STARTING JAVASCRIPT

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STARTING JAVASCRIPT

Post by Admin on Tue Jul 28, 2015 6:03 pm

Now, lets continue javascripting.


JavaScript can be placed in the <body> and the <head> sections of an HTML page.


The <script> Tag

In HTML, JavaScript code must be inserted between <script> and </script> tags.




Example



Code:
<script>
document.getElementById("demo").innerHTML = "My First JavaScript";
</script>



[th][/th]
Older examples may use a type attribute: <script type="text/javascript">.
The type attribute is not required. JavaScript is the default scripting language in HTML.


JavaScript Functions and Events

A JavaScript function is a block of JavaScript code, that can be executed when "asked" for.
For example, a function can be executed when an eventoccurs, like when the user clicks a button.
You will learn much more about functions and events in later chapters.


JavaScript in <head> or <body>

You can place any number of scripts in an HTML document.
Scripts can be placed in the <body>, or in the <head> section of an HTML page, or in both.


[th][/th]
Keeping all code in one place, is always a good habit.


JavaScript in <head>

In this example, a JavaScript function is placed in the <head> section of an HTML page.
The function is invoked (called) when a button is clicked:


Example



Code:
<!DOCTYPE html>
<html>
<head>
<script>
function myFunction() {
    document.getElementById("demo").innerHTML = "Paragraph changed.";
}
</script>
</head>
<body>
<h1>My Web Page</h1>
<p id="demo">A Paragraph</p>
<button type="button" onclick="myFunction()">Try it</button>
</body>
</html>






JavaScript in <body>

In this example, a JavaScript function is placed in the <body> section of an HTML page.
The function is invoked (called) when a button is clicked:


Example



Code:
<!DOCTYPE html>
<html>
<body> 
<h1>My Web Page</h1>
<p id="demo">A Paragraph</p>
<button type="button" onclick="myFunction()">Try it</button>
<script>
function myFunction() {
   document.getElementById("demo").innerHTML = "Paragraph changed.";
}
</script>
</body>
</html>


[th][/th]
It is a good idea to place scripts at the bottom of the <body> element.
This can improve page load, because HTML display is not blocked by scripts loading.


External JavaScript

Scripts can also be placed in external files.
External scripts are practical when the same code is used in many different web pages.
JavaScript files have the file extension .js.
To use an external script, put the name of the script file in the src (source) attribute of the <script> tag:


Example



Code:
<!DOCTYPE html>
<html>
<body>
<script src="myScript.js"></script>
</body>
</html>


You can place an external script reference in <head> or <body> as you like.
The script will behave as if it was located exactly where the <script> tag is located.
[th][/th]
External scripts cannot contain <script> tags.


External JavaScript Advantages

Placing JavaScripts in external files has some advantages:

  • It separates HTML and code.


  • It makes HTML and JavaScript easier to read and maintain


  • Cached JavaScript files can speed up page loads. Very Happy


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