Javascript can be placed in three ways.
- Between <head> and </head>
- Between <body> and </body>
- External JS File
How To Define Javascript
In HTML, JavaScript code must be inserted between <script> and </script> tags.
<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:
<html>
<head>
<script>
function myfun()
{
document.getElementById("line").innerHTML = "The Content Is Changed By Javascript"
}
</script>
</head>
<body>
<h2>Java Script Can Change HTML Content</h2>
<p id="line">This Is HTML Content</p>
<button type="button" onclick='myfun()'>Click Me to Change</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>
<head>
</head>
<body>
<h2>Java Script Can Change HTML Content</h2>
<p id="line">This Is HTML Content</p>
<button type="button" onclick='myfun()'>Click Me to Change</button>
<script>
function myfun()
{
document.getElementById("line").innerHTML = "The Content Is Changed By Javascript"
}
</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. Script file have to save with .js extension.
sc1.js
function myfun()
{
document.getElementById("line").innerHTML = "The Content Is Changed By Javascript"
}
xternal 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="sc1.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:
<script src="sc1.js"></script> <script src="sc2.js"></script>
You can also specify path for java script from folder in your system or from web.