Format numbers in JavaScript similar to C#

Generally

• Formatting numbers in JavaScript
• Formatting numbers for currency display and more.

In jQuery

• autoNumeric (a decent number formatter & input helper with locale support for jQuery 1.5+)
• jQuery Format (a clientSide implementation of Java's SimpleDateFormat and NumberFormat)
• jquery-numberformatter (number formatter with locale support)

May I suggest numbro for locale based formatting and number-format.js for the general case. A combination of the two depending on use-case may help.

To further jfriend00's answer (I dont't have enough points to comment) I have extended his/her answer to the following:

function log(args) {
    var str = "";
    for (var i = 0; i < arguments.length; i++) {
        if (typeof arguments[i] === "object") {
            str += JSON.stringify(arguments[i]);
        } else {
            str += arguments[i];
        }
    }
    var div = document.createElement("div");
    div.innerHTML = str;
    document.body.appendChild(div);
}

Number.prototype.addCommas = function (str) {
    if (str === undefined) {
     str = this;
    }
    
    var parts = (str + "").split("."),
        main = parts[0],
        len = main.length,
        output = "",
        first = main.charAt(0),
        i;
    
    if (first === '-') {
        main = main.slice(1);
        len = main.length;    
    } else {
       first = "";
    }
    i = len - 1;
    while(i >= 0) {
        output = main.charAt(i) + output;
        if ((len - i) % 3 === 0 && i > 0) {
            output = "," + output;
        }
        --i;
    }
    // put sign back
    output = first + output;
    // put decimal part back
    if (parts.length > 1) {
        output += "." + parts[1];
    }
    return output;
}

var testCases = [
    1, 12, 123, -1234, 12345, 123456, -1234567, 12345678, 123456789,
    -1.1, 12.1, 123.1, 1234.1, -12345.1, -123456.1, -1234567.1, 12345678.1, 123456789.1
];
 
for (var i = 0; i < testCases.length; i++) {
 log(testCases[i].addCommas());
}
 
/*for (var i = 0; i < testCases.length; i++) {
    log(Number.addCommas(testCases[i]));
}*/

You can do it in the following way: So you will not only format the number but you can also pass as a parameter how many decimal digits to display, you set a custom decimal and mile separator.

function format(number, decimals = 2, decimalSeparator = '.', thousandsSeparator = ',') {
    const roundedNumber = number.toFixed(decimals);
    let integerPart = '', fractionalPart = '';
    if (decimals == 0) {
        integerPart = roundedNumber;
        decimalSeparator = '';
    } else {
        let numberParts = roundedNumber.split('.');
        integerPart = numberParts[0];
        fractionalPart = numberParts[1];
    }
    integerPart = integerPart.replace(/(\d)(?=(\d{3})+(?!\d))/g, `$1${thousandsSeparator}`);
    return `${integerPart}${decimalSeparator}${fractionalPart}`;
}

Use:

let min = 1556454.0001;
let max = 15556982.9999;
console.time('number format');
for (let i = 0; i < 15000; i++) {
    let randomNumber = Math.random() * (max - min) + min;

    let formated = format(randomNumber, 4, ',', '.'); // formated number

    console.debug('number: ', randomNumber, 'formated: ', formated);
}
console.timeEnd('number format');