Difference Between Class Properties and Function Prototype in Javascript

Question

Hi,

I like to learn the difference between Class Property and Prototype in Javascript what I mean is shown in the code :

function Rectangle(x, y) {
    this.width = x;
    this.height = y;
}

Rectangle.UNIT = new Rectangle(1, 1);

Rectangle.prototype.UNIT = new Rectangle(1, 1);

The thing I know is prototype is working like inherit object which means UNIT will be shown all the instances from now on but Rectangle.UNIT = new Rectangle(1, 1); code doesn't do the same thing ?

Answer 1

Rectangle.UNIT is a static class property. It can only ever be accessed on the Rectangle class object. It won't be accessible on any instances of Rectangle.

Rectangle.prototype.UNIT is a prototype property and can be accessed on instances of Rectangle.

If you make a class Square that inherits from Rectangle, any instances of Square will share the same prototype property, but not any static class properties.

You may find these articles helpful (though maybe a little obscure):

• http://phrogz.net/JS/Classes/OOPinJS.html
• http://phrogz.net/JS/Classes/OOPinJS2.html
• http://javascript.crockford.com/prototypal.html

Answer 2

Rectangle.UNIT is like a class static member, whereas Rectangle.prototype.UNIT is a local member. Looks like you want Rectangle.UNIT in your case, because you only need one static instance of the object.

Answer 3

Class-Based vs. Prototype-Based Languages @ Mozilla Developer Center. Neat!