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#

{{jsxref("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 example-bad class Base { constructor() {} }

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

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

### Valid cases

```js example-good
class Base {
  constructor() {}
}

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

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

See also#

• Classes
• {{jsxref("Classes/extends", "extends")}}
• {{jsxref("new")}}