JavaScript: The Definitive Guide, Fourth Edition

Language Version Compatibility

The previous section discussed general compatibility techniques that are useful for coping with incompatibilities between different versions of browsers from different vendors running on different platforms. This section addresses another compatibility concern: how to use new features of the JavaScript language in a way that does not cause errors on browsers that do not support those features. Our goals are simple: we need to prevent JavaScript code from being interpreted by browsers that don’t understand it, and we need to display special messages on those browsers that inform users that their browsers cannot run the scripts.

The language Attribute

The first goal is easy. As we saw in Chapter 12, we can prevent a browser from attempting to run code that it cannot understand by setting the language attribute of the <script> tag appropriately. For example, the following <script> tag specifies that the code it contains uses features of JavaScript 1.1 and that browsers that do not support that version of the scripting language should not attempt to run it:

<script language="JavaScript1.1">
    // JavaScript 1.1 code goes here
</script>

Note that the use of the language attribute is a general technique. When set to the string “JavaScript1.2”, the attribute prevents JavaScript 1.0 or 1.1 browsers from attempting to run the code. At the time of this writing, the latest browsers (Netscape 6 and IE 6) support language versions 1.0, 1.1, 1.2, 1.3, 1.4, and ...