experimental

experimental.openTelemetry 4.0.11+

FEEDBACK

Please leave feedback regarding this feature in a GitHub Discussion.

• Type:

interface OpenTelemetryOptions {
  enabled: boolean
  /**
   * A path to a file that exposes an OpenTelemetry SDK for Node.js.
   */
  sdkPath?: string
  /**
   * A path to a file that exposes an OpenTelemetry SDK for the browser.
   */
  browserSdkPath?: string
}

• Default: { enabled: false }

This option controls OpenTelemetry support. Vitest imports the SDK file in the main thread and before every test file, if enabled is set to true.

PERFORMANCE CONCERNS

OpenTelemetry may significantly impact Vitest performance; enable it only for local debugging.

You can use a custom service together with Vitest to pinpoint which tests or files are slowing down your test suite.

For browser mode, see the Browser Mode section of the OpenTelemetry guide.

An sdkPath is resolved relative to the root of the project and should point to a module that exposes a started SDK instance as a default export. For example:

otel.js

import { getNodeAutoInstrumentations } from '@opentelemetry/auto-instrumentations-node'
import { OTLPTraceExporter } from '@opentelemetry/exporter-trace-otlp-proto'
import { NodeSDK } from '@opentelemetry/sdk-node'

const sdk = new NodeSDK({
  serviceName: 'vitest',
  traceExporter: new OTLPTraceExporter(),
  instrumentations: [getNodeAutoInstrumentations()],
})

sdk.start()
export default sdk

vitest.config.js

import { defineConfig } from 'vitest/config'

export default defineConfig({
  test: {
    experimental: {
      openTelemetry: {
        enabled: true,
        sdkPath: './otel.js',
      },
    },
  },
})

WARNING

It's important that Node can process sdkPath content because it is not transformed by Vitest. See the guide on how to work with OpenTelemetry inside of Vitest.

experimental.importDurations 4.1.0+

FEEDBACK

Please leave feedback regarding this feature in a GitHub Discussion.

• Type:

interface ImportDurationsOptions {
  /**
   * When to print import breakdown to CLI terminal.
   * - false: Never print (default)
   * - true: Always print
   * - 'on-warn': Print only when any import exceeds warn threshold
   */
  print?: boolean | 'on-warn'
  /**
   * Fail the test run if any import exceeds the danger threshold.
   * When enabled and threshold exceeded, breakdown is always printed.
   * @default false
   */
  failOnDanger?: boolean
  /**
   * Maximum number of imports to collect and display.
   */
  limit?: number
  /**
   * Duration thresholds in milliseconds for coloring and warnings.
   */
  thresholds?: {
    /** Threshold for yellow/warning color. @default 100 */
    warn?: number
    /** Threshold for red/danger color and failOnDanger. @default 500 */
    danger?: number
  }
}

• Default: { print: false, failOnDanger: false, limit: 0, thresholds: { warn: 100, danger: 500 } } (limit is 10 if print or UI is enabled)

Configure import duration collection and display.

The print option controls CLI terminal output. The limit option controls how many imports to collect and display. Vitest UI can always toggle the breakdown display regardless of the print setting.

• Self: the time it took to import the module, excluding static imports;
• Total: the time it took to import the module, including static imports. Note that this does not include transform time of the current module.

Note that if the file path is too long, Vitest will truncate it at the start until it fits 45 character limit.

experimental.importDurations.print

• Type: boolean | 'on-warn'
• Default: false

Controls when to print import breakdown to CLI terminal after tests finish. This only works with default, verbose, or tree reporters.

• false: Never print breakdown
• true: Always print breakdown
• 'on-warn': Print only when any import exceeds the thresholds.warn value

experimental.importDurations.failOnDanger

• Type: boolean
• Default: false

Fail the test run if any import exceeds the thresholds.danger value. When enabled and the threshold is exceeded, the breakdown is always printed regardless of the print setting.

This is useful for enforcing import performance budgets in CI:

vitest --experimental.importDurations.failOnDanger

experimental.importDurations.limit

• Type: number
• Default: 0 (or 10 if print, failOnDanger, or UI is enabled)

Maximum number of imports to collect and display in CLI output, Vitest UI, and third-party reporters.

experimental.importDurations.thresholds

• Type: { warn?: number; danger?: number }
• Default: { warn: 100, danger: 500 }

Duration thresholds in milliseconds for coloring and warnings:

• warn: Threshold for yellow/warning color (default: 100ms)
• danger: Threshold for red/danger color and failOnDanger (default: 500ms)

INFO

Vitest UI shows a breakdown of imports automatically if at least one file took longer than the danger threshold to load.

experimental.viteModuleRunner 4.1.0+

FEEDBACK

Please leave feedback regarding this feature in a GitHub Discussion.

• Type: boolean
• Default: true

Controls whether Vitest uses Vite's module runner to run the code or fallback to the native import.

If this option is defined in the root config, all projects will inherit it automatically.

Consider disabling the module runner if you are running tests in the same environment as your code (server backend or simple scripts, for example). However, we still recommend running jsdom/happy-dom tests with Vite's module runner or in the browser since it doesn't require any additional configuration.

