feat(experimental): allow disabling the module runner

docs/config/experimental.md

@@ -195,3 +195,126 @@ Note that if the file path is too long, Vitest will truncate it at the start unt
 ::: info
 [Vitest UI](/guide/ui#import-breakdown) shows a breakdown of imports automatically if at least one file took longer than 500 milliseconds to load. You can manually set this option to `false` to disable this.
 :::
+
+## experimental.viteModuleRunner <Version type="experimental">4.0.16</Version> {#experimental-vitemodulerunner}
+
+- **Type:** `boolean`
+- **Default:** `true`
+
+Controls whether Vitest uses Vite's [module runner](https://vite.dev/guide/api-environment-runtimes#modulerunner) to run the code or fallback to the native `import`.
+
+If this option is defined in the root config, all [projects](/guide/projects) will inherit it automatically.
+
+We recommend 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 the module runner or in [the browser](/guide/browser/) as 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.
+:::
+
+### Module Runner
+
+By default, Vitest runs tests in a very permissive module runner sandbox powered by Vite's [Environment API](https://vite.dev/guide/api-environment.html#environment-api). Every file is categorized as either an "inline" module or an "external" module.
+
+Module runner runs all "inline" 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 is counter-productive as it 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.
+
+### Limitations
+
+Some Vitest features rely on files being transformed. Vitest uses synchronous [Node.js Loaders API](https://nodejs.org/api/module.html#customization-hooks) to transform certain files to support these features:
+
+- [`import.meta.vitest`](/guide/in-source)
+- [`vi.mock`](/api/vi#vi-mock)
+- [`vi.hoisted`](/api/vi#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.
+:::
+
+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 results.
+
+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
+- no `alias`: aliases are not applied because there is no transformation phase
+
+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:
+
+```ts
+import * as module from './some-module.js'
+import { vi } from 'vitest'
+
+vi.spyOn(module, 'function').mockImplementation(() => 42)
+```
+
+However, Vitest supports autospying 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:
+
+```ts
+import * as module from './some-module.js'
+import { vi } from 'vitest'
+
+vi.mock('./some-module.js', { spy: true })
+
+module.function.mockImplementation(() => 42)
+```
+
+### TypeScript
+
+If you are using Node.js 22.18/23.6 or higher, TypeScript will be [transformed natively](https://nodejs.org/en/learn/typescript/run-natively) by Node.js.
+
+::: warning 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:
+
+```shell
+NODE_OPTIONS="--experimental-strip-types" vitest
+```
+
+Note that Node.js will print an experimental warning for every test file; you can silence the warning by providing `--no-warnings` flag:
+
+```shell
+NODE_OPTIONS="--experimental-strip-types --no-warnings" 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](https://nodejs.org/api/module.html#customization-hooks) via `execArgv` flag
+
+```ts
+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 "${tsxCli}";tsx.register()`,
+    ],
+    experimental: {
+      viteModuleRunner: false,
+    },
+  },
+})
+```
+
+If you are running tests in Deno, TypeScript files are processed by the runtime without any additional configurations.
+
+## experimental.nodeLoader <Version type="experimental">4.0.16</Version> {#experimental-nodeloader}
+
+- **Type:** `boolean`
+- **Default:** `true`
+
+If module runner is disabled, Vitest uses a 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.

examples/native/jsSetup.js

@@ -0,0 +1,3 @@
+import { initJsSetup } from './src/setups.ts'
+
+initJsSetup()

examples/native/package.json

@@ -0,0 +1,22 @@
+{
+  "name": "@vitest/example-native",
+  "type": "module",
+  "private": true,
+  "license": "MIT",
+  "main": "index.js",
+  "scripts": {
+    "test": "vitest",
+    "test:ui": "vitest --ui",
+    "test:run": "vitest run"
+  },
+  "devDependencies": {
+    "@types/node": "^24.10.1",
+    "@vitest/ui": "latest",
+    "tinyspy": "^4.0.4",
+    "vite": "latest",
+    "vitest": "latest"
+  },
+  "stackblitz": {
+    "startCommand": "npm run test:ui"
+  }
+}

examples/native/src/__mocks__/redirect.ts

@@ -0,0 +1 @@
+export default true

examples/native/src/basic.ts

@@ -0,0 +1,4 @@
+export const squared = (n: number) => n * n
+export * from './dependency.ts'
+export { helloMe as hello } from './index.ts'
+export default 'hello world'

examples/native/src/dependency.ts

@@ -0,0 +1,3 @@
+export function add(a: number, b: number) {
+  return a + b
+}

examples/native/src/from-async.ts

@@ -0,0 +1,4 @@
+import { answer } from './mock-async.ts'
+
+const topLevelAnswer = answer
+export { topLevelAnswer }

examples/native/src/in-source/add.ts

@@ -0,0 +1,13 @@
+export function add(...args: number[]) {
+  return args.reduce((a, b) => a + b, 0)
+}
+
+// in-source test suites
+if (import.meta.vitest) {
+  const { it, expect } = import.meta.vitest
+  it('add', () => {
+    expect(add()).toBe(0)
+    expect(add(1)).toBe(1)
+    expect(add(1, 2, 3)).toBe(6)
+  })
+}

