formatNumberWithCommas.js

/**
 * Turns a number (in number or string form) into a string with commas used as
 * a thousands separator.
 *
 * Example: 10000000 -> 10,000,000
 *
 * @param {String|Number} num
 * @return {String}
 */
function formatNumberWithCommas(num) {
    var chunks = _.chain(num.toString())
        .toArray()
        .reverse()
        .chunk(3)
        .value();

    var reversed = _.map(chunks, function (n) {
        return n.join('')
    }).join();

    return _(reversed).toArray().reverse().join('');
}

x-example.js

var input = [
    100000000,
    10000000,
    1000000,
    100000,
    10000,
    1000,
    100,
    10,
    1,
    0
];

_.map(input, formatNumberWithCommas);

/*
100,000,000
10,000,000
1,000,000
100,000
10,000
1,000
100
10
1
0
*/