Improving Performance

By default Vitest runs every test file in an isolated environment based on the pool:

• threads pool runs every test file in a separate Worker
• forks pool runs every test file in a separate forked child process
• vmThreads pool runs every test file in a separate VM context, but it uses workers for parallelism

This greatly increases test times, which might not be desirable for projects that don't rely on side effects and properly cleanup their state (which is usually true for projects with node environment). In this case disabling isolation will improve the speed of your tests. To do that, you can provide --no-isolate flag to the CLI or set test.isolate property in the config to false. If you are using several pools at once with poolMatchGlobs, you can also disable isolation for a specific pool you are using.

CLI

vitest --no-isolate

vitest.config.js

import { defineConfig } from 'vitest/config';

export default defineConfig({
  test: {
    isolate: false,
    // you can also disable isolation only for specific pools
    poolOptions: {
      forks: {
        isolate: false,
      },
    },
  },
});

TIP

If you are using vmThreads pool, you cannot disable isolation. Use threads pool instead to improve your tests performance.

For some projects, it might also be desirable to disable parallelism to improve startup time. To do that, provide --no-file-parallelism flag to the CLI or set test.fileParallelism property in the config to false.

CLI

vitest --no-file-parallelism

vitest.config.js

import { defineConfig } from 'vitest/config';

export default defineConfig({
  test: {
    fileParallelism: false,
  },
});