Typescript: TS2409 is an invalid error.

[michaelhartmayer]

TypeScript Version: 3.3.0-dev.20181222

Search Terms: TS2409

Code

class A {
  constructor() {
    // return an arrow function
    return message => {
      this._message = message;

      return this;
    };
  }

  bar() {
    console.log(this._message);
  }
}

const a = new A();
a('hello world').bar(); // 'hello world'

Expected behavior:
No error.

Actual behavior:
src/test.ts(4,5): error TS2409: Return type of constructor signature must be assignable to the instance type of the class.

I should be able to override the object returned from a constructor.

Normally constructors don't return a value, but they can choose to do so if they want to override the normal object creation process.

Souce: MDN - new operator

[weswigham]

Functions can, classes cannot:

[weswigham]

(This may "work" downlevel where classes are compiled to functions, however with real classes it does not)

[michaelhartmayer]

Functions can, classes cannot:

@weswigham I believe you're mistaken. Class constructors cannot return _other primitives_. They can however return Objects.

[weswigham]

Mmm. I suppose we just make the distinction to make a class easier to typecheck.

[AhmedBHameed]

I have the same issue. Is there a solution around? It seems that Typescript will prevent any class chaining when you want to chain your methods which is bad IMO!!

[towozy]

@weswigham
Hi. I'm wondering about this issue's result.
I know this is (probably) not an error in ESLint. However, it is an error in typescript. So I can't choose which is better. What do you think about this issue now in 2020?

[willemmulder]

Real life example where returning something other that this is actually very helpful is when we want to make the constructor of a class T async. Due to the nature of how async/await is implemented, the constructor would need to return a Promise<T> instead of T.

class T {
    constructor() {
        let that = this;
        return (async () => {
            // ... All async code here
            return that; 
        })();
    }
}

However Typescript complains that 'Return type of constructor signature must be assignable to the instance type of the class.' while I believe it should not indicate the above method of returning a Promise<T> as erroneous.