examples/native/src/in-source/fibonacci.ts

@@ -0,0 +1,24 @@
+import { add } from './add.ts'
+
+export function fibonacci(n: number): number {
+  if (n < 2) {
+    return n
+  }
+  return add(fibonacci(n - 1), fibonacci(n - 2))
+}
+
+if (import.meta.vitest) {
+  const { it, expect } = import.meta.vitest
+  it('fibonacci', () => {
+    expect(fibonacci(0)).toBe(0)
+    expect(fibonacci(1)).toBe(1)
+    expect(fibonacci(2)).toBe(1)
+    expect(fibonacci(3)).toBe(2)
+    expect(fibonacci(4)).toBe(3)
+    expect(fibonacci(5)).toBe(5)
+    expect(fibonacci(6)).toBe(8)
+    expect(fibonacci(7)).toBe(13)
+    expect(fibonacci(8)).toBe(21)
+    expect(fibonacci(9)).toMatchInlineSnapshot('34')
+  })
+}

examples/native/src/in-source/index.ts

@@ -0,0 +1,2 @@
+export * from './add.ts'
+export * from './fibonacci.ts'

examples/native/src/index.ts

@@ -0,0 +1,2 @@
+export { add, hello, squared } from './basic.ts'
+export const helloMe = 'world'

examples/native/src/minus.ts

@@ -0,0 +1,3 @@
+export function minus(a: number, b: number): number {
+  return a - b
+}

examples/native/src/mock-async.ts

@@ -0,0 +1 @@
+export const answer = 0

examples/native/src/mock-js.d.ts

@@ -0,0 +1 @@
+export declare function mockJs(): number

examples/native/src/mock-js.js

@@ -0,0 +1,3 @@
+export function mockJs() {
+  return 0
+}

examples/native/src/mock-sync.ts

@@ -0,0 +1,3 @@
+export function syncMock() {
+  return 0
+}

examples/native/src/no-mock.ts

@@ -0,0 +1,3 @@
+export function notMocked() {
+  return true
+}

examples/native/src/redirect.ts

@@ -0,0 +1,2 @@
+throw new Error('Should be redirected!')
+export default false

examples/native/src/setups.ts

@@ -0,0 +1,19 @@
+import { vi } from 'vitest'
+
+let jsSetup = false
+let tsSetup = false
+
+export const initJsSetup = vi.fn(() => {
+  jsSetup = true
+})
+
+export const initTsSetup = vi.fn(() => {
+  tsSetup = true
+})
+
+export function getSetupStates() {
+  return {
+    jsSetup,
+    tsSetup,
+  }
+}

examples/native/test/__snapshots__/suite.test.ts.snap

@@ -0,0 +1,7 @@
+// Vitest Snapshot v1, https://vitest.dev/guide/snapshot.html
+
+exports[`suite name > snapshot 1`] = `
+{
+  "foo": "bar",
+}
+`;

examples/native/test/automock.test.ts

@@ -0,0 +1,9 @@
+import { expect, test, vi } from 'vitest'
+import { squared } from '../src/basic.ts'
+
+vi.mock(import('../src/basic.ts'))
+
+test('squared is mocked', () => {
+  expect(vi.isMockFunction(squared)).toBe(true)
+  expect(squared(2)).toBe(undefined)
+})

examples/native/test/autospy.test.ts

@@ -0,0 +1,10 @@
+import { expect, test, vi } from 'vitest'
+import { squared } from '../src/basic.ts'
+
+vi.mock(import('../src/basic.ts'), { spy: true })
+
+test('squared is mocked', () => {
+  expect(vi.isMockFunction(squared)).toBe(true)
+  expect(squared(2)).toBe(4)
+  expect(squared).toHaveBeenCalled()
+})

examples/native/test/basic.test.ts

@@ -0,0 +1,33 @@
+import { assert, expect, test } from 'vitest'
+import { squared } from '../src/basic.ts'
+// importing from the barrel file
+import { add } from '../src/in-source/index.ts'
+
+// Edit an assertion and save to see HMR in action
+
+test('Math.sqrt()', () => {
+  expect(Math.sqrt(4)).toBe(2)
+  expect(Math.sqrt(144)).toBe(12)
+  expect(Math.sqrt(2)).toBe(Math.SQRT2)
+})
+
+test('add', () => {
+  expect(add(1, 2)).toBe(3)
+})
+
+test('Squared', () => {
+  expect(squared(2)).toBe(4)
+  expect(squared(12)).toBe(144)
+})
+
+test('JSON', () => {
+  const input = {
+    foo: 'hello',
+    bar: 'world',
+  }
+
+  const output = JSON.stringify(input)
+
+  expect(output).eq('{"foo":"hello","bar":"world"}')
+  assert.deepEqual(JSON.parse(output), input, 'matches original')
+})