The JavaScript exception "is not a non-null object" occurs when an object is expected
somewhere and wasn't provided. null is not an object and won't work.
Message
TypeError: Property description must be an object: x (V8-based)
TypeError: Property descriptor must be an object, got "x" (Firefox)
TypeError: Property description must be an object. (Safari)
TypeError: Invalid value used in weak set (V8-based)
TypeError: WeakSet value must be an object, got "x" (Firefox)
TypeError: Attempted to add a non-object value to a WeakSet (Safari)
Error type
What went wrong?
An object is expected somewhere and wasn't provided. null is not an
object and won't work. You must provide a proper object in the given situation.
Examples
Property descriptor expected
When methods like Object.create or Object.defineProperty and Object.defineProperties are used, the optional descriptor parameter expects a property descriptor object. Providing no object (like just a number), will throw an error:
Object.defineProperty({}, "key", 1);
// TypeError: 1 is not a non-null object
Object.defineProperty({}, "key", null);
// TypeError: null is not a non-null object
A valid property descriptor object might look like this:
Object.defineProperty({}, "key", { value: "foo", writable: false });
WeakMap and WeakSet objects require object or symbol keys
WeakMap and WeakSet objects store object or symbol keys. You can't use other types as keys.
const ws = new WeakSet();
ws.add("foo");
// TypeError: "foo" is not a non-null object
Use objects instead:
ws.add({ foo: "bar" });
ws.add(window);