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JS
JavaScript error

TypeError

The JavaScript exception "_x_ is (not) _y_" occurs when there was an unexpected type. Oftentimes, unexpected undefined or null values.

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: Cannot read properties of undefined (reading 'x') (V8-based)
As listed by MDN Web Docs
TypeError: "x" is undefined (Firefox)
As listed by MDN Web Docs
TypeError: "undefined" is not an object (Firefox)
As listed by MDN Web Docs
TypeError: undefined is not an object (evaluating 'obj.x') (Safari)
As listed by MDN Web Docs
TypeError: "x" is not a symbol (V8-based & Firefox)
As listed by MDN Web Docs
TypeError: Symbol.keyFor requires that the first argument be a symbol (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.