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;

Note, the Car function in your code is not a class. It is a constructor function. You may call it "pseudo-class". — Šime Vidas
– Šime Vidas, Commented Dec 1, 2010 at 17:01

Sean Patrick Floyd · Accepted Answer · 2010-12-01 17:11:46Z

13

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

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

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

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

edited Dec 1, 2010 at 17:11

answered Dec 1, 2010 at 17:06

Sean Patrick Floyd

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4 Comments

capdragon Over a year ago

Thanks i was aware of the prototype one i was just wondering if the 1st option was okay. If i could mark 2 answers i would. thanks for the info.

Sean Patrick Floyd Over a year ago

no prob, upvotes are actually better than accepts. Only upvotes get you tag badges :-)

Šime Vidas Over a year ago

P.Floyd I think it's better to attach non-function properties to the object itself (this.needsWork = true inside the constructor) rather then to the object's prototype.

Sean Patrick Floyd Over a year ago

@Šime Vidas sure, it's pretty much the same thing, only I come from a Java background and feel safer if I can actually see the properties I define in the class outline

Šime Vidas · Accepted Answer · 2010-12-01 16:58:52Z

8

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

answered Dec 1, 2010 at 16:58

Šime Vidas

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Comments

Adam · Accepted Answer · 2010-12-01 16:59:38Z

1

Yes

There is nothing wrong with that.

See Object Augmentation here: http://www.crockford.com/javascript/inheritance.html

answered Dec 1, 2010 at 16:59

Adam

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