Add ES6 Support

[Constellation]

Supporting ES6 block scopes and some special scopes (such as arraw function and method)

[nzakas]

Are you still going to do this? ESLint has a bug related to let scoping that could use this: https://github.com/eslint/eslint/issues/917

[Constellation]

Oh, OK. I'll work on it on weekend :)

[Constellation]

semantics in ES6 spec may be mature enough :)

[nzakas]

They are. I've documented it in my book: https://github.com/nzakas/understandinges6/blob/master/manuscript/01-The-Basics.md#block-bindings

[jlongster]

This would be awesome to have!

[nzakas]

Just checking in - any chance you're working on this? :)

[Constellation]

Hi @nzakas, Sorry for my further late reply.

I'm now gradually supporting ES6 in the tools, now escodegen supports ES6 Syntax completely. So, next, I'm planning to support ES6 in estraverse since it is used in the escope. Then, support scoping in escope.

Since there's significant difference between ES5 scopes and ES6 scopes, maybe I'll introduce ecmaVersion option.

[nzakas]

That sounds awesome!

[elwayman02]

@Constellation what's the status on this? Is there anything we can do to help out? I'm working on trying to get an ES6-supported version of ESlint using the harmony branch of esprima, so my next step is to approach ES6 support in escope and estraverse. :)

[Constellation]

OK, so, let's start to support ES6. I'll start it with estraverse.

[Constellation]

Support ES6 traversing in estraverse. https://github.com/Constellation/estraverse/commit/19f7a5c5dbb7bcaca3226d3cd608e59dbea1a275

[StoneCypher]

This would really help for getting eslint running on jsx

[sebmck]

Any update on this? Definently keen to use this for 6to5. Reference 6to5/6to5#175

[Constellation]

Currently, we're investigating the ES6 spec and Esprima AST nodes. And We're now updating escodegen and estraverse. Does anyone knows the issue about them? I think estraverse now completely support ES6 syntax. If it's correct, my mental model of ES6 AST may be stable.

[sebmck]

Main issues you'll run into is implementing temporal dead zones. Default parameters have their own scope and you can't access let/const variables before they're initialised.

[Constellation]

Main issues you'll run into is implementing temporal dead zones. Default parameters have their own scope and you can't access let/const variables before they're initialised.

Nice, it's very useful information. And we need to consider destructuring assigned variables.

var { a, b, c } = obj;  // it materialize a, b, c variables.

[Constellation]

And we need to ensure,

for (let i = 0; i < 10; ++i) { console.log(i); }

scope behavior.

[Constellation]

We're now implementing scopes for ES6. But be careful, it's experimental and the behavior might be changed

[Constellation]

@sebmck:

Do you know the actual section in the ES6 draft (rev28)? I'm now investigating it.

[sebmck]

@Constellation For what specifically?

[Constellation]

@sebmck I think sticking to the formal spec is very important ;) It provides a clear reason to "why it is so implemented". And makes the tool stable.

[sebmck]

@Constellation Yeah I understand that. I was referring to what section specifically that you were looking for.

[Constellation]

@sebmck Ah, I've misunderstood.

Default parameters have their own scope.

I'm looking for it. Now seeing 9.2.13, http://people.mozilla.org/~jorendorff/es6-draft.html#sec-functiondeclarationinstantiation

[Constellation]

Added class declaration / expression scope support.

[Constellation]

Remaining TODO is,

• Destructuring assignments
  • [x] VariableDeclaration
  • [x] Assignment
  • [x] Parameters
• Initializers
  • [ ] Default parameters
  • [ ] Initializers in Destructuring Assignments
• [x] MethodDefinition's strictness
• [x] import declaration
• [x] export declaration
• [x] For/ForIn/ForOf special block scope
• [x] Class
• [x] TemplateLiteral

Please feel free to modify this list. Now, 4:00 JST, so I'll sleep today...

[nzakas]

There's also the import statement.

[Constellation]

@nzakas Nice. I've added it to TODO list.

[Constellation]

Destructuring assignments support is partially done. Initializers / Default parameters are not supported yet. Maybe need to change Esprima's AST architecture.

