Prototype an object with secret methods which self-destruct

this-method-will-self-destruct.js

function SecretMethods () {} (function () {
  
  var add = function (a,b) {
    return a + b;
  }

  var mul = function (a,b) {
    return a * b;
  }
  
  var once = function (f) {
      return function () {
        var fn = f;
        f = null;
        return fn.apply(this, arguments);
      };
    }
  
  var z = once(add);
  
  SecretMethods.prototype = {
    secret: z
  };
  
}());

var g = new SecretMethods();

console.log(g.secret(3,4)); // Works, as expected

console.log(g.secret(3,4)); // Throws an error because IT HAS SELF DESTRUCTED!