The this Keyword
In Actionscript this represents what can be considered the current scope. If you putting code in a timeline, say _root, then using this will allow you to reference _root directly. For a function, if the function is defined in _root, the this keyword will again reference _root. If the function was defined in some generic object created in _root, this in that function would be the generic object.

Lets add a function to an array object. Using this, we can access that array directly, and in doing so, use the function to alter the array’s contents.

myArray = [1,2];

myArray.switchValues = function(){

var temp = this[0];

this[0] = this[1];

this[1] = temp;

};

trace(myArray); // traces 1,2

myArray.switchValues();

trace(myArray); // traces 2,1

As you can see, this function uses this to directly access the current object’s (myArray’s) array elements as this refers to the array itself. Then, using a local temporary variable temp, it is able to internally switch the two values of those array elements. The trace after the function call confirms its success on the myArray variable.

Even though the this keyword references the current object, a this in a function may not always reference the same object. Since function variables are references to a function object, there can be separate definitions of the same function in different objects. Depending on which object initiates that function call determines which object the this in the function represents. This behavior is quite advantageous as it can save you the need to repeatedly redefine the same function for multiple objects. Instead, you can define the function once and then assign it to each object requiring it. For example

increaseNum = function(){

this.num++;

trace(this.num);

};

 

objectA = {num: 0};

objectA.increaseNum = increaseNum;

 

objectB = {num: 10};

objectB.increaseNum = increaseNum;

 

objectA.increaseNum(); // traces 1

objectB.increaseNum(); // traces 11

Here, two separate objects, objectA and objectB each have their separate num values assigned to them. Each also have assigned to them a increaseNum property which is set to be the previously defined increaseNum function. When increaseNum is run for each, the this object in the function block refers to the object which called the function. It’s the same function, only the this value is representative of what object called it.

The this keyword is used in excess when dealing with objects and functions, or methods, associated with such objects. Each method of an object gains access to that object through the use of this so be prepared to write it consistently especially when writing methods to be used with all instances of an object type.

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