Disabling this flag will disable all file transforms:

• test files and your source code are not processed by Vite
• your global setup files are not processed
• your custom runner/pool/environment files are not processed
• your config file is still processed by Vite (this happens before Vitest knows the viteModuleRunner flag)

WARNING

At the moment, Vitest still requires Vite for certain functionality like the module graph or watch mode.

Also note that this option only works with forks or threads pools.

Module Runner

By default, Vitest runs tests in a very permissive module runner sandbox powered by Vite's Environment API. Every file is categorized as either an "inline" module or an "external" module.

Module runner runs all "inlined" modules. It provides import.meta.env, require, __dirname, __filename, static import, and has its own module resolution mechanism. This makes it very easy to run code when you don't want to configure the environment and just need to test that the bare JavaScript logic you wrote works as intended.

All "external" modules run in native mode, meaning they are executed outside of the module runner sandbox. If you are running tests in Node.js, these files are imported with the native import keyword and processed by Node.js directly.

While running JSDOM/happy-dom tests in a permissive fake environment might be justified, running Node.js tests in a non-Node.js environment can hide and silence potential errors you may encounter in production, especially if your code doesn't require any additional transformations provided by Vite plugins.

Known Limitations

Some Vitest features rely on files being transformed. Vitest uses synchronous Node.js Loaders API to transform test files and setup files to support these features:

• import.meta.vitest
• vi.mock
• vi.hoisted

WARNING

This means that Vitest requires at least Node 22.15 for those features to work. At the moment, they also do not work in Deno or Bun.

Vitest will only detect vi.mock and vi.hoisted inside of test files, they will not be hoisted inside imported modules.

This could affect performance because Vitest needs to read the file and process it. If you do not use these features, you can disable the transforms by setting experimental.nodeLoader to false. Vitest only reads test files and setup files while looking for vi.mock or vi.hoisted. Using these in other files won't hoist them to the top of the file and can lead to unexpected behavior.

Some features will not work due to the nature of viteModuleRunner, including:

• no import.meta.env: import.meta.env is a Vite feature, use process.env instead
• no plugins: plugins are not applied because there is no transformation phase, use customization hooks via execArgv instead
• no alias: aliases are not applied because there is no transformation phase
• istanbul coverage provider doesn't work because there is no transformation phase, use v8 instead
• vi.resetModules(): there is no API to invalidate ES modules from the module cache

Coverage Support

At the moment Vitest supports coverage via v8 provider as long as files can be transformed into JavaScript. To transform TypeScript, Vitest uses module.stripTypeScriptTypes which is available in Node.js since v22.13. If you are using a custom module loader, Vitest is not able to reuse it to transform files for analysis.

With regards to mocking, it is also important to point out that ES modules do not support property override. This means that code like this won't work anymore:

import * as fs from 'node:fs'
import { vi } from 'vitest'

vi.spyOn(fs, 'readFileSync').mockImplementation(() => '42') // ❌

However, Vitest supports auto-spying on modules without overriding their implementation. When vi.mock is called with a spy: true argument, the module is mocked in a way that preserves original implementations, but all exported functions are wrapped in a vi.fn() spy:

import * as fs from 'node:fs'
import { vi } from 'vitest'

vi.mock('node:fs', { spy: true })

fs.readFileSync.mockImplementation(() => '42') // ✅

Factory mocking is implemented using a top-level await. This means that mocked modules cannot be loaded with require() in your source code:

vi.mock('node:fs', async (importOriginal) => {
  return {
    ...await importOriginal(),
    readFileSync: vi.fn(),
  }
})

const fs = require('node:fs') // throws an error

This limitation exists because factories can be asynchronous. This should not be a problem because Vitest doesn't mock builtin modules inside node_modules, which is similar to how Vitest works by default.

TypeScript

If you are using Node.js 22.18/23.6 or higher, TypeScript will be transformed natively by Node.js.

TypeScript with Node.js 22.6-22.18

If you are using Node.js version between 22.6 and 22.18, you can also enable native TypeScript support via --experimental-strip-types flag:

NODE_OPTIONS="--experimental-strip-types" vitest

If you are using TypeScript and Node.js version lower than 22.6, then you will need to either:

• build your test files and source code and run those files directly
• import a custom loader via execArgv flag

import { defineConfig } from 'vitest/config'

const tsxApi = import.meta.resolve('tsx/esm/api')

export default defineConfig({
  test: {
    execArgv: [
      `--import=data:text/javascript,import * as tsx from "${tsxApi}";tsx.register()`,
    ],
    experimental: {
      viteModuleRunner: false,
    },
  },
})

If you are running tests in Deno, TypeScript files are processed by the runtime without any additional configurations.

experimental.vcsProvider 4.1.1+

• Type: VCSProvider | string

interface VCSProvider {
  findChangedFiles(options: VCSProviderOptions): Promise<string[]>
}

interface VCSProviderOptions {
  root: string
  changedSince?: string | boolean
}

• Default: 'git'

Custom provider for detecting changed files. Used with the --changed flag to determine which files have been modified.

