JavaScript format number

format_number.js

/**
 * Format number
 *
 * (1234567.8).format(2)
 * => "1,234,567.80"
 *
 * precision: decimal precision
 */

// Method 0 (http://stackoverflow.com/a/14428340/1877620)

if (typeof Number.prototype.format === 'undefined') {
    Number.prototype.format = function (precision) {
        if (!isFinite(this)) {
            return this.toString();
        }
        
        var a = this.toFixed(precision).split('.');
        a[0] = a[0].replace(/\d(?=(\d{3})+$)/g, '$&,');
        return a.join('.');
    }
}

// Method 1

if (typeof Number.prototype.format1 === 'undefined') {
    Number.prototype.format1 = function (precision) {
        if (!isFinite(this)) {
            return this.toString();
        }
        
        var a = this.toFixed(precision).split('.'),
            // skip the '-' sign
            head = Number(this < 0);
        
        // skip the digits that's before the first thousands separator 
        head += (a[0].length - head) % 3 || 3;
        
        a[0] = a[0].slice(0, head) + a[0].slice(head).replace(/\d{3}/g, ',$&');
        return a.join('.');
    };
}

// Method 2

if (typeof Number.prototype.format2 === 'undefined') {
    Number.prototype.format2 = function (precision) {
        if (!isFinite(this)) {
            return this.toString();
        }
        
        var a = this.toFixed(precision).split('.');
        
        a[0] = a[0]
            .split('').reverse().join('')
            .replace(/\d{3}(?=\d)/g, '$&,')
            .split('').reverse().join('');
            
        return a.join('.');
    };
}

// Method 3

if (typeof Number.prototype.format3 === 'undefined') {
    Number.prototype.format3 = function (precision) {
        if (!isFinite(this)) {
            return this.toString();
        }
        
        var a = this.toFixed(precision).split('');
        a.push('.');
        
        var i = a.indexOf('.') - 3;
        while (i > 0 && a[i-1] !== '-') {
            a.splice(i, 0, ',');
            i -= 3;
        }
        
        a.pop();
        return a.join('');
    };
}

format_number_test.js

// Test suite

function assertEqual(a, b) {
    if (a !== b) {
        throw a + ' !== ' + b;
    }
}

function test(format_function) {
    console.log(format_function);
    assertEqual('NaN', format_function.call(NaN, 0))
    assertEqual('Infinity', format_function.call(Infinity, 0))
    assertEqual('-Infinity', format_function.call(-Infinity, 0))
    
    assertEqual('0', format_function.call(0, 0))
    assertEqual('0.00', format_function.call(0, 2))
    assertEqual('1', format_function.call(1, 0))
    assertEqual('-1', format_function.call(-1, 0))
    // decimal padding
    assertEqual('1.00', format_function.call(1, 2))
    assertEqual('-1.00', format_function.call(-1, 2))
    // decimal rounding
    assertEqual('0.12', format_function.call(0.123456, 2))
    assertEqual('0.1235', format_function.call(0.123456, 4))
    assertEqual('-0.12', format_function.call(-0.123456, 2))
    assertEqual('-0.1235', format_function.call(-0.123456, 4))
    // thousands separator
    assertEqual('1,234', format_function.call(1234.123456, 0))
    assertEqual('12,345', format_function.call(12345.123456, 0))
    assertEqual('123,456', format_function.call(123456.123456, 0))
    assertEqual('1,234,567', format_function.call(1234567.123456, 0))
    assertEqual('12,345,678', format_function.call(12345678.123456, 0))
    assertEqual('123,456,789', format_function.call(123456789.123456, 0))
    assertEqual('-1,234', format_function.call(-1234.123456, 0))
    assertEqual('-12,345', format_function.call(-12345.123456, 0))
    assertEqual('-123,456', format_function.call(-123456.123456, 0))
    assertEqual('-1,234,567', format_function.call(-1234567.123456, 0))
    assertEqual('-12,345,678', format_function.call(-12345678.123456, 0))
    assertEqual('-123,456,789', format_function.call(-123456789.123456, 0))
    // thousands separator and decimal
    assertEqual('1,234.12', format_function.call(1234.123456, 2))
    assertEqual('12,345.12', format_function.call(12345.123456, 2))
    assertEqual('123,456.12', format_function.call(123456.123456, 2))
    assertEqual('1,234,567.12', format_function.call(1234567.123456, 2))
    assertEqual('12,345,678.12', format_function.call(12345678.123456, 2))
    assertEqual('123,456,789.12', format_function.call(123456789.123456, 2))
    assertEqual('-1,234.12', format_function.call(-1234.123456, 2))
    assertEqual('-12,345.12', format_function.call(-12345.123456, 2))
    assertEqual('-123,456.12', format_function.call(-123456.123456, 2))
    assertEqual('-1,234,567.12', format_function.call(-1234567.123456, 2))
    assertEqual('-12,345,678.12', format_function.call(-12345678.123456, 2))
    assertEqual('-123,456,789.12', format_function.call(-123456789.123456, 2))
}

console.log('======== Testing ========');
test(Number.prototype.format);
test(Number.prototype.format1);
test(Number.prototype.format2);
test(Number.prototype.format3);

// Example

console.log('======== Demo ========')
var n = 0;
for (var i=1; i<20; i++) {
    n = (n * 10) + (i % 10)/100;
    console.log(n.format(2), (-n).format(2));
}

// Benchmark

function benchmark(f) {
    var start = new Date().getTime();
    f();
    return new Date().getTime() - start;
}

function benchmark_format(f) {
    console.log(f);
    time = benchmark(function () {
        for (var i = 0; i < 100000; i++) {
            f.call(123456789, 0);
            f.call(123456789, 2);
        }
    });
    console.log(time.format(0) + 'ms');
}

async = [];
function next() {
    setTimeout(function () {
        var f = async.shift();
        f && f();
        next();
    }, 10);
}

console.log('======== Benchmark ========');
async.push(function () { benchmark_format(Number.prototype.format); });
async.push(function () { benchmark_format(Number.prototype.format1); });
async.push(function () { benchmark_format(Number.prototype.format2); });
async.push(function () { benchmark_format(Number.prototype.format3); });
next();

If we want custom thousands separator or decimal separator, use replace():

123456.78.format(2).replace(',', ' ').replace('.', ' ');

... or replace the function with this one :

Number.prototype.format = function (precision, thousandSep, decimalSep) {
  thousandSep = thousandSep || ',';
  decimalSep = decimalSep || '.';

  if (!isFinite(this)) {
    return this.toString();
  }

  var a = this.toFixed(precision).split('.');
  a[0] = a[0].replace(/\d(?=(\d{3})+$)/g, '$&' + thousandSep);
  return a.join(decimalSep);
}

I took half a line of your code and it fixed a problem i just faced that when I had let's say 1234 my code converted it to $ 1,234.00 just fine, but when I had 4000, it came off as 4,0.00, so it was shrinking the thousans portion to just 1 zero.
Here's my code just in case:
function formatToMoney(amount) {
return "$ " + amount.toString().replace(/\B(?=(\d{3})+(?!\d))/g, ",").slice(0, -4);
}