Pad a number with leading zeros in JavaScript [duplicate]

Pad a number with leading zeros in JavaScript [duplicate]

Possible Duplicate:
How can I create a Zerofilled value using JavaScript?

In JavaScript, I need to have padding.
For example, if I have the number 9, it will be “0009”. If I have a number of say 10, it will be “0010”. Notice how it will always contain four digits.
One way to do this would be to subtract the number minus 4 to get the number of 0s I need to put.
Is there was a slicker way of doing this?


Solution 1:

Not a lot of “slick” going on so far:

function pad(n, width, z) {
  z = z || '0';
  n = n + '';
  return n.length >= width ? n : new Array(width - n.length + 1).join(z) + n;

When you initialize an array with a number, it creates an array with the length set to that value so that the array appears to contain that many undefined elements. Though some Array instance methods skip array elements without values, .join() doesn’t, or at least not completely; it treats them as if their value is the empty string. Thus you get a copy of the zero character (or whatever “z” is) between each of the array elements; that’s why there’s a + 1 in there.

Example usage:

pad(10, 4);      // 0010
pad(9, 4);       // 0009
pad(123, 4);     // 0123

pad(10, 4, '-'); // --10

Solution 2:

function padToFour(number) {
  if (number<=9999) { number = ("000"+number).slice(-4); }
  return number;

Something like that?

Bonus incomprehensible-but-slicker single-line ES6 version:

let padToFour = number => number <= 9999 ? `000${number}`.slice(-4) : number;


  • let is a block scoped variable (as opposed to var’s functional scoping)
  • => is an arrow function that among other things replaces function and is prepended by its parameters
  • If a arrow function takes a single parameter you can omit the parentheses (hence number =>)
  • If an arrow function body has a single line that starts with return you can omit the braces and the return keyword and simply use the expression
  • To get the function body down to a single line I cheated and used a ternary expression

Solution 3:


String.prototype.lpad = function(padString, length) {
    var str = this;
    while (str.length < length)
        str = padString + str;
    return str;

Now test:

var str = "5";
alert(str.lpad("0", 4)); //result "0005"
var str = "10"; // note this is string type
alert(str.lpad("0", 4)); //result "0010"


In ECMAScript 8 , we have new method padStart and padEnd which has below syntax.

‘string’.padStart(targetLength [,padString]):

So now we can use

const str = "5";
str.padStart(4, '0'); // '0005'

Solution 4:

Funny, I recently had to do this.

function padDigits(number, digits) {
    return Array(Math.max(digits - String(number).length + 1, 0)).join(0) + number;

Use like:

padDigits(9, 4);  // "0009"
padDigits(10, 4); // "0010"
padDigits(15000, 4); // "15000"

Not beautiful, but effective.

Solution 5:

You did say you had a number-

String.prototype.padZero= function(len, c){
    var s= '', c= c || '0', len= (len || 2)-this.length;
    while(s.length<len) s+= c;
    return s+this;
Number.prototype.padZero= function(len, c){
    return String(this).padZero(len,c);

Solution 6:

You could do something like this:

function pad ( num, size ) {
  return ( Math.pow( 10, size ) + ~~num ).toString().substring( 1 );

Edit: This was just a basic idea for a function, but to add support for larger numbers (as well as invalid input), this would probably be better:

function pad ( num, size ) {
  if (num.toString().length >= size) return num;
  return ( Math.pow( 10, size ) + Math.floor(num) ).toString().substring( 1 );

This does 2 things:

  1. If the number is larger than the specified size, it will simply return the number.
  2. Using Math.floor(num) in place of ~~num will support larger numbers.