2.1 JavaScript in HTML

JavaScript code may be embedded in HTML files in the form of scripts, event handlers and URLs, as detailed below.

2.1.1 The <script> tag

Most JavaScript code appears in HTML files between a <script> tag and a </script> tag. For example:

<script>
document.write("The time is: " + new Date());
</script>

In JavaScript 1.1 and later you can use the src attribute of the <script> tag to specify the URL of an external script to be loaded and executed. Files of JavaScript code typically have a .js extension. Note that the </script> tag is still required when this attribute is used:

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

HTML allows scripts to be written in languages other than JavaScript, and some browsers, such as Internet Explorer, support languages such as VBScript. You can use the language attribute to specify what language a script is written in. This attribute defaults to "JavaScript" in all browsers, so you do not usually have to set it. You can also use attribute values such as "JavaScript1.3" and "JavaScript1.5" to specify the version of JavaScript your code uses. Browsers that do not support the specified version of the language simply ignore the script.

HTML 4 does not actually recognize the language attribute of the <script> tag. Instead, the official way to specify the language a script is written in is with the type attribute. For JavaScript, set this attribute to the MIME type "text/javascript":

<script src="functions.js"
        language="JavaScript1.5"
        type="text/javascript"></script>

2.1.2 Event handlers

JavaScript code may also appear as the value of an event handler attribute of an HTML tag. Event handler attribute names always begin with "on". The code specified by one of these attributes is executed when the named event occurs. For example, the following HTML creates a button, and the onclick attribute specifies an event handler that displays an alert (a dialog box) when the user clicks the button:

<input type="button" value="Press Me"
       onclick="alert('Hello World!');">

A list of other available event handler attributes is included later in this section.

2.1.3 JavaScript URLs

JavaScript code may appear in a URL that uses the special javascript: pseudo-protocol. The content of such a URL is determined by evaluating the JavaScript code and converting the resulting value to a string. If you want to use a JavaScript URL to execute JavaScript code without returning any document content that would overwrite the current document, use the void operator:

<form action="javascript:void validate( )">