JavaScript error“SyntaxError: Identifier 'x' has already been declared”

Why the name is taken, and the fix

1. 01

   Two declarations in the same scope

   “let declarations cannot be in the same scope as any other declaration, including let, const, class, function, var, and import declaration.” MDN, let: Redeclarations

   Often from pasting code, or an AI edit that added a block without removing the old one. Keep one declaration and assign to it.

   // wrong
   let total = 0;
   let total = items.length;
   
   // right
   let total = 0;
   total = items.length;

2. 02

   A local name that clashes with an import or a parameter

   “A let declaration within a function's body cannot have the same name as a parameter.” MDN, let: Redeclarations

   import { user } from './store' and then const user = ... in the same module, or a function parameter redeclared with let. Rename the local value, or import under another name.

   import { user as storeUser } from './store';
   const user = storeUser ?? guest;

3. 03

   The same name in two switch cases

   “To avoid the error, wrap each case in a new block statement.” MDN, let: Redeclarations

   All cases of a switch share one block, so let message in two cases is a redeclaration.

   switch (status) {
     case 'ok': {
       const message = 'Saved';
       break;
     }
     case 'error': {
       const message = 'Try again';
       break;
     }
   }

4. 04

   A script loaded twice, or two scripts using one name

   “if you declare a let or const variable in the first script, declaring it again in the second script will cause a redeclaration error.” MDN, let: Temporal dead zone

   Classic script tags share one top-level scope. If a page includes the same file twice, or two files both declare const API_URL at the top level, the second one throws. Load the file once, or make the scripts modules (type="module"), which each get their own scope. Modules also stop top-level names becoming globals, so check that nothing reads them from window.

   <script type="module" src="/js/app.js"></script>

Checked against the vendor’s docs on November 8, 2026: MDN: let.

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Related: JavaScript “x is not defined” · “Unexpected identifier” · “Invalid or unexpected token”