Pad a number with leading zeros in JavaScript [duplicate]

Answer 1

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.