This function extends String adding minimal *printf-like* functionality.

javascript-string-format.js

String.prototype.format = function (format_object) {
    // TODO: Find better way to do this. Works great for now.
    // I don't know how efficient this is, but it works.

    var string_current = this.valueOf();
    var string_new     = '';
    var format_matches = string_current.match(/\%\([a-zA-Z0-9_]+\)/g); // Definitely has edge cases.
    var replace_object = {};

    if (format_matches.length > 0) {
        for (var i = 0, j = format_matches.length; i < j; i++) {
            var format_variable_length = format_matches[i].length;
            var format_object_key      = format_matches[i].substring(2, (format_variable_length - 1));

            if (format_object_key in format_object) {
                replace_object[format_matches[i]] = format_object[format_object_key];
            }
        }

        string_new = string_current;

        for (var key in replace_object) {
            string_new = string_new.replace(key, replace_object[key]);
        }

        return string_new;
    } else { return string_current; }
};

javascript-string-format.min.js

String.prototype.format=function(a){var b=this.valueOf(),c="",d=b.match(/\%\([a-zA-Z0-9_]+\)/g),e={};if(d.length>0){for(var f=0,g=d.length;g>f;f++){var h=d[f].length,i=d[f].substring(2,h-1);i in a&&(e[d[f]]=a[i])}c=b;for(var j in e)c=c.replace(j,e[j]);return c}return b};

This makes it possible to do things like the following.

'%(a) is %(c).'.format({
    'c': 2,
    'a': 'this'
}); // => "this is 2."