Can I add a property dynamically in javascript?

Question

Is it okay to add properties to an object at runtime? It seems to run okay but are there any issues I should be aware of?

I'm using a 3rd party javascript API which has an object class, which I've instantiated and added my own property to after instantiation, like the code below:

For example can I do this:

var Car = function (id, type) {
    this.id = id;
    this.type = type;
};

var myCar = new Car(1,"Nissan");

// CAN I DO THIS: (needsWork not a property of object Car)
myCar.needsWork = true;

Answer 1

Actually, you have two ways to do that in JavaScript:

1. add a method or property to an instance (this car only)

   var myCar = new Car(1,"Nissan");
   myCar.needsWork = true;
   

2. add a method or property to the car prototype (all cars, even already existing ones)

   var myCar = new Car(1, "Nissan");
   var biggerCar = new Car(2, "Hummer");
   Car.prototype.needsWork = true;
   alert( myCar.needsWork && biggerCar.needsWork 
           ?  "We need work"
           : "Something wrong here"
   );
   

Reference:

• Object.prototype

Answer 2

Yea, this is called object augmentation. It is a key feature in JavaScript.