formatThousands.js

var formatThousands = function (input, separator) {
    var output = [];

    input = String(input).split('').reverse();

    if (separator === void 0 || separator == null) {
        separator = ',';
    }

    for (var i = 0, len = input.length, lastIndex = len - 1; i < len; i++) {
        output.push(input[i]);

        if (!((i + 1) % 3) && i < lastIndex) {
            output.push(separator);
        }
    }

    return output.reverse().join('');
};

formatThousands(1000);
// 1,000

formatThousands(3000, '.');
// 3.000