The <script> tag is used to define a client-side script (JavaScript).

The <script> element either contains script statements, or it points to an external script file through the srcattribute.

Common uses for JavaScript are image manipulation, form validation, and dynamic changes of content.

To select an HTML element, JavaScript most often uses the document.getElementById() method.

This JavaScript example writes "Hello JavaScript!" into an HTML element with id="demo":

example : 

<script>

document.getElementById("demo").innerHTML = "Hello JavaScript!";

</script>

A Taste of JavaScript

Here are some examples of what JavaScript can do:

1) 

JavaScript can change HTML content

document.getElementById("demo").innerHTML = "Hello JavaScript!";

2) 

JavaScript can change HTML styles

document.getElementById("demo").style.fontSize = "25px";

document.getElementById("demo").style.color = "red";

document.getElementById("demo").style.backgroundColor = "yellow";

3) 

JavaScript can change HTML attributes

document.getElementById("image").src = "picture.gif";