The <script> Tag

In HTML, JavaScript code is insert between <script> and </script> tags.

html

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

JavaScript Functions and Events

A JavaScript function is a block of JavaScript code, that can be executed when “called” for.

For example, a function can be called when an event occurs, 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.

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 :

html

<!DOCTYPE html>
<html>
	<head>
		<script> 
			function myFunction(){
				document.getElementById("demo").innerHTML = "Paragraph changed ."; 
			}
		</script> 
	</head>
	<body>
		<h2> Demo JavaScript in head</h2>
		<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:

html

<!DOCTYPE html> 
<html>
	<body>

	<h2>Demo JavaScript in Body</h2>
	
	<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>

Placing scripts at the bottom of the <body> element improves the display speed, because script interpretation slows down the display.

External JavaScript

Scripts can also be placed in external files:

javascript

function myFunction(){
	document.getElementById("demo").innerHTML = "Paragraph changed."; 
}

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 a <script> tag:

html

<script src = "myScript.js"></script>

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.

External scripts cannot contain <script> tags.

External JavaScript Advantages

Placing scripts external files has some advantage:

It separates HTML and code.
It makes HTML and JavaScript easier to read and maintain.
Cached JavaScript files can speed up page loads.

To add several script files to one page - use several script tags:

Example

html

<script src="myScript1.js"></script>
<script src="myScript2.js"></script>

External References

An external script can be referenced in 3 different ways:

With a full URL (a full web address)
With a file path (like /js/)
Without any path

This example uses a full URL to link to myScript.js:

html

<script src="https://www.w3schools.com/js/myScript.js"></script>

This example uses a file path to link myScript.js:

html

<script src="/js/myScript.js"></script>

This example uses no path to link to myScript.js:

html

<script src="myScript.js"></script>

Where to Place JavaScript in HTML Documents

The <script> Tag

JavaScript Functions and Events

JavaScript in <head> or <body>

JavaScript in <head>

JavaScript in <body>

External JavaScript

External JavaScript Advantages

External References

Nguyen Bao Huy

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