The <script> Tag
In HTML, JavaScript code is inserted between <script> and </script> tags.
Example
<script> document.getElementById("demo").innerHTML = "My First JavaScript";
</script>Old JavaScript 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 "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:
Example
<!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>Live preview
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
<!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>Live preview
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:
External file: myScript.js
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:
<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 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
To add several script files to one page - use several script tags:
Example
<script src="myScript1.js"></script>
<script src="myScript2.js"></script>