SUGGESTION: add support for writeonly properties on interfaces

I'd like to resurrect an old discussion around the desire to have getters/setters support for interfaces: #11878

Obviously we can use readonly to express a property on an interface that has just a getter but there's no way of expressing that a property should have just a setter. In issue #11878 there was an idea proposed to add the concept of a writeonly property designed to cover this need but the consensus was that there wasn't enough real-world scenarios to justify such a feature. So let me try and add one.

We have a situation where we have a child object that we want to publish data to a parent but while we want the parent to know about its child we don't want the child to know about its parent. We've ruled out the use of events because we only want a single subscriber and we need to return a Promise to the child to let them know when we're done. Instead we've opted to establish what we would have called a "weak reference" between the child and parent back in the days of COM. The interface in TypeScript looks something like this:

interface Adapter {
     onDataReceived: (data: any) => Promise<void>;
     publishData(data: any): Promise<void>;
}

As you can see data flows bidirectionally between the parent and child and while we've received a couple of questions about why the interface is the way it is, it's generally easy enough to grok from a TypeScript perspective.

The issue we just ran into, however, is that a developer on our team just created a class in ES6 that implements this interface and the result ended up being.... yuck :(

If we literally implement this interface in a declarative way in ES6 it looks something like:

export class WebAdapter {
     get onDataReceived() {
          return this.callback;
     }
     set onDataReceived(cb) {
          this.callback = cb;
     }
     postData(data) {
     }
}

Not only is it crappy that you have to define a getter and a setter, the fact of the matter is we're never going to ask for the callback back so the getter is pointless here. So what did our dev do? He did this:

export class WebAdapter {
     onDataReceived(data) {
          // will be replaced by parent
     }
     postData(data) {
     }
}

That technically works and what's nice is you have some sense of the signature of the handler but it makes my skin crawl to look at it. If I was to mirror that in my TypeScript interface you'd have zero clue that onDataReceived() was something I expect you to override. What I really want the developer to have to write implementation wise is this:

export class WebAdapter {
     set onDataReceived(cb) {
          this.callback = cb;
     }
     postData(data) {
     }
}

That's the proper contract for a weak reference but I have no way of expressing it in TypeScript. While it's very rare that you need to do this it doesn't make it any less valid a scenario. The addition of "writeonly" properties would give me a way to express this.

Start by reading the linked discussion in issue #11878 and the interface and ES6 examples in this issue. Define what syntax and assignability rules a write-only property requires, then verify that an interface can express the setter-only contract and that the shown JavaScript implementation satisfies it.