By default, Vitest uses Git to detect changed files. You can provide a custom implementation of the VCSProvider interface to use a different version control system:

vitest.config.ts

import { defineConfig } from 'vitest/config'

export default defineConfig({
  test: {
    experimental: {
      vcsProvider: {
        async findChangedFiles({ root, changedSince }) {
          // return paths of changed files
          return []
        },
      },
    },
  },
})

You can also pass a string path to a module with a default export that implements the VCSProvider interface:

vitest.config.js

import { defineConfig } from 'vitest/config'

export default defineConfig({
  test: {
    experimental: {
      vcsProvider: './my-vcs-provider.js',
    },
  },
})

my-vcs-provider.js

export default {
  async findChangedFiles({ root, changedSince }) {
    // return paths of changed files
    return []
  },
}

experimental.nodeLoader 4.1.0+

• Type: boolean
• Default: true

If module runner is disabled, Vitest uses a native Node.js module loader to transform files to support import.meta.vitest, vi.mock and vi.hoisted.

If you don't use these features, you can disable this to improve performance.

experimental.preParse 4.1.3+

• Type: boolean
• Default: false

Parses test specifications before running them. This applies the .only modifier, the -t test name pattern, --tags-filter, test lines, and test IDs across all files without executing them. For example, if only a single test is marked with .only, Vitest will skip all other tests in all files.

TIP

This option is recommended when using .only, the -t flag, or --tags-filter.

Enabling it unconditionally may slow down your test runs due to the additional parsing step.

WARNING

Pre-parsing uses static analysis (AST parsing) instead of executing your test files. This means that test names, tags, and modifiers (.only, .skip, .todo) must be statically analyzable. Dynamic test names (e.g., names stored in variables or returned from function calls) and non-literal tags will not be resolved correctly.

// ✅ works — static string literal
test('adds numbers', () => {})

// ✅ works — static tags
test('my test', { tags: ['unit'] }, () => {})

// ❌ won't match correctly — dynamic name
const name = getName()
test(name, () => {})

// ❌ won't match correctly — dynamic tags
const tags = getTags()
test('my test', { tags }, () => {})

experimental.diagnostics 5.0.0+

• Type:

interface DiagnosticsOptions {
  /**
   * Hint when `isolate: true` spends a significant amount of time spawning
   * a fresh worker (and re-creating the environment) for every test file,
   * estimating how much `isolate: false` could save.
   * @default true
   */
  isolate?: boolean
  /**
   * Hint when re-creating a DOM environment for every test file dominates
   * the run and a `vm` pool would set it up once per worker.
   * @default true
   */
  environment?: boolean
  /**
   * Hint when test files repeatedly evaluate the same module graph
   * (typical for barrel-file imports) and `isolate: false` would
   * evaluate it once per worker.
   * @default true
   */
  import?: boolean
  /**
   * Hint when transforming modules dominates the run and
   * `fsModuleCache` would persist the results across runs.
   * @default true
   */
  transform?: boolean
}

• Default: true

Print performance hints after the run when the collected timings show that a configuration change would make the run significantly faster:

Environment  jsdom was created 40 times · 23.80s total, 79% of tracked time
             create it once per worker with pool: 'vmThreads' (keeps per-file isolation) or isolate: false (shares it across files)
             learn more: https://vitest.dev/guide/improving-performance#test-environments

Hints never suggest changing an option that was set explicitly: if the config defines pool, other pools are not suggested, and an explicitly configured isolate is never suggested to be disabled. Hints are also printed in CI. Set the option to false to disable all hints, or disable them individually.

To measure the impact of a configuration change instead of estimating it, run vitest doctor.

experimental.diagnostics.isolate

• Type: boolean
• Default: true

Hint when isolate: true spends a significant amount of time spawning a fresh worker (and re-creating the environment) for every test file, estimating how much isolate: false could save. Reused workers also keep evaluated modules alive, so files stop re-evaluating the module graph they share. Per-module evaluation times are only collected when experimental.importDurations is enabled; without it the estimate counts the worker startups alone and is reported as a lower bound ("at least").

experimental.diagnostics.environment

• Type: boolean
• Default: true

Hint when re-creating a DOM environment for every test file dominates the run and a vm pool would set it up once per worker.

experimental.diagnostics.import

• Type: boolean
• Default: true

Hint when test files repeatedly evaluate the same module graph and isolate: false would evaluate it once per worker. This is typical for barrel-file imports: every test file imports a few symbols through an index file and evaluates the whole graph behind it. The duplication is measured from how often each module was served to the workers, so suites whose test files import mostly disjoint modules stay quiet: reusing workers would not reduce their import work.

Import  837 modules were evaluated 16740 times · 15.69s total, 64% of tracked time
        ~850ms faster with isolate: false — shared modules are evaluated once per worker instead of once per file
        learn more: https://vitest.dev/guide/improving-performance#test-isolation

experimental.diagnostics.transform

• Type: boolean
• Default: true

Hint when transforming modules dominates the run. Without a persistent cache every vitest run transforms the whole module graph from scratch; fsModuleCache stores the results on disk so repeated runs skip them. The hint estimates the time the next run would save. On CI the hint includes a note that the cache directory must be persisted between runs for the cache to take effect.