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Dynamic HTML #14: JavaScript - Part III

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أستاذ المادة حيدر محمد حبيب الخميس       18/12/2012 08:40:03
JavaScript Functions
To keep the browser from executing a script when the page loads, you can put your script into a function.
A function contains code that will be executed by an event or by a call to the function.
You may call a function from anywhere within a page (or even from other pages if the function is embedded in an external .js file).
Functions can be defined both in the and in the section of a document. However, to assure that a function is read/loaded by the browser before it is called, it could be wise to put functions in the section.

How to Define a Function
Syntax
function functionname(var1,var2,...,varX)
{
some code
}
The parameters var1, var2, etc. are variables or values passed into the function. The { and the } defines the start and end of the function.

Note: A function with no parameters must include the parentheses () after the function name.

Note: Do not forget about the importance of capitals in JavaScript! The word function must be written in lowercase letters, otherwise a JavaScript error occurs! Also note that you must call a function with the exact same capitals as in the function name.

The return Statement
The return statement is used to specify the value that is returned from the function.
So, functions that are going to return a value must use the return statement.

The Lifetime of JavaScript Variables
If you declare a variable within a function, the variable can only be accessed within that function. When you exit the function, the variable is destroyed. These variables are called local variables. You can have local variables with the same name in different functions, because each is recognized only by the function in which it is declared.
If you declare a variable outside a function, all the functions on your page can access it. The lifetime of these variables starts when they are declared, and ends when the page is closed.

JavaScript Loops
Often when you write code, you want the same block of code to run over and over again in a row. Instead of adding several almost equal lines in a script we can use loops to perform a task like this.
In JavaScript, there are two different kind of loops:
• for - loops through a block of code a specified number of times
• while - loops through a block of code while a specified condition is true

The for Loop
The for loop is used when you know in advance how many times the script should run.
Syntax
for (var=startvalue;var<=endvalue;var=var+increment)
{
code to be executed
}


The while Loop
The while loop loops through a block of code while a specified condition is true.
Syntax
while (var<=endvalue)
{
code to be executed
}
Note: The <= could be any comparing operator.

The do...while Loop
The do...while loop is a variant of the while loop. This loop will execute the block of code ONCE, and then it will repeat the loop as long as the specified condition is true.
Syntax
do
{
code to be executed
}
while (var<=endvalue);

JavaScript document.getElementById
If you want to quickly access the value of an HTML input give it an id to make your life a lot easier. This small script below will check to see if there is any text in the text field "myText". The argument that getElementById requires is the id of the HTML element you wish to utilize.

Changing Text with innerHTML
Each HTML element has an innerHTML property that defines both the HTML code and the text that occurs between that element s opening and closing tag. By changing an element s innerHTML after some user interaction, you can make much more interactive pages.
However, using innerHTML requires some preparation if you want to be able to use it easily and reliably. First, you must give the element you wish to change an id. With that id in place you will be able to use the getElementById function, which works on all browsers.


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