Although basic function definition and invocation should not present a challenge to experienced programmers, some of the more advanced features of ActionScript functions require some explanation. You call a function by using its identifier followed by the parentheses operator. You use the parentheses operator to enclose any function parameters you want to send to the function.
For example, the trace function is a top-level function in ActionScript 3. If you are calling a function with no parameters, you must use an empty pair of parentheses. For example, you can use the Math. There are two ways to define a function in ActionScript 3. The technique you choose depends on whether you prefer a more static or dynamic programming style. Define your functions with function statements if you prefer static, or strict mode, programming. Define your functions with function expressions if you have a specific need to do so.
Function expressions are more often used in dynamic, or standard mode, programming. Function statements are the preferred technique for defining functions in strict mode.
A function statement begins with the function keyword, followed by:. The function body—that is, the ActionScript code to be executed when the function is called, enclosed in curly brackets. The second way to declare a function is to use an assignment statement with a function expression, which is also sometimes called a function literal or an anonymous function. This is a more verbose method that is widely used in earlier versions of ActionScript.
An assignment statement with a function expression begins with the var keyword, followed by:. For example, the following code declares the traceParameter function using a function expression:. Notice that you do not specify a function name, as you do in a function statement. Another important difference between function expressions and function statements is that a function expression is an expression rather than a statement.
This means that a function expression cannot stand on its own as a function statement can. A function expression can be used only as a part of a statement, usually an assignment statement. The following example shows a function expression assigned to an array element:.
As a general rule, use a function statement unless specific circumstances call for the use of an expression. Function statements are less verbose, and they provide a more consistent experience between strict mode and standard mode than function expressions. Function statements are easier to read than assignment statements that contain function expressions.
Function statements make your code more concise; they are less confusing than function expressions, which require you to use both the var and function keywords. Function statements provide a more consistent experience between the two compiler modes in that you can use dot syntax in both strict and standard mode to call a method declared using a function statement.
This is not necessarily true for methods declared with a function expression. For example, the following code defines a class named Example with two methods: methodExpression , which is declared with a function expression, and methodStatement , which is declared with a function statement. In strict mode, you cannot use dot syntax to call the methodExpression method. Function expressions are considered better suited to programming that focuses on run-time, or dynamic, behavior.
If you prefer to use strict mode, but also need to call a method declared with a function expression, you can use either of two techniques. First, you can call the method using square brackets [] instead of the dot. The following method call succeeds in both strict mode and standard mode:. Second, you can declare the entire class as a dynamic class.
Although this allows you to call the method using the dot operator, the downside is that you sacrifice some strict mode functionality for all instances of that class. For example, the compiler does not generate an error if you attempt to access an undefined property on an instance of a dynamic class. There are some circumstances in which function expressions are useful. One common use of function expressions is for functions that are used only once and then discarded.
Another less common use is for attaching a function to a prototype property. For more information, see The prototype object. There are two subtle differences between function statements and function expressions that you should take into account when choosing which technique to use.
The first difference is that function expressions do not exist independently as objects with regard to memory management and garbage collection. In other words, when you assign a function expression to another object, such as an array element or an object property, you create the only reference to that function expression in your code. If the array or object to which your function expression is attached goes out of scope or is otherwise no longer available, you no longer have access to the function expression.
If the array or object is deleted, the memory that the function expression uses becomes eligible for garbage collection, which means that the memory is eligible to be reclaimed and reused for other purposes.
The following example shows that for a function expression, once the property to which the expression is assigned is deleted, the function is no longer available. The class Test is dynamic, which means that you can add a property named functionExp that holds a function expression. The functionExp function can be called with the dot operator, but once the functionExp property is deleted, the function is no longer accessible.
If, on the other hand, the function is first defined with a function statement, it exists as its own object and continues to exist even after you delete the property to which it is attached. The delete operator only works on properties of objects, so even a call to delete the function stateFunc itself does not work. The second difference between function statements and function expressions is that function statements exist throughout the scope in which they are defined, including in statements that appear before the function statement.
Function expressions, by contrast, are defined only for subsequent statements. For example, the following code successfully calls the scopeTest function before it is defined:. Function expressions are not available before they are defined, so the following code results in a run-time error:. To return a value from your function, use the return statement followed by the expression or literal value that you want to return.
For example, the following code returns an expression representing the parameter:. Notice that the return statement terminates the function, so that any statements below a return statement are not executed, as follows:. In strict mode, you must return a value of the appropriate type if you choose to specify a return type. For example, the following code generates an error in strict mode, because it does not return a valid value:. You can nest functions, which means that functions can be declared within other functions.
A nested function is available only within its parent function unless a reference to the function is passed to external code. Here, the File method is the constructor function corresponding to the class of the same name File.
Only those properties and methods in the public namespace are available in JavaScript. For example, the File class in the flash. Both are available as properties of a JavaScript variable that instantiates a File object via the runtime. File constructor method. Any code that instantiates an object of a certain type can access the public properties and methods in the class that defines that type. Also, any code can access the public static properties and methods of a public class. Properties and methods designated as private are only available to code within the class.
They cannot be accessed as properties or methods of an object defined by that class. Properties and methods in the private namespace are not available in JavaScript. Properties and methods designated as protected are only available to code in the class definition and to classes that inherit that class.
Properties and methods in the protected namespace are not available in JavaScript. Properties and methods designated as internal are available to any caller within the same package. Classes, properties, and methods belong to the internal namespace by default. Additionally, custom classes can use other namespaces that are not available to JavaScript code. In both ActionScript 3. The following ActionScript 3. It also includes the p parameter, which is optional, with a default value of An event listener is a function.
When an object dispatches an event, the event listener responds to the event. The event, which is an ActionScript object, is passed to the event listener as a parameter of the function. For example, when you call the load method of a Sound object to load an mp3 file , the Sound object attempts to load the sound. Then the Sound object dispatches any of the following events:. Any class that can dispatch events either extends the EventDispatcher class or implements the IEventDispatcher interface.
In each class listing for these classes in the ActionScript Language Reference, there is a list of events that the class can dispatch. You can register an event listener function to handle any of these events, using the addEventListener method of the object that dispatches the event. For example, in the case of a Sound object, you can register for the progress and complete events, as shown in the following ActionScript code:. The following code illustrates this.
Adobe Flash Platform. View Help PDF 5. ActionScript basics for JavaScript developers. Sound file ; sound. Data types corresponding to custom classes An ActionScript 3. The void data type The void data type is used as the return value for a function that, in fact, returns no value. Runtime classes The runtime includes built-in classes, many of which are also included in standard JavaScript, such as the Array, Date, Math, and String classes and others.
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