数字前加0

 //迭代方式实现
 function padding1(num, length) {
  for(var len = (num + "").length; len < length; len = num.length) {
   num = "0" + num;
  }
  return num;
 }
 //递归方式实现
 function padding2(num, length) {
  if((num + "").length >= length) {
   return num;
  }
  return padding2("0" + num, length)
 }
 //转为小数
 function padding3(num, length) {
  var decimal = num / Math.pow(10, length);
  //toFixed指定保留几位小数
  decimal = decimal.toFixed(length) + "";
  return decimal.substr(decimal.indexOf(".")+1);
 }
 //填充截取法
 function padding4(num, length) {
  //这里用slice和substr均可
  return (Array(length).join("0") + num).slice(-length);
 }
 //填充截取法
 function padding5(num, length) {
  var len = (num + "").length;
  var diff = length - len;
  if(diff > 0) {
   return Array(diff).join("0") + num;
  }
  return num;
 }

num：数字
length：数字最后的长度