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Book Excerpt: Object-Oriented JavaScript
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This chapter excerpt from
Microsoft AJAX Library Essentials by Cristian Darie, Bogdan
Brinzarea, is printed with permission from Packt
Publishing, Copyright 2007.
Prototypes
You learned earlier that in JavaScript you should defi ne "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:
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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.
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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:
//Table class
function Table (rows, columns)
{
// save parameter values to class properties
this.rows = rows;
this.columns = columns;
}
//Table.getCellCount returns the number of table cells
Table.prototype.getCellCount = function()
{
return this.rows * this.columns;
};
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