|
Book Excerpt: Object-Oriented JavaScript
|
|
This chapter excerpt from
Microsoft AJAX Library Essentials by Cristian Darie, Bogdan
Brinzarea, is printed with permission from Packt
Publishing, Copyright 2007.
Anonymous Functions
Anonymous functions can be created adhoc and used instead of a named function.
Although this can hinder readability when the function is more complex, you can
do this if you don't intend to reuse a function's code. In the following
example we pass such an anonymous function to DisplayGreeting(), instead of
passing GetCurrentHour():
// call DisplayGreeting
DisplayGreeting(
function()
{
return (new Date()).getHours();
}
);
|
This syntax is sure to look strange if this is the first time you have worked
with anonymous functions. You can compact it on a single line if it helps
understanding it better:
DisplayGreeting( function() { return (new Date()).getHours(); } );
This code can be tested online at http://www.cristiandarie.ro/asp-ajax/
AnonymousFunction.html.
Inner Functions and JavaScript Closures
JavaScript functions implement the concept of closures, which are functions that
are defi ned inside other functions, and use contextual data from the parent
functions to execute. You can fi nd a complete and technically accurate defi
nition of closures at
http://en.wikipedia.org/wiki/Closure_(computer_science).
In JavaScript a function can be regarded as a named block of code that you can
execute, but it can also be used as a data member inside another function, in
which case it is referred to as an inner functions. In other words, a
JavaScript function can contain other functions.
Say that we want to upgrade the initial ShowHelloWorld() function by separating
the code that displays the greeting message into a separate function inside
ShowHelloWorld(). This is a possible implementation, and the output continues
to be the same as before:
// call function to display greeting message
ShowHelloWorld();
// "Hello, World" function
function ShowHelloWorld()
{
// declaring new variables
var date = new
Date();
var hour =
date.getHours();
// call DisplayGreeting
supplying the current hour as parameter
DisplayGreeting(hour);
// display greeting
function DisplayGreeting(hour)
{
if (hour >= 22 ||
hour <= 5)
document.write("Goodnight, world!");
else
document.write("Hello, world!");
}
}
Here, we created a function named DisplayGreeting() inside ShowHelloWorld(),
which displays a greeting message depending on the hour parameter it receives.
The execution rules apply here as well. This new function needs to be called
explicitly from its parent function in order to execute.
This code can be tested online at
http://www.cristiandarie.ro/asp-ajax/JavaScriptClosure.html.
Related Links
>Object
Oriented Programming Interview Questions
Concurrency is the system's ability to act with several requests simultaneously,
such a way that threads don't corrupt the state of objects when they gain
access at the same time............
What is the relation between Classes and Objects? Explain different properties
of Object Oriented Systems. What is difference between Association, Aggregation
and Inheritance relationships? Explain the features of an abstract class in
NET. Difference between abstract classes and interfaces Similarities and
difference between Class and structure in .NET Features of Static/Shared
classes. What is Operator Overloading in .NET?.............
What
is object oriented programming (OOP)?
The object oriented programming is commonly known as OOP. Most of the languages
are developed using OOP concept. Object-oriented programming (OOP) is a
programming concept that uses "objects" to develop a system.........
What
are the various elements of OOP?
Various elements of OOP are.........
|
