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JavaScript: The Definitive Guide

Previous Appendix E
A Preview of Navigator 4.0

E.2 Client-side JavaScript

Navigator 4.0 includes some new ways that client-side JavaScript is embedded within HTML documents.

  • JavaScript 1.2 include support for "conditional comments", a way of combining an HTML JavaScript entity with an HTML comment. If the JavaScript expression within the entity evaluates to true, then the comment will be ignored, and any JavaScript code within it will be executed (and any HTML text will be displayed). On the other hand, if the expression evaluates to false, then the conditional comment behaves like a normal HTML comment, and its contents are ignored. In this way, you can write JavaScript code that will only run on platforms that can support it. The code below, for example, will only run if the navigator.platform property (also new in JavaScript 1.2) is equal to the string "win95".

    <!--&{navigator.platform == "win95"}; 
        ...  // JavaScript code goes here

  • Navigator 4.0 allows JavaScript code to be embedded in an HTML file with the new ARCHIVE attribute of the <SCRIPT> tag. This is much like the ARCHIVE attribute of the <APPLET> tag that is used to embed Java applets in a Web page. This attribute specifies an archive file, in Java JAR format (the JAR format is a ZIP file with the addition of a standardized manifest file). The advantage of a JAR file for storing JavaScript code is that files in a JAR archive can have digital signatures attached to them. A digital signature guarantees the authenticity of the signed code, and, if you trust the signer, this allows you to trust their code as well.

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