Answer 1

100%高度的 after 加上 inline-block

<div class="box">
    <div class="center">
</div>

.box {
  width: 500px;
  height: 500px;
  text-align: center; /*center*/
}
.box::after {
  content: '';
  display: inline-block;
  width: 0;
  height: 100%;
  vertical-align: middle;
}
.center {
  display: inline-block;
  width: 100px;
  height: 100px;
  vertical-align: middle;/*center*/
}

Answer 2

//倒序循环可避免
const arr = [0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 1, 2, 3, 0, 0, 0, 1, 9, 9, 9, 0, 0, 0, 0, 1, 0, 3, 12, 0, 0, 0, 0];
    const len = arr.length;
    console.log(len)
    for (let i = len; i >= 0; i--) {
      if (arr[i] === 0) {
        arr.splice(i, 1);
        arr.push(0)
      }
    }
    console.log(arr)

Answer 3

@MillionQW 因为数组被转换成字符串了啊。。。