printf()/format() for JavaScript String

String.format.js

/*
* Extend JavaScript String object with printf() like functionality
* "{0} is dead, but {1} is alive!".format("This", "that");
*/
String.prototype.format = function() {
    var formatted = this;
    for(arg in arguments) {
        formatted = formatted.replace("{" + arg + "}", arguments[arg]);
    }
    return formatted;
};