JavaScript: The Definitive Guide, 6th Edition

Chapter 11. JavaScript Subsets and Extensions

Until now, this book has described the complete and official JavaScript language, as standardized by ECMAScript 3 and ECMAScript 5. This chapter instead describes subsets and supersets of JavaScript. The subsets have been defined, for the most part, for security purposes: a script written using only a secure language subset can be executed safely even if it comes from an untrusted source such as an ad server. JavaScript Subsets describes a few of these subsets.

The ECMAScript 3 standard was published in 1999 and a decade elapsed before the standard was updated to ECMAScript 5 in 2009. Brendan Eich, the creator of JavaScript, continued to evolve the language during that decade (the ECMAScript specification explicitly allows language extensions) and, with the Mozilla project, released JavaScript versions 1.5, 1.6, 1.7, 1.8, and 1.8.1 in Firefox 1.0, 1.5, 2, 3, and 3.5. Some of the features of these extensions to JavaScript have been codified in ECMAScript 5, but many remain nonstandard. Future versions of ECMAScript are expected to standardize at least some of the remaining nonstandard features.

The Firefox browser supports these extensions, as does the Spidermonkey JavaScript interpreter that Firefox is based on. Mozilla’s Java-based JavaScript interpreter, Rhino, (see Scripting Java with Rhino) also supports most of the extensions. Because these language extensions are nonstandard, however, they will not be useful to web developers who ...