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JS
JavaScript error

ReferenceError

The JavaScript strict mode-only exception "Assignment to undeclared variable" occurs when the value has been assigned to an undeclared variable.

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

The error messages

ReferenceError: x is not defined (V8-based)
As listed by MDN Web Docs
ReferenceError: assignment to undeclared variable x (Firefox)
As listed by MDN Web Docs
ReferenceError: Can't find variable: x (Safari)
As listed by MDN Web Docs

Details

Category
ReferenceError in strict mode only
Severity
error
Runtime
both

Related errors

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