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How to Resolve Jest Error: "TypeError: a transform must export a process function" in JavaScript

The TypeError: Jest: a transform must export a process function is a common error in projects using TypeScript with Jest. This error almost always means there is a major version mismatch between your jest and ts-jest packages.

This guide will explain why this version mismatch causes the error and provide a clear, step-by-step process to diagnose and fix it by aligning your package versions.

The Core Problem: Breaking Changes in Jest's Transformer API​

Jest uses "transformers" to process files before running tests. For TypeScript, ts-jest acts as a transformer, converting your TypeScript code into JavaScript that Jest can understand.

The API that Jest uses to communicate with transformers can have breaking changes between major versions. The ts-jest package is versioned to follow Jest's major versions. For example:

  • ts-jest version 29.x.x is designed to work with jest version 29.x.x.
  • ts-jest version 28.x.x is designed to work with jest version 28.x.x.

The "a transform must export a process function" error occurs when you use a version of ts-jest that was built for a different major version of Jest. The ts-jest module you have installed does not have the process function that your version of Jest is expecting, leading to the TypeError.

Step 1: How to Diagnose the Version Mismatch​

You can quickly check for a version mismatch in your project.

Option A: Check package.json​

Open your package.json file and look at the devDependencies.

Example of a Mismatch:

"devDependencies": {
  "jest": "^28.1.3",
  "ts-jest": "^29.0.3" // This will cause the error!
}
note

In this example, jest is on version 28 while ts-jest is on version 29, which is incompatible.

Option B: Use npm list​

Run the following commands in your terminal to see the installed versions.

npm list jest
npm list ts-jest
note

This will show you the exact versions your project is currently using.

Step 2: The Solution - Aligning Your Package Versions​

The fix is to ensure that jest, ts-jest, and @types/jest (if you use it) are all aligned to the same major version. The easiest way to do this is to install the latest version of each.

Solution: run the following command in your project's root directory:

npm install --save-dev jest@latest ts-jest@latest @types/jest@latest

This command will update all three packages to their latest available versions, which are designed to be compatible with each other, resolving the version mismatch.

Step 3 (If Needed): Troubleshooting Caching and Dependency Issues​

If the error persists after aligning the versions, the problem is likely due to caching or a corrupted dependency tree.

Action 1: Clear the Jest Cache​

Jest caches transformed files to speed up test runs. Sometimes, this cache can become stale.

npx jest --clearCache

After clearing the cache, try running your tests again.

Action 2: The "Nuclear" Option - Reinstall Everything​

If you suspect a deeper issue with your node_modules folder, a full re-installation is the most reliable fix.

  1. Delete node_modules and package-lock.json:
    • On macOS or Linux: 
      rm -rf node_modules
      rm -f package-lock.json
    • On Windows (CMD): 
      rd /s /q "node_modules"
      del package-lock.json
  2. Reinstall all packages from scratch: 
    npm install
note

This ensures you have a completely fresh set of dependencies that correctly match your package.json.

Conclusion​

The TypeError: Jest: a transform must export a process function is almost always a sign of an incompatible dependency.

  • The root cause is a major version mismatch between jest and ts-jest.
  • The solution is to align these versions by running npm install --save-dev jest@latest ts-jest@latest @types/jest@latest.
  • If the problem continues, clear the Jest cache (npx jest --clearCache) or perform a full re-installation of your node_modules to resolve any deeper dependency conflicts.

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