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JavaScript Introduction

JS Introduction

JavaScript Where To

 

JavaScripts can be put in the <body> and in the <head> sections of an HTML page.

JavaScript in <body>

The example below writes the current date into an existing <p> element when the page loads:

Example

<html>
<body>

<h1>My First Web Page</h1>

<p id="demo"></p>

<script type="text/javascript">
document.getElementById("demo").innerHTML=Date();
</script>

</body>
</html>


Note that the JavaScript is placed at the bottom of the page to make sure it is not executed before the <p> element is created.

JavaScript Functions and Events

JavaScripts in an HTML page will be executed when the page loads. This is not always what we want.

Sometimes we want to execute a JavaScript when an event occurs, such as when a user clicks a button. When this is the case we can put the script inside a function.

Events are normally used in combination with functions (like calling a function when an event occurs).

You will learn more about JavaScript functions and events in later chapters.


JavaScript in <head>

The example below calls a function when a button is clicked:

Example

<html>

<head>
<script type="text/javascript">
function displayDate()
{
document.getElementById("demo").innerHTML=Date();
}
</script>
</head>

<body>

<h1>My First Web Page</h1>

<p id="demo"></p>

<button type="button" onclick="displayDate()">Display Date</button>

</body>
</html>



Scripts in <head> and <body>

You can place an unlimited number of scripts in your document, and you can have scripts in both the body and the head section at the same time.

It is a common practice to put all functions in the head section, or at the bottom of the page. This way they are all in one place and do not interfere with page content.


Using an External JavaScript

JavaScript can also be placed in external files.

External JavaScript files often contain code to be used on several different web pages.

External JavaScript files have the file extension .js.

Note: External script cannot contain the <script></script> tags!

To use an external script, point to the .js file in the "src" attribute of the <script> tag:

Example

 

<html>
<head>


<script type="text/javascript" src="xxx.js"></script>


</head>
<body>
</body>
</html>

 


Note: Remember to place the script exactly where you normally would write the script!

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