result = result.concat(arr[i]);
    }
  }
  return result;
}

const a = [1, [2, [3, 4]]];
console.log(flatten(a));  //[1, 2, 3, 4]