Prototypes for Properties
Though the prototype object, though commonly used for methods, is not solely for methods alone. Variable properties of other more simple values can be defined there as well. These can be variables that you wish consistent for all object instances or also values to serve as defaults for otherwise unique properties in objects.

An example of such a property as a default would be Button and MovieClip objects’ useHandCursor property. By default, each Button and Movieclip has a useHandCursor value of true. This means when button actions such as onPress are assigned to either a button or movieclip, the default arrow cursor will change to a hand icon when the cursor is over the button or movieclip. This true value is stored in the button and movieclip prototypes and therefore is set for each button and movieclip created.

When you want individual instances to have their own useHandCursor value, you can just define a new useHandCursor value for that instance itself. Properties defined directly to object instances will have precedence over prototype values. With this, realize that assigning a value to a property which may not exist in an object instance but does exist in the instance’s class prototype won’t mean that you are assigning the value to that prototype variable. You would actually then be creating a new variable specific to the object that will then have precedence over the prototype’s variable value when referenced for that object. So a value referenced and a value assigned may not always be the same variable. If you want a prototype value to change, you will need to do so directly from the prototype object itself, otherwise you would be defining a value in an instance.

The following example will assume a movieclip called clipA in the timeline.
 

// trace default value for movieclips’ useHandCursor

trace(MovieClip.prototype.useHandCursor); // traces true

trace(clipA.useHandCursor); // traces true

 

// change the prototype and see it reflected in clipA

MovieClip.prototype.useHandCursor = false;

trace(MovieClip.prototype.useHandCursor); // traces false

trace(clipA.useHandCursor); // traces false

 

// reset the prototype

MovieClip.prototype.useHandCursor = true;

trace(clipA.useHandCursor); // traces true

 

// now set a unique useHandCursor for clipA that overrides the prototype

clipA.useHandCursor = false;

trace(clipA.useHandCursor); // traces false

trace(MovieClip.prototype.useHandCursor); // traces true

When incorporating such defaults into your own classes, be sure not to set anything for the value you wish to default or it will override what is set in the prototype. For example, using the house class, in defaulting the siding, siding should only be set at all in the house constructor if the siding is passed. When not, the prototype value will be used.
 

// class definition

House = function(siding){

this.floors = 4;

if (siding != undefined){

this.siding = siding;

}

};

// default siding property value

House.prototype.siding = "Red";

// outputSiding method

House.prototype.outputSiding = function(){

trace(this.siding);

};

 

// create an instance of the House class

myHouse = new House();

myHouse.outputSiding(); // traces "Red"

Since no siding was passed in creating myHouse, the if statement skipped the assignment of siding altogether in the constructor. As such, the siding traced in the outputSiding method is the siding referenced from the house prototype object.

For an online collection of prototype functions, see http://proto.layer51.com.

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