So why would you really care much about Javascript 1.7 anyway? Simple – because it’s a fully supported part of the upcoming Firefox 2.0 browser.

JavaScript 1.7 is a language update introducing several new features, in particular generators, iterators, array comprehensions, let expressions, and destructuring assignment. It also includes all the features of JavaScript 1.6.
JavaScript 1.7 support is available starting in Firefox 2 Beta 1, as well as in current Minefield (trunk) builds.
The code samples included in this article can be experimented with in the JavaScript shell. Read Introduction to the JavaScript shell to learn how to build and use the shell. Source: Mozilla Dev Center