Ensuring type when Duck Typing in Javascript a.k.a. Duck Checking

In Javascript there is no such thing as an interface, at least not yet, but sometimes it can be really useful to know if an object is similar to another, i.e. checking if object A implements all methods in object B, to ensure nothing goes wrong when Duck Typing.

Therefor I've written this nifty little method which does this kind of typechecking:

Object.prototype.doesImplement = function (interfaceObj) {
  for (var key in interfaceObj) {
    if (interfaceObj.hasOwnProperty(key) && typeof this[key] !== typeof interfaceObj[key]) {
      return false;
    }
  }
  return true;
};

The doesImplement method can then be used like this:

/* Interfaces */
var IRunnable = {
      run: function () {}
    },
     IUser = {
      username: '',
    };

var myRunnable = {
  run: function () {
    alert("I'm running!");
  }
};

var myUser = {username: 'user1'};

console.log(myRunnable.doesImplement(IRunnable)); /* true */
console.log(myRunnable.doesImplement(IUser)); /* false */
console.log(myUser.doesImplement(IUser)); /* true */
console.log(myUser.doesImplement(IRunnable)); /* false */

I hope you can find it useful!