js手写数组扁平化

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