Implement the updated JS decorators proposal

[arackaf]

Suggestion

Implement Decorators!

🔍 Search Terms

TypeScript Decorators

List of keywords you searched for before creating this issue. Write them down here so that others can find this suggestion more easily and help provide feedback.

Decorators

✅ Viability Checklist

My suggestion meets these guidelines:

• This wouldn't be a breaking change in existing TypeScript/JavaScript code
• This wouldn't change the runtime behavior of existing JavaScript code
• This could be implemented without emitting different JS based on the types of the expressions
• This isn't a runtime feature (e.g. library functionality, non-ECMAScript syntax with JavaScript output, new syntax sugar for JS, etc.)
• This feature would agree with the rest of TypeScript's Design Goals.

Not sure - this would be badly breaking against your legacy decorators feature, but I think it meets the goal to align TS with JS

⭐ Suggestion

The TypeScript proposal is now Stage 3!!!

https://github.com/tc39/proposal-decorators

We've all seen the issue from hell requesting better typing support for decorators

#4881

but now that the proposal is moving forward, currently at Stage 3, I thought I'd open this issue to see if there are plans to finally, fully implement (and type) the ES version of decorators.

📃 Motivating Example

N/A - just implementing new JS features, to keep JS and TS aligned.

💻 Use Cases

All the various ways decorators can be used.

[DanielRosenwasser]

I think the first step will be implementing decorators. We're not going to try to model the "decorators can change the types of things" portion at first, but I think we can get something good and working.

[arackaf]

@DanielRosenwasser that sounds perfect.

But just to be clear, annotating how a decorator change’s a type will be on the roadmap long term?

[glen-84]

Shhh, don't say "long term". 😉

[wycats]

@DanielRosenwasser the biggest "type-related" transformation that comes up for me in the current implementation of decorators is communicating that a field decorator initializes the field's value.

I think that this would just fall out of accessor decorators (since the decorator turns into a getter, the concept of an uninitialized value no longer makes sense). Is that right?

Also, a regular field decorator might still want to communicate that the value is initialized (for example, a decorator that implements a default by returning a new initializer). Is it possible to address the issue of communicating that a decorator initializes the value on a different timeframe than arbitrary type transformation, or is it just as hard?

[Jamesernator]

We're not going to try to model the "decorators can change the types of things" portion at first, but I think we can get something good and working.

Simple transform decorators should be easy enough to support right?

Like:

function wrapInBox<R>(f: () => R): () => { value: R } {
    return () => { value: f() };
}

class Foo {
    @wrapInBox
    method(): number {
        return 3;
    }
}

const foo = new Foo();
foo.method(); // should be { value: number }, same as regular function wrapping

Has very little difference to (type inference wise):

class Foo {
    method = wrapInBox((): number => {
        return 3;
    });
}

(Ignoring the prototype vs own property distinction here).

Isn't it primarily the metadata-based type-mutations that are hard to actually support? (given they need to communicate types across calls)