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JS
JavaScript error

TypeError

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 WeakSet or as a value in a WeakMap.

Reference entryJavaScriptFetched
This is a reference entry from MDN Web Docs. A full explanation with fixes has not been written yet.

The error messages

TypeError: Invalid value used as weak map key (V8-based)
As listed by MDN Web Docs
TypeError: WeakMap key 1 must be an object or an unregistered symbol (Firefox)
As listed by MDN Web Docs
TypeError: WeakMap keys must be objects or non-registered symbols (Safari)
As listed by MDN Web Docs
TypeError: Invalid value used in weak set (V8-based)
As listed by MDN Web Docs
TypeError: WeakSet value 1 must be an object or an unregistered symbol (Firefox)
As listed by MDN Web Docs
TypeError: WeakSet values must be objects or non-registered symbols (Safari)
As listed by MDN Web Docs

Details

Category
TypeError
Severity
error
Runtime
both

Related errors

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Source: MDN Web Docs. Data fetched 2026-09-25T01:48:37.274Z.