TypeError: derived class constructor returned invalid value x

The JavaScript exception "derived class constructor returned invalid value x" occurs when a derived class constructor returns a value that is not an object or undefined.

Message

TypeError: Derived constructors may only return object or undefined (V8-based)
TypeError: derived class constructor returned invalid value 1 (Firefox)
TypeError: Cannot return a non-object type in the constructor of a derived class. (Safari)

Error type

TypeError

What went wrong?

Typically, a constructor does not need to return anything—the value of this is automatically returned when the class is constructed. A constructor can also return an object, and this object will override this as the newly constructed instance. However, returning something that's neither an object nor undefined is usually a mistake, because that value is ignored. In base classes and function constructors (using the function syntax), returning such a value is silently ignored, while in derived classes, it throws an error.

Examples

Invalid cases

js
class Base {
  constructor() {}
}

class Derived extends Base {
  constructor() {
    return 2;
  }
}

new Derived(); // TypeError: derived class constructor returned invalid value 2

Valid cases

js
class Base {
  constructor() {}
}

class Derived extends Base {
  constructor() {
    return { x: 1 };
  }
}

new Derived(); // { x: 1 }

See also