New operator should return object returned by constructor

Search Terms

new, operator, constructor, return, object

Suggestion

in javascript, calling with the operator new a function that returns an object does not resolves to the created instance but to the object returned by the function

Typescript should do the same.

That could offer a better compliance with the ECMAScript standard.

Use Cases

This can for example allow to create async constructor (see examples)

Examples

function foo(returnDate = false) {
  this.bar = 1;
  if(returnDate) return new Date()
}
const obj1 = new foo() // ES resolves to a foo instance ; typescript get error "ts(2350)"
const obj2 = new foo(true) // ES resolves to a Date     ; typescript get error "ts(2350)"
obj1 instanceof foo // true
obj2 instanceof foo // false

// obj1 and obj2 type should be foo | Date

Use case example: async constructor:

class Foo {
  constructor() {
    return Promise.resolve().then(() => this);
  }
}

const foo1 = await new Foo();
const foo2 = new Foo(); // foo2 is Promise<Foo> ; ts consider foo2 as Foo

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, etc.)
• This feature would agree with the rest of TypeScript's Design Goals.

Beginne mit der Überprüfung des TypeScript-Typprüfpfads für den new-Operator und die Rückgabetypen von Konstruktoren; das Issue nennt keine Datei, keinen Test und keinen Einstiegspunkt. Verwende die JavaScript-Beispiele als Verhaltensreferenz, einschließlich Konstruktoren, die ein Date oder Promise zurückgeben, und stelle sicher, dass die inferierten Typen mit dem Laufzeitergebnis übereinstimmen, ohne das erzeugte JavaScript zu ändern.