js数组中如何过滤empty 空值

  let arr = ['3', '2', '1']
  arr.length = 4
  // console.log(arr);
  let arr1 = arr.filter(d => d)
  console.log(arr);        // ["3", "2", "1", empty]
  console.log(arr1);        // ["3", "2", "1"]
注意：filter不会改变原数组