I used the idea from Kerry's answer, but I simplified it since I was just looking for something simple for my specific purpose. Here is what I have:

function numberWithCommas(x) {
    return x.toString().replace(/\B(?=(\d{3})+(?!\d))/g, ",");
}

function numberWithCommas(x) {
    return x.toString().replace(/\B(?<!\.\d*)(?=(\d{3})+(?!\d))/g, ",");
}

function test(x, expect) {
    const result = numberWithCommas(x);
    const pass = result === expect;
    console.log(`${pass ? "✓" : "ERROR ====>"} {result}`);
    return pass;
}

let failures = 0;
failures += !test(0,        "0");
failures += !test(100,      "100");
failures += !test(1000,     "1,000");
failures += !test(10000,    "10,000");
failures += !test(100000,   "100,000");
failures += !test(1000000,  "1,000,000");
failures += !test(10000000, "10,000,000");
if (failures) {
    console.log(`${failures} test(s) failed`);
} else {
    console.log("All tests passed");
}

.as-console-wrapper {
    max-height: 100% !important;
}

The regex uses two lookahead assertions:

• a positive one to look for any point in the string that has a multiple of 3 digits in a row after it,
• a negative assertion to make sure that point only has exactly a multiple of 3 digits. The replacement expression puts a comma there.

For example, if you pass it 123456789.01, the positive assertion will match every spot to the left of the 7 (since 789 is a multiple of 3 digits, 678 is a multiple of 3 digits, 567, etc.).

The negative assertion checks that the multiple of 3 digits does not have any digits after it. 789 has a period after it so it is exactly a multiple of 3 digits, so a comma goes there. 678 is a multiple of 3 digits, but it has a 9 after it, so those 3 digits are part of a group of 4, and a comma does not go there. Similarly for 567.

456789 is 6 digits, which is a multiple of 3, so a comma goes before that. 345678 is a multiple of 3, but it has a 9 after it, so no comma goes there. And so on. The \B keeps the regex from putting a comma at the beginning of the string.

neu-rah mentioned that this function adds commas in undesirable places if there are more than 3 digits after the decimal point. If this is a problem, you can use this function:

function numberWithCommas(x) {
    var parts = x.toString().split(".");
    parts[0] = parts[0].replace(/\B(?=(\d{3})+(?!\d))/g, ",");
    return parts.join(".");
}

function numberWithCommas(x) {
    var parts = x.toString().split(".");
    parts[0] = parts[0].replace(/\B(?=(\d{3})+(?!\d))/g, ",");
    return parts.join(".");
}

function test(x, expect) {
    const result = numberWithCommas(x);
    const pass = result === expect;
    console.log(`${pass ? "✓" : "ERROR ====>"} {result}`);
    return pass;
}

let failures = 0;
failures += !test(0              , "0");
failures += !test(0.123456       , "0.123456");
failures += !test(100            , "100");
failures += !test(100.123456     , "100.123456");
failures += !test(1000           , "1,000");
failures += !test(1000.123456    , "1,000.123456");
failures += !test(10000          , "10,000");
failures += !test(10000.123456   , "10,000.123456");
failures += !test(100000         , "100,000");
failures += !test(100000.123456  , "100,000.123456");
failures += !test(1000000        , "1,000,000");
failures += !test(1000000.123456 , "1,000,000.123456");
failures += !test(10000000       , "10,000,000");
failures += !test(10000000.123456, "10,000,000.123456");
if (failures) {
    console.log(`${failures} test(s) failed`);
} else {
    console.log("All tests passed");
}

.as-console-wrapper {
    max-height: 100% !important;
}

T.J. Crowder pointed out that now that JavaScript has lookbehind (support info), it can be solved in the regular expression itself:

function numberWithCommas(x) {
    return x.toString().replace(/\B(?<!\.\d*)(?=(\d{3})+(?!\d))/g, ",");
}

function numberWithCommas(x) {
    return x.toString().replace(/\B(?<!\.\d*)(?=(\d{3})+(?!\d))/g, ",");
}

function test(x, expect) {
    const result = numberWithCommas(x);
    const pass = result === expect;
    console.log(`${pass ? "✓" : "ERROR ====>"} {result}`);
    return pass;
}

let failures = 0;
failures += !test(0,               "0");
failures += !test(0.123456,        "0.123456");
failures += !test(100,             "100");
failures += !test(100.123456,      "100.123456");
failures += !test(1000,            "1,000");
failures += !test(1000.123456,     "1,000.123456");
failures += !test(10000,           "10,000");
failures += !test(10000.123456,    "10,000.123456");
failures += !test(100000,          "100,000");
failures += !test(100000.123456,   "100,000.123456");
failures += !test(1000000,         "1,000,000");
failures += !test(1000000.123456,  "1,000,000.123456");
failures += !test(10000000,        "10,000,000");
failures += !test(10000000.123456, "10,000,000.123456");
if (failures) {
    console.log(`${failures} test(s) failed`);
} else {
    console.log("All tests passed");
}

.as-console-wrapper {
    max-height: 100% !important;
}

(?<!\.\d*) is a negative lookbehind that says the match can't be preceded by a . followed by zero or more digits. The negative lookbehind is faster than the split and join solution (comparison), at least in V8.

The $(this).html().replace(',', '') shouldn't actually modify the page. Are you sure the commas are being removed in the page?

If it is, this addCommas function should do the trick.

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;
}

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,''))