string.format para javascript

Código:

String.prototype.format = function() {
  var str = this;
  for (var i = 0; i <= arguments.length - 1; i++) {       
    var reg = new RegExp("\{" + i + "\}", "gm");             
    str = str.replace(reg, arguments[i]);
  }
  return str;
};

Uso:

console.log("Hello, {0}, today is {1}!".format("Tom", "Monday"));