Most programming languages have keywords or reserved words: names which can’t be used to name variables. Keywords have special meaning, so using them as variable names would conflict with such use. Reserved words are keywords of the future: names which might eventually be given special meaning, so they can’t be used now to ease future adoption.

JavaScript and the ECMAScript standard that underlie it historically have had an excessively large set of keywords and reserved words “inherited” from Java. ES5 partially loosened ES3‘s past keyword restrictions. For example, byte, char, and int were reserved in ES3 but aren’t in ES5.

Many years ago, before work started on ECMAScript after ES3, a few browsers stopped reserving all of ES3’s reserved words. In response browsers generally started to un-reserve many of these names. As it turned out this un-reservation went too far: ES5 un-reserved many of these words, but it didn’t un-reserve all of them. In particular, while some implementations un-reserved the names class, enum, export, extends, import, and super, ES5 did not.

Firefox un-reserved these names then along with some other browsers. But as ES5 corrects the over-reservation of ES3 without un-reserving these names, we are moving to align with ES5 by re-reserving class, enum, export, extends, import, and super in all code. (Firefox 4 reserves these names only in strict mode code.)

You can experiment with a version of Firefox with these changes by downloading a TraceMonkey nightly build. Trunk’s still locked down for Firefox 4, so it hasn’t picked up these changes just yet. (Don’t forget to use the profile manager if you want to keep the settings you use with your primary Firefox installation pristine.)