Jeff Walden, one of the Peer members of the Mozilla JavaScript module (SpiderMonkey) has brought to light some changes coming to their JavaScript interpreter in Firefox 3.7. These change will bring SpiderMonkey in line with the next edition of the JavaScript standard, ECMA-262. These count not only break existing code, but also introduce a new opt-in mode of execution for JavaScript called strict mode.

Jeff mentions that not only does the new standard add to it many features that where merely extensions to JavaScript present the current browsers, but they don’t behave the same way unless strict mode is turned on. He also elaborates how turning on strict mode can complicate things for existing code as well. As the saying, “if you do, and if you don’t.”

Jeff describes changes in detail and examples of where they are relevant, but also states that this may be only a small minority of JavaScript code currently in use on the Web.  The truth is, we will only know when browsers start rolling out versions with these new changes.  You can read more details about it on Jeff’s blog posting.