StringFormat.js

// Only add if it doesn't exist yet!
if (!String.prototype.format) {
    String.prototype.format = function() {
        var str = this.toString();
        if (!arguments.length)
            return str;
        var args = typeof arguments[0],
            args = (("string" == args || "number" == args) ? arguments : arguments[0]);
        for (arg in args)
            str = str.replace(RegExp("\\{" + arg + "\\}", "gi"), args[arg]);
        return str;
    }
}

Usage is :

'{0} {1}!'.format('Hello', 'World');

results in

Hello World!

or

'{greeting} {name}!'.format({ greeting:'Hello', name:'World'});

results in

Hello World!