Putting a zero before a number to match a digit

//You can do this.
var str = "123";
str = "0000" + str;
str = str.slice(-5);
alert(str);

var number = 257;
var length = 5;

Convert number=number+"//number to string
var str=""
for(var i=0;i<length-number.length;i++){
  str=str+"0";
}
str=str+number;
console.log(str);

function pad(n, width) {
  n = n + '';
  return n.length >= width ? n : new Array(width - n.length + 1).join('0') + n;
}
console.log(pad(5,7));