I just tried in Firebug console,
let (X=10) X/2
and
[x,y]=[y,x]
These are features supported by SpiderMonkey, I guess V8 has its own share.
Where can I learn of features that are not yet included in ECMAScript, but work in various browsers? Is there a place where these are collected together?
ECMAScript 6 (a.k.a. ECMAScript 2015) is the current standard for JavaScript, but engines have yet to implement it completely:
Starting with ECMAScript 2016, there will be yearly releases and a new release process:
Blog post explaining the new release process and the features that are candidates for ES2016: http://www.2ality.com/2015/11/tc39-process.html
Official list of proposed features (that may or may not be accepted for the ECMAScript standard): https://github.com/tc39/ecma262
Feature table for ES2016: http://kangax.github.io/compat-table/es7/
If you want to use any of the new features even on older engines, you can transpile them to ES5 via Babel: https://babeljs.io/
And here is a article covering various resources around Harmony/ES6/Javascript.next:
http://addyosmani.com/blog/ecmascript-6-resources-for-the-curious-javascripter/
That first feature is known as a "let expression" and it is nonstandard; it was dropped from Firefox 41, and the similarly nonstandard "let block" was dropped from Firefox 44: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Statements/let#Non-standard_let_extensions
I was surprised to find that this particular non-standard JS was not mentioned in Kangax's table, but I guess he had to restrict this list to non-standard JS extensions that are supported by multiple engines: https://kangax.github.io/compat-table/non-standard/
If you want to go deeper down the rabbit-hole, and Kangax and MDN haven't satisfied your curiosity, this old reference may tell you about curiosities in older browsers: help.dottoro.com/ljsdaoxj.php
Beyond that, the browser-makers usually document the quirks of their own browsers (MDN is also good about documenting non-Mozilla quirks, but it's not perfect); speaking of quirks, Peter-Paul Koch documents both standard and non-standard DOM methods here: quirksmode.org/dom/
Anyway, these aren't just "not yet" in the standards, but likely "not ever" and you shouldn't use them in your own code.
For current ECMAScript 264 implementation here is a list of features supported by different browser vendors: http://kangax.github.com/es5-compat-table/
For the next generation ECMAScript Harmony some resources:
http://addyosmani.com/blog/ecmascript-6-resources-for-the-curious-javascripter/
