Allow to specify return type of constructor

Search Terms

constructor return type

Suggestion

Add an ability to specify return type of a class constructor.
Right now the compiler produces the following error: "Type annotation cannot appear on a constructor declaration."

Use Cases

Consider this example:

interface MyInterface {
  readonly data: ReadonlyArray<string>

  add(item: string): void
  remove(item: string): void

  // ...more
}

class MyClass implements MyInterface {
  data = [] as string[]
  
  constructor(): MyInterface {} // right now it's an error

  add(item: string) {
    this.data.push(item)
  }

  // ...more
}

Current behavior:

const c = new MyClass() // typeof c == MyClass
c.data = [] // oops! no error from the compiler

Suggested behavior:

const c = new MyClass() // typeof c == MyInterface
c.data = [] // the compiler generates an error here, correct behavior

Of course, there are several ways to achieve this result (like using private constructor with factory function, or using a type converter like this:
type AsInterface<T, C> = C extends new (...args: infer A) => T ? new (...args: A) => T : never
But I think that the proposed feature is more concise

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. new expression-level syntax)

No implementation files or tests are named. Start by reviewing the constructor return-type examples and the current compiler error, then trace how new MyClass() is typed. Done means the proposed annotation is accepted, the constructed value is exposed as MyInterface, and invalid writes such as c.data = [] are rejected without changing emitted JavaScript.