TypeError: WeakSet key/WeakMap value 'x' must be an object or an unregistered symbol

The JavaScript exception "WeakSet key (or WeakMap value) 'x' must be an object or an unregistered symbol" occurs when a value of invalid type is used as a key in a {{jsxref("WeakSet")}} or as a value in a {{jsxref("WeakMap")}}.

Message#

TypeError: Invalid value used as weak map key (V8-based)
TypeError: WeakMap key 1 must be an object or an unregistered symbol (Firefox)
TypeError: WeakMap keys must be objects or non-registered symbols (Safari)

TypeError: Invalid value used in weak set (V8-based)
TypeError: WeakSet value 1 must be an object or an unregistered symbol (Firefox)
TypeError: WeakSet values must be objects or non-registered symbols (Safari)

Error type#

{{jsxref("TypeError")}}

What went wrong?#

{{jsxref("WeakSet")}} and {{jsxref("WeakMap")}} require the keys to be garbage collectable. Only objects and non-registered symbols (that is, symbols not returned by {{jsxref("Symbol.for()")}}) are valid. For more information, see Memory management. If you want to add keys that are strings, numbers, or other primitive values, you should store them in a regular Set or Map instead.

Examples#

Invalid cases#

```js example-bad new WeakSet().add(1); // TypeError new WeakMap().set(1, {}); // TypeError new WeakSet([1]); // TypeError new WeakMap([[1, {}]]); // TypeError

### Valid cases

```js example-good
new WeakSet().add({}); // OK
new WeakMap().set({}, 1); // OK

new Set([1]); // OK
new Map([[1, {}]]); // OK

See also#

• Memory management
• {{jsxref("WeakSet")}}
• {{jsxref("WeakMap")}}
• {{jsxref("Set")}}
• {{jsxref("Map")}}