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JavaScript function declared the same parameter twice

SyntaxError: duplicate formal argument x

JavaScript rejected a function definition because the same parameter name appeared more than once.

SyntaxError: duplicate formal argumentsyntax

Observed message

SyntaxError: duplicate formal argument x

Why it happens

  • A function parameter list repeats the same identifier.
  • A refactor renamed one parameter but not another.
  • Generated or copied code introduced a duplicate argument name.

How to fix it

  1. Rename duplicate parameters so each one is unique.
  2. Check generated or transpiled code if the source looks correct.
  3. Use linting to catch duplicate parameters before runtime.

Official reference

MDN: SyntaxError: duplicate formal argument x

Related errors

SyntaxError: missing ) after argument listJavaScript call or expression is missing a closing parenthesisSyntaxError: missing = in const declarationJavaScript const declaration is missing an initializerSyntaxError: Unexpected tokenJavaScript parser hit an unexpected tokenSyntaxError: return not in functionJavaScript return statement is outside a function

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JavaScript duplicate formal argument: causes and fixes | Error Atlas