Prototypes

You learned earlier that in JavaScript you should define "class methods" outside the body of the "class", in order to prevent their multiplication for each instantiated object. Prototyping is a JavaScript language feature that allows attaching functions and properties to the "blueprint" of a function.

When functions are added to a class (function) prototype, they are not replicated for each object of the class (function). This reflects quite well the behavior of classes in C#, although the core mechanism and the specific implementation details differ greatly. A few facts that you should keep in mind about prototypes are:

• Every JavaScript function has a property named prototype. Adding members to the function's prototype is implemented by adding them to the prototype property of the function.
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• Private variables of a function aren't accessible through functions added to its prototype.
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• You can add members to a function's prototype at any time, but this won't affect objects that were already created. It will affect only any new ones.
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• You can add members to a function's prototype only after the function itself has been defined.

The Table "class" from the previous example contains a "method" named getCellCount(). The following code creates the same class, but this time adding getCellCount() to its prototype:

First Page: Microsoft AJAX Library - Creating Object Members on the Fly