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.

Commencez par examiner le chemin de vérification des types de TypeScript pour l’opérateur new et les types de retour des constructeurs ; l’issue ne nomme aucun fichier, test ou point d’entrée. Utilisez les exemples JavaScript comme référence comportementale, notamment les constructeurs qui renvoient un Date ou une Promise, et assurez-vous que les types inférés correspondent au résultat à l’exécution sans modifier le JavaScript généré.