pool

• Type: 'threads' | 'forks' | 'vmThreads' | 'vmForks'
• Default: 'forks'
• CLI: --pool=threads

Pool used to run tests in.

threads

Enable multi-threading. When using threads you are unable to use process related APIs such as process.chdir(). Some libraries written in native languages, such as Prisma, bcrypt and canvas, have problems when running in multiple threads and run into segfaults. In these cases it is advised to use forks pool instead.

forks

Similar as threads pool but uses child_process instead of worker_threads. Communication between tests and main process is not as fast as with threads pool. Process related APIs such as process.chdir() are available in forks pool.

vmThreads

Run tests using VM context (inside a sandboxed environment) in a threads pool.

This makes tests run faster, but the VM module is unstable when running ESM code. Your tests will leak memory - to battle that, consider manually editing vmMemoryLimit value.

WARNING

Running code in a sandbox has some advantages (faster tests), but also comes with a number of disadvantages.

• The globals within native modules, such as (fs, path, etc), differ from the globals present in your test environment. As a result, any error thrown by these native modules will reference a different Error constructor compared to the one used in your code:

try {
  fs.writeFileSync('/doesnt exist')
}
catch (err) {
  console.log(err instanceof Error) // false
}

• Importing ES modules caches them indefinitely which introduces memory leaks if you have a lot of contexts (test files). There is no API in Node.js that clears that cache.
• Accessing globals takes longer in a sandbox environment.

Please, be aware of these issues when using this option. Vitest team cannot fix any of the issues on our side.

vmForks

Similar as vmThreads pool but uses child_process instead of worker_threads. Communication between tests and the main process is not as fast as with vmThreads pool. Process related APIs such as process.chdir() are available in vmForks pool. Please be aware that this pool has the same pitfalls listed in vmThreads.