Python's.format for Javascript

format.js

String.prototype.format = function (obj) {
    /**
     * Does the same as a simple form of python "".format
     * that will not work for anything else than number or string.
     * Array form :
     *      "I have {} replacements to {}".format("two", "make")
     *      "I have {} replacements to {}".format(["two", "make"])
     *      "I have {0} replacements to {1}".format(["two", "make"])
     * Dict form :
     *      "Hello {name} !".format({name: "Bob"});
     *      "I am {age} years old".format({age: 12});
     *      "Pi is {pi} ! ".format({"pi": Math.PI});
     * No error is outputed in case of malformed input. It just isn't replaced.
     **/
    var source;
    if (typeof obj !== "object")
    {
        source = arguments;
    } else {
        source  = obj;
    }
    var i = 0;
    return this.replace(/{([^{}]*)}/g,
        function (a, b) {
            if (b === "")
            {
                b = i ++;
            }
            var r = source[b];
            return typeof r === 'string' || typeof r === 'number' ? r : a;
        }
    );
};