Javascript Thousand Separator / string format

Update (7 years later)

The reference cited in the original answer below was wrong. There is a built in function for this, which is exactly what kaiser suggests below: toLocaleString

So you can do:

(1234567.89).toLocaleString('en')              // for numeric input
parseFloat("1234567.89").toLocaleString('en')  // for string input

The function implemented below works, too, but simply isn't necessary.

(I thought perhaps I'd get lucky and find out that it was necessary back in 2010, but no. According to this more reliable reference, toLocaleString has been part of the standard since ECMAScript 3rd Edition [1999], which I believe means it would have been supported as far back as IE 5.5.)

Original Answer

According to this reference there isn't a built in function for adding commas to a number. But that page includes an example of how to code it yourself:

function addCommas(nStr) {
    nStr += '';
    var x = nStr.split('.');
    var x1 = x[0];
    var x2 = x.length > 1 ? '.' + x[1] : '';
    var rgx = /(\d+)(\d{3})/;
    while (rgx.test(x1)) {
            x1 = x1.replace(rgx, '$1' + ',' + '$2');
    }
    return x1 + x2;
}

Edit: To go the other way (convert string with commas to number), you could do something like this:

parseFloat("1,234,567.89".replace(/,/g,''))

Updated using look-behind support in line with ECMAScript2018 changes.
For backwards compatibility, scroll further down to see the original solution.

A regular expression may be used - notably useful in dealing with big numbers stored as strings.

const format = num => 
    String(num).replace(/(?<!\..*)(\d)(?=(?:\d{3})+(?:\.|$))/g, '$1,')

;[
    format(100),                           // "100"
    format(1000),                          // "1,000"
    format(1e10),                          // "10,000,000,000"  
    format(1000.001001),                   // "1,000.001001"
    format('100000000000000.001001001001') // "100,000,000,000,000.001001001001
]
    .forEach(n => console.log(n))

» Verbose regex explanation (regex101.com)

This original answer may not be required but can be used for backwards compatibility.

Attempting to handle this with a single regular expression (without callback) my current ability fails me for lack of a negative look-behind in Javascript... never the less here's another concise alternative that works in most general cases - accounting for any decimal point by ignoring matches where the index of the match appears after the index of a period.

const format = num => {
    const n = String(num),
          p = n.indexOf('.')
    return n.replace(
        /\d(?=(?:\d{3})+(?:\.|$))/g,
        (m, i) => p < 0 || i < p ? `${m},` : m
    )
}

;[
    format(100),                           // "100"
    format(1000),                          // "1,000"
    format(1e10),                          // "10,000,000,000"  
    format(1000.001001),                   // "1,000.001001"
    format('100000000000000.001001001001') // "100,000,000,000,000.001001001001
]
    .forEach(n => console.log(n))

» Verbose regex explanation (regex101.com)

There's a nice jQuery number plugin: https://github.com/teamdf/jquery-number

It allows you to change any number in the format you like, with options for decimal digits and separator characters for decimal and thousand:

$.number(12345.4556, 2);          // -> 12,345.46
$.number(12345.4556, 3, ',', ' ') // -> 12 345,456

You can use it inside input fields directly, which is nicer, using same options like above:

$("input").number(true, 2);

Or you can apply to a whole set of DOM elements using selector:

$('span.number').number(true, 2);

If is about localizing thousands separators, delimiters and decimal separators, go with the following:

// --> numObj.toLocaleString( [locales [, options] ] )
parseInt( number ).toLocaleString();

There are several options you can use (and even locales with fallbacks):

number = 123456.7089;

result  = parseInt( number ).toLocaleString() + "<br>";
result += number.toLocaleString( 'de-DE' ) + "<br>";
result += number.toLocaleString( 'ar-EG' ) + "<br>";
result += number.toLocaleString( 'ja-JP', { 
  style           : 'currency',
  currency        : 'JPY',
  currencyDisplay : 'symbol',
  useGrouping     : true
} ) + "<br>";
result += number.toLocaleString( [ 'jav', 'en' ], { 
  localeMatcher            : 'lookup',
  style                    : 'decimal',
  minimumIntegerDigits     : 2,
  minimumFractionDigits    : 2,
  maximumFractionDigits    : 3,
  minimumSignificantDigits : 2,
  maximumSignificantDigits : 3
} ) + "<br>";

var el = document.getElementById( 'result' );
el.innerHTML = result;

<div id="result"></div>

Details on the MDN info page.

Edit: Commentor @I like Serena adds the following:

To support browsers with a non-English locale where we still want English formatting, use value.toLocaleString('en'). Also works for floating point.