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 orundefined.
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
What went wrong?
Typically, a constructor does not need to return anything—the value ofthis is automatically returned when the class is constructed. A constructor can also return an object, and this object will overridethis as the newly constructed instance. However, returning something that's neither an object norundefined is usually a mistake, because that value is ignored. In base classes and function constructors (using thefunction 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 2Valid cases
js
class Base { constructor() {}}class Derived extends Base { constructor() { return { x: 1 }; }}new Derived(); // { x: 1 }