Edit in JSFiddle

if(!String.format){
    String.format = function(){
        var argv = Array.prototype.slice.call(arguments),
            string = argv[0],
            i = 0;

        string = string.replace("%%", "<<percent>>", "g");

        while(argv[i++]){
            string = string.replace(/%[a-z]/, argv[i]);
        }

        string = string.replace("<<percent>>", "%", "g");

        return string;
    }
}

txt.value = String.format(
    "Hello %s!\n" +
    "The answer to %s is %d.\n" +
    "Here's an array: %a\n" +
    "Here's a percentage sign: %%\n" +
    "Here's a percentage sign and the letter d: %d\n" +
    "Here it is again but escaped properly: %%d\n" +
    "Here's the format string itself, just to go extra-meta:\n\n" +
    "Hello %%s!\\nThe answer to %%s is %%d\\nHere's an array: %%a\\nHere's a percentage sign: %%%%\\nHere's a percentage sign and the letter d: %%d\\nHere it is again but escaped properly: %%%%d\\nHere's the format string itself, just kidding.", 
    
    "world", 
    "Life, the Universe, and Everything",
    42,
    [1,2,3,4,5]);

<textarea id=txt></textarea>

#txt{
    width:100%;
    height:25em;
    background:inherit;
    border:none;
}