Question

Is there any function in Javascript for formatting number and strings ?

I am looking for a way for thousand separator for string or numbers…
(Like String.Format In c#)

Answer 1

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.)

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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,''))