PHP's number_format() implementation with JavaScript.

Number.prototype.toNumberFormat.js

/**
 * To number format.
 * @param  {Integer} decimals?
 * @param  {String}  decimalsSeparator?
 * @param  {String}  thousandsSeparator?
 * @return {String}
 * @links  http://php.net/number_format, https://stackoverflow.com/q/2901102
 */
Number.prototype.toNumberFormat = function(decimals, decimalsSeparator, thousandsSeparator) {
    decimalsSeparator = decimalsSeparator || '.';
    thousandsSeparator = thousandsSeparator || ',';
    var numbers = this.toFixed(decimals).split('.');
    numbers[0] = numbers[0].replace(/\B(?=(\d{3})+(?!\d))/g, thousandsSeparator);
    return numbers.join(decimalsSeparator);
};
// minified
Number.prototype.toNumberFormat=function(t,r,e){r=r||".",e=e||",";var o=this.toFixed(t).split(".");return o[0]=o[0].replace(/\B(?=(\d{3})+(?!\d))/g,e),o.join(r)};

// examples
var x = Math.random() * 0xffffff;
console.log(x, x.toNumberFormat());
console.log(x, x.toNumberFormat(2));
console.log(x, x.toNumberFormat(2, ',', '.'));
console.log(x, x.toNumberFormat(2, ',', ' '));

function numberFormat(num, dec = 0, dsep = '.', tsep = ',') {
    if (!Number.isFinite(num = Number(num))) {
        return;
    }

    var [base, decs] = num.toFixed(dec).split('.', 2);
    var seps = '';

    while (base.length > 3) {
        seps = tsep + base.slice(base.length - 3) + seps;
        base = base.slice(0, base.length - 3);
    }

    return base + seps + (dec ? ('.' + decs) : '');
}