[Constellation]

Seeing the ClassDefinitionEvaluation1, we attempt to create class body scope only when the className is provided. However, in this case, computed property names are evaluated in the upper scope, so sometimes it is not strict scope. But spec says, ClassBody is strict code. I think it may be a spec bug. In this time, I always create the scope for strictness of scope. And I'll open the issue to TC39 bugzilla.

edit: http://people.mozilla.org/~jorendorff/es6-draft.html#sec-strict-mode-code I'll investigate more.

[Constellation]

@ariya @michaelficarra Found Esprima's bug,

esprima.parse('try{}catch({a,b,c}){}').body[0].handlers[0].param

Should return ObjectPattern, but returns ObjectExpression.

[Constellation]

ForIn/ForOf with let / const's runtime evaluation spec http://people.mozilla.org/~jorendorff/es6-draft.html#sec-runtime-semantics-forin-div-ofbodyevaluation

[Constellation]

ES6 ForOf/ForIn scopes are supported :)

edit:

Oops, TDZ scope support is not landed. I'll reconstruct the logic.

edit: Done.

[Constellation]

Introduced ForStatement iteration scope for LetOrConstDeclaration.

[Constellation]

I'll add TDZ options. I think creating TDZ scope sometimes bothering users. Making it switchable with options is preferrable.

[nzakas]

One piece of complexity I just came across: variables can be referenced from within template strings. Just a special case to keep in mind. :)

[Constellation]

@nzakas Nice! Thank you for your note. I would appreciate if you would inform me the edge cases :) Yay! escope's ES6 template literal supports it :) https://github.com/estools/escope/blob/master/test/es6-template-literal.coffee#L28

[Constellation]

Some complicated scoping semantics requires fine-grained traversing order control of the child nodes. So I've created the esrecurse https://github.com/estools/esrecurse, it is recursive style visitor. I'll switch to it in escope.

[Constellation]

Switching traversing function from estraverse to esrecurse is done. Since escope needs fine-grained traversing order control, esrecurse is more suitable for escope. Current escope tests are passed, and integration tests with esmangle, which heavily uses escope, are also passed :)

[nzakas]

Nice!

[Constellation]

Implemented ImportDeclaration supports.

[SimonDegraeve]

Awesome, any idea how I can use the new escope with eslint? I am stuck with an error in escope.Scope.isScopeRequired.

[Constellation]

@SimonDegraeve,

new escope is drastically changed, so Scope.isScopeRequired API is now dropped. So please modify the caller not to use this.

This is because, now, escope cannot determine whether the node requires scope easily. For example, in ecmaVersion: 6 mode, BlockStatement requires scope, but not in ecmaVersion: 5. However, some BlockStatement in the AST tree doesn't require scope. In the current Mozilla AST, FunctionExpression, FunctionDeclaration has BlockStatement as it's body. But in this case, this BlockStatement doesn't require scope since the function scope is created for the parent node FunctionExpression.

[Constellation]

ExportDeclaration is now completely supported.

[Constellation]

@SimonDegraeve,

Seeing the code, I think it can be rewritten with https://github.com/estools/escope/blob/master/escope.js#L832 and https://github.com/estools/escope/blob/master/escope.js#L814 :)

[SimonDegraeve]

Ok I will have a look. Thank you.

[Constellation]

@SimonDegraeve

Great! If you find issues, please inform me. escope ES6 support is now heavily under the development. So feedbacks are welcome :)

[Constellation]

@SimonDegraeve

I'll extend acquire by using WeakMap. It will provide more good functionality for you. Please wait for a moment...

[Constellation]

@SimonDegraeve

I've added ScopeManager#acquireAll. It acquires all scopes bound to the specified node. And now, escope can use ScopeManager#acquire and ScopeManager#release without ScopeManager#attach and ScopeManager#detach.

[SimonDegraeve]

Si I had a look, and even if I managed to run it without Error. The scope are totally wrong. (Lack of knowledge how tree parsing works, I guess).... maybe I will try another attempt when I have free time. Thanks

[Constellation]

@nzakas Except for default parameters and destructuring assignment initializers, ES6 scopes are basically supported. (of cource, still it is experimental phase.) And these features require some updates for esprima, (or espree).

In the meantime, I've released it as 2.0.0. It shipps major changes. So to stabilize it, feedbacks are welcome. And futher release may include the drastic changes. (Maybe, TDZ scopes should be treated more nicely, and parameter scopes are needed when default parameters are supported.)