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Let, const, class or import name cannot be redeclared within the same lexical scope. Also, loading the same file twice, re-adding the script via AJAX, or redefining a global variable in a header/footer combination can trigger this error.
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SyntaxError: Identifier 'config' has already been declared
Identifier 'data' has already been declared
Cannot redeclare block-scoped variableLet, const, class or import name cannot be redeclared within the same lexical scope. Also, loading the same file twice, re-adding the script via AJAX, or redefining a global variable in a header/footer combination can trigger this error.
A SyntaxError may stop all subsequent scripts; First resolve the first red record in the Console.
Determine whether the error is source or build with Source map, pretty print and Network URL.
Reduce the issue to a small area with node --check, ESLint, TypeScript or JSON validator.
Check the PHP render, API body, CDN cache and module loader result in the production answer.
Don't convert let to var for global variable resolution blindly.
Meaning: The same name was redeclared within the same scope.
Possible cause: Two let/const/class/import.
Meaning: Block-scope variable redeclared.
Possible cause: TypeScript global script or duplicate file.
Meaning: The same global code ran twice.
Possible cause: Header/footer or bundle pair.
Meaning: Inline script in HTML part is added every time it opens.
Possible cause: Dynamic content has global let/const.
Meaning: The imported name is the same as the local variable.
Possible cause: Name collision.
Meaning: Parameter was redeclared with let in the body.
Possible cause: Scope rule violation.
Meaning: Can be reported again but conflicts with let/const.
Possible cause: Mixed declaration type.
Meaning: Module files do not pollute the global scope.
Possible cause: Classic script structure.
No records matching this expression were found.
grep -RnoE '\b(let|const|class|function)\s+[A-Za-z_$][A-Za-z0-9_$]*' src | sort | uniq -c | sort -nr | head -n 80Helps to scan notifications by name and file basis.
curl -sL https://example.com | grep -ioE '<script[^>]+src=[^>]*>'Lists external scripts loaded on the page.
curl -sL https://example.com | grep -ioE '<script[^>]+src="[^"]+' | sort | uniq -c | sort -nrIndicates that the same script source is being reloaded.
npx eslint src --rule 'no-redeclare:error'The linter catches duplicate notifications.
npx tsc --noEmitIt reports global and module scope conflicts.
Array.from(document.scripts).map(s => s.src || '[inline]').reduce((a,x)=>(a[x]=(a[x]||0)+1,a),{})Displays the number of script sources loaded in the DevTools Console.
const durum = true;
const durum = false;let durum = true;
durum = false;function kaydet(veri) { let veri = temizle(veri); }function kaydet(veri) { const temizVeri = temizle(veri); }import { config } from './config.js';
const config = {};import { config as uygulamaConfig } from './config.js';
const localConfig = {};const application = new App();window.EkaApp ??= new App();Chrome DevTools Console, Sources, and Network panels should be examined together with real file, line, and server response.
Node version, package.json module type, build output, and the actual file run should comply with the same module system.
PHP warnings, theme HTML, redirects, and double script loading JavaScript errors may appear.
It occurs when the JavaScript parser, JSON parser, or module loader cannot find the expected grammar.
The line is usually where the parser detects the error. Missing quotes, parentheses, or commas may be in the previous line.
Use source map and DevTools pretty print; correct the original source file and rebuild.
PHP warning, HTML error page or unpadded data pressed may break JavaScript/JSON syntax.
Most parsers find most errors; however, incorrect server response, double script loading, and runtime module configuration should also be checked.
Deploy, minify, cache, package update, Node version, PHP output or half-loaded file behavior may have changed.
Provides diagnostic and safe repair steps; changes should be tested in the staging environment and reverted using Git.
We examine Console, Fetch/AJAX, PHP JSON responses, ES Modules, TypeScript build and WISECP script loading problems without disrupting the production structure.