TypeError Jest: a transform must export a `process` function

# TypeError Jest: a transform must export a process function

The error "TypeError: Jest: a transform must export a process function" occurs when your jest and ts-jest versions don't match.

To solve the error, make sure to install matching versions of jest and ts-jest because major versions of ts-jest always follow jest major versions.

Open your package.json file and check if the major versions of the jest and ts-jest packages in your devDependencies object differ.

For example, the error occurs if you use jest v26 with ts-jest v27.

You can also use the following commands to check your jest and ts-jest versions.

Copied!
npm list jest --depth=0

npm list ts-jest --depth=0

Notice that jest and ts-jest are both version 29 in my case.

If you open the NPM pages of ts-jest and jest, you will see that the major versions match.

The major versions of both packages are 29 at the moment.

You can install a specific major version of jest and ts-jest by running the following command.

Copied!
npm install --save-dev jest@29 ts-jest@29

The command installs version 29 of both packages.

If you get an error when running the command, you might have to rerun it will the --legacy-peer-deps flag.

Copied!
npm install --save-dev jest@29 ts-jest@29 --legacy-peer-deps

You can also update your global jest module to the same version.

Copied!
npm install -g jest@29

Try to clear your jest cache after installing the same major version for both packages.

Copied!
npx jest --clearCache

# Installing the latest version of jest and ts-jest

If the error persists, try to install the latest version of the jest and ts-jest packages.

Open your terminal in your project's root directory (where your package.json file is) and run the following command.

Copied!
npm install --save-dev jest@latest ts-jest@latest

If you get an error, rerun the command with the --force flag.

Copied!
npm install --save-dev jest@latest ts-jest@latest --force

You should also update your global jest module to the latest version.

Copied!
npm install -g jest@latest

If updating the module globally fails, you might have to prefix the command with sudo (macOS and Linux) or open Command Prompt as an administrator (Windows).

Try to clear your jest cache if the issue persists.

Copied!
npx jest --clearCache

# Delete your node_modules and reinstall your dependencies

If the error persists, your node_modules might've glitched.

You might have other packages that depend on different versions of the jest and ts-jest modules.

If you are on macOS or Linux, run the following commands in bash or zsh to delete your node_modules folder.

Copied!
#  for macOS and Linux
rm -rf node_modules
rm -f package-lock.json
rm -f yarn.lock

# 👇️ clean the cache
npm cache clean --force
npx jest --clearCache

# 👇️ install packages
npm install

If you are on Windows, run the following commands in CMD.

Copied!
# for Windows
rd /s /q "node_modules"
del package-lock.json
del -f yarn.lock

# 👇️ clean npm cache
npm cache clean --force
npx jest --clearCache

# 👇️ install packages
npm install

Try to restart your test server if the issue persists.

# Check if other packages reference older versions of jest or ts-jest

If the error persists, check if other packages reference older versions of jest or ts-jest by issuing the following commands.

Copied!
npm list jest

npm list ts-jest

For example, the screenshot above shows that the react-scripts package references jest version 23.6.0.

You can try to update the packages that depend on older versions of jest and ts-jest.

Make sure to replace react-scripts with the output from issuing the 2 commands above.

Copied!
npm install react-scripts@latest

Try to clear your jest cache if the issue persists.

Copied!
npx jest --clearCache

# Additional Resources

You can learn more about the related topics by checking out the following tutorials:

• How to clear or disable the Cache in Jest [4 Ways]
• Received: serializes to the same string Jest error [Solved]
• Consider using the "jsdom" test environment error [Solved]
• Jest: Cannot read properties of undefined (reading 'testEnvironmentOptions')
• Error: Test environment jest-environment-jsdom cannot be found
• Testing exceptions in Jest (Sync and Async code)
• TypeError Jest: a transform must export a process function