写一个方法，当给定数字位数不足8位时，则在左边补充0以补足8位数的方法

function padNumber(n, targetLen, placeholder) {  const arr = ("" + n).split("");  const diff = arr.length - targetLen;  if (diff < 0) {    return Array(0 - diff)      .fill(placeholder, 0, 0 - diff + 1)      .concat(arr)      .join("");  } else {    return arr.join("");  }}console.log(padNumber(3458, 8, "0")); //'00003458'console.log(padNumber(90990, 3, "-")); //'90990'