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JS
JavaScript error

SyntaxError

The JavaScript exception "redeclaration of formal parameter" occurs when the same variable name occurs as a function parameter and is then redeclared using a let assignment in a function body again.

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

The error messages

SyntaxError: Identifier "x" has already been declared (V8-based)
As listed by MDN Web Docs
SyntaxError: redeclaration of formal parameter "x" (Firefox)
As listed by MDN Web Docs
SyntaxError: Cannot declare a let variable twice: 'x'. (Safari)
As listed by MDN Web Docs

Details

Category
SyntaxError
Severity
error
Runtime
both

Related errors

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