Milestone: Decorator Support

[JamesHenry]

This issue it to track progress against supporting decorators within the final AST that the parser produces. We currently remove them during parsing.

We need to ensure that the following are accurately carried over from the original TypeScript AST:

• Parameter Decorators
  • On instance member
  • On static member
  • In constructor
• Method Decorators
  • On instance member
  • On static member
• Accessor Decorators
  • On instance member
  • On static member
• Property Decorators
  • On instance member
  • On static member
• Class Decorators

We also need to account for the difference between decorators and decorator factories:

Decorator:

Decorator Factory:

At the AST-level, the decorator node has an Identifier TSNode as an expression, whereas the decorator factory has a CallExpression TSNode.

Decorators are not currently covered by the ESTree spec as they are still a fairly early-stage es-proposal, but Angular 2, for example, has adopted them as a core aspect of the framework.

TypeScript adds a decorators array to the node in question, and I propose we do the same.

Resources:
https://www.typescriptlang.org/docs/handbook/decorators.html

[nzakas]

TypeScript adds a decorators array to the node in question, and I propose we do the same.

Agreed.

It's probably worth looking at the AST that Babel produces for decorators, which I'm guessing is similar.

[JamesHenry]

Good thinking!

astexplorer shows that babel-eslint produces near identical output, with both the decorator and decorator factory nodes (within a decorators array on the MethodDefinition) having a type of Decorator and an expression property.

As described above, the expression is an Identifier for the decorator, and a CallExpression for the decorator factory.

Here is a PR #64 for a Parameter Decorator and a Parameter Decorator Factory on an instance method to get the ball rolling and confirm the format with you.

Feel free to leave it open until more cases are added, but I need to break off now as it's quite late here.

[nzakas]

Makes sense. Bonus points for the animated demo. :)

[nzakas]

It looks like Flow does support decorators, and it's with the decorators array with Identifier or CallExpression as items, so probably best to mimic that.

[JamesHenry]

Ok, cool thanks. Do you have a reference for that you can link to?

I will update that first PR now

[nzakas]

I just used astexplorer.net.

[JamesHenry]

Method decorator support (static and instance methods) now complete in #64

[JamesHenry]

Class decorator support now also complete. I branched from the method decorator PR to avoid conflicts. PR is here #67

[JamesHenry]

It appears that flow does not currently support Parameter Decorators.

I think it makes sense to just follow the same approach as with the other decorator types - a decorators array on the parameter identifier node directly

[nzakas]

Agreed.

[JamesHenry]

Cool, thanks. In which case PR for parameter decorators is ready #69

[JamesHenry]

Property decorators up next! #72

[nzakas]

You are rocking!

[JamesHenry]

🎸

[nzakas]

@JamesHenry you are awesome. Thanks for all your work on this.

[JamesHenry]

Thanks so much @nzakas! Your patience and guidance has been invaluable throughout this process