Duplicate identifier error in TypeScript [Solved]

# Duplicate identifier error in TypeScript [Solved]

To solve the "Duplicate identifier error":

1. Make sure you don't have any clashing names of identifiers.
2. Set the skipLibCheck option to true in your tsconfig.json file.
3. Update the version of your typings, e.g. npm i -D @types/node@latest.

Sometimes the error is caused when we use a name that clashes with a globally defined object, a global interface or a member with the same name in a different file (if not using ES modules).

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// ⛔️ Duplicate identifier 'Employee'.ts(2300)
class Employee {}

// ⛔️ Duplicate identifier 'Node'.ts(2300)
class Node {} // 👈️ name of global interface

// ⛔️ Duplicate identifier 'Array'.ts(2300)
class Array {} // 👈️ name of global interface

The Employee class causes the error because this file is not an ES module. A module is a file that contains at least 1 import or export statement.

# Add an export {} statement

If you don't have anything to export, simply export {}.

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// ✅ It's an ES module now
class Employee {}

export {};

We used the export {} line in our file to mark it as an ES module.

Make sure you haven't used any names of global interfaces or objects like Node, Array, String in your code.

# Pulling in typings from multiple versions of a package

Another common cause of the error is that your project is pulling in typings from multiple versions of a package.

Set the skipLibCheck option to true in your tsconfig.json.

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{
  "compilerOptions": {
    "skipLibCheck": true,
    // ... rest
  },
  "include": ["src/**/*"],
  "exclude": ["node_modules"]
}

The skipLibCheck option instructs the compiler to skip type-checking for declaration files.

This is needed because sometimes two libraries define two copies of the same type.

Make sure to add node_modules to your exclude array (like in the code sample above) if you haven't.

# Update your typings

If your error message shows the name of a typings package, e.g. /node_modules/typescript/lib....

You have to update the version of the types package. For example, this is how you would update the version of @types/node.

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npm i -D @types/node@latest

Make sure to install the latest version of the @types package because you might have an older version (or multiple versions).

If you have the @types/node package in your dependencies, definitely run npm i -D @types/node@latest as it often causes the error.

# Delete your node_modules and reinstall your dependencies

If the error is not resolved, try to delete your node_modules and package-lock.json files, re-run npm install and restart your IDE.

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# 👇️  (Windows) delete node_modules and package-lock.json
rd /s /q "node_modules"
del package-lock.json
del -f yarn.lock

# 👇️  (macOS/Linux) delete node_modules and package-lock.json
rm -rf node_modules
rm -f package-lock.json
rm -f yarn.lock

# 👇️ clean npm cache
npm cache clean --force

npm install

Restart your IDE and your development server if the error persists.