A simple function to add thousands separators to a number.

fn.format_number.js

function format_number(number) {
    // split integer and decimal parts
    var parts = ('' + number).split('.');
    var significand = parts[0];
  
    // check if there is a decimal part
    var decimal = typeof parts[1] !== 'undefined' ? parts[1] : false;
  
    // reverse the integer part
    var reversed = (significand).split('').reverse().join('');
    // split the integer part in chunks of 3 max numbers
    var chunks = reversed.match(/.{1,3}/g);
    // join the chunks using the thousands separator and reverse the number again
    var with_separator = chunks.join('.').split('').reverse().join('');
    // build the decimal part
    var decimal_suffix = decimal ? (',' + decimal) : '';

    // return the full number
    return with_separator + decimal_suffix;
}