support ES6 features

[clausreinke]

Several valuable ES6 features appear to be stable (arrow functions, classes (modulo private members), block scoping) and this month's (May 2013) tc39 meeting seems to be the deadline for a wiki-complete module system spec. Transpilers and engines are already prototyping support, and it is important that tern is not left behind.
If I recall correctly, the esprima and spidermonkey folks have started to agree on the syntax/AST extensions, but tern also needs to take the scoping, typing, and semantics into account (eg desugaring class syntax to ES5 features while keeping ES6 locations).

[marijnh]

Yes, this is a project that's on the horizon. It might be huge, and will be tricky as long as the features are non-final and poorly documented. I'm not going to work on it yet, but if you want to, the first thing would be to add an ES6 mode to Acorn that parses at least the stable parts of the new syntax.

[RReverser]

Which is to be closed by acornjs/acorn#110.

[marijnh]

@RReverser note that this is the Tern repository, and your PR adds ES6 support to Acorn. There's going to be a bunch of additional work to be done to teach Tern to understand the new constructs.

[RReverser]

Sure, my reply was rather for

the first thing would be to add an ES6 mode to Acorn that parses at least the stable parts of the new syntax.

[bvalosek]

What would be the next steps for getting ES6 support in Tern now that acorn is parsing the es6 features?

I'd like to contribute in some way if possible, or even put a bounty for this feature.

[RReverser]

There is still need to fix template literals in loose parser which is somewhat more tricky, but in general parser-side is ready.

[tomitrescak]

Great news and a great job! How can we get it to work? I am using Sublime 3 with TernJS plugin which for now somehow marginally works with ES6 while the tern_for_sublime crashes. Thanks!

[marijnh]

@tomitrescak Tern does not work with ES6 yet, only Acorn (the parser) does.

How does tern_for_sublime crash (i.e. what code, what error message)? It shouldn't -- theoretically it should just ignore the things it can't understand.

[mk0x9]

Is there any roadmap to support ES6 features?

I'm especially interested in classes and module imports.

[angelozerr]

Is there any roadmap to support ES6 features?

Tern starts supporting ES6 with ecma6.json https://github.com/marijnh/tern/blob/master/defs/ecma6.json

@marijnh has managed Promise, but ecma6.json must be improved again like Iterator support.

But I think Acorn should be customized to support ES6 inside tern.

[RReverser]

@angelozerr

But I think Acorn should be customized to support ES6 inside tern.

I don't really understand why it should be customized to support ES6 differently inside Tern? How is that different from already implemented ES6 support?

[angelozerr]

@RReverser I don't know acorn, but with acorn you can set ecmaVersion http://marijnhaverbeke.nl/acorn/#section-4 although inside tern it is never done.

I suppose setting ecmaVersion gives features according the version (perhaps acorn is able to support iterator protocal?

But perhaps I'm wrong?

[marijnh]

Yes, we are currently not enabling ES6 parsing when calling acorn.parse because Tern does not understand the ES6 node types. The way to start supporting this would be to extend the AST walkers in infer.js so that they properly handle the new syntactic constructs.

I will to work on this eventually, but I'll need time and funding. The second can probably be addressed with another crowd-funder, which I intend to start at some point when I am less busy.

[RReverser]

@marijnh I was also just thinking about getting to work on this as soon as finish better loose support in JSX, as I've got some free time, but same problem with 2nd part of your statement :)