feat(experimental): option to disable the module runner

docs/api/vi.md

@@ -37,6 +37,8 @@ function mock<T>(
 
 Substitutes all imported modules from provided `path` with another module. You can use configured Vite aliases inside a path. The call to `vi.mock` is hoisted, so it doesn't matter where you call it. It will always be executed before all imports. If you need to reference some variables outside of its scope, you can define them inside [`vi.hoisted`](#vi-hoisted) and reference them inside `vi.mock`.
 
+It is recommended to use `vi.mock` or `vi.hoisted` only inside test files. If Vite's [module runner](/config/experimental#experimental-vitemodulerunner) is disabled, they will not be hoisted. This is a performance optimisation to avoid ready unnecessary files.
+
 ::: warning
 `vi.mock` works only for modules that were imported with the `import` keyword. It doesn't work with `require`.
 

docs/config/experimental.md

@@ -8,7 +8,7 @@ outline: deep
 ## experimental.fsModuleCache <Version type="experimental">4.0.11</Version> {#experimental-fsmodulecache}
 
 ::: tip FEEDBACK
-Please, leave feedback regarding this feature in a [GitHub Discussion](https://github.com/vitest-dev/vitest/discussions/9221).
+Please leave feedback regarding this feature in a [GitHub Discussion](https://github.com/vitest-dev/vitest/discussions/9221).
 :::
 
 - **Type:** `boolean`
@@ -30,9 +30,9 @@ DEBUG=vitest:cache:fs vitest --experimental.fsModuleCache
 
 ### Known Issues
 
-Vitest creates persistent file hash based on file content, its id, vite's environment configuration and coverage status. Vitest tries to use as much information it has about the configuration, but it is still incomplete. At the moment, it is not possible to track your plugin options because there is no standard interface for it.
+Vitest creates a persistent file hash based on file content, its id, Vite's environment configuration and coverage status. Vitest tries to use as much information as it has about the configuration, but it is still incomplete. At the moment, it is not possible to track your plugin options because there is no standard interface for it.
 
-If you have a plugin that relies on things outside the file content or the public configuration (like reading another file or a folder), it's possible that the cache will get stale. To workaround that, you can define a [cache key generator](/api/advanced/plugin#definecachekeygenerator) to specify dynamic option or to opt-out of caching for that module:
+If you have a plugin that relies on things outside the file content or the public configuration (like reading another file or a folder), it's possible that the cache will get stale. To work around that, you can define a [cache key generator](/api/advanced/plugin#definecachekeygenerator) to specify a dynamic option or to opt out of caching for that module:
 
 ```js [vitest.config.js]
 import { defineConfig } from 'vitest/config'
@@ -66,7 +66,7 @@ export default defineConfig({
 
 If you are a plugin author, consider defining a [cache key generator](/api/advanced/plugin#definecachekeygenerator) in your plugin if it can be registered with different options that affect the transform result.
 
-On the other hand, if your plugin should not affect the cache key, you can opt-out by setting `api.vitest.experimental.ignoreFsModuleCache` to `true`:
+On the other hand, if your plugin should not affect the cache key, you can opt out by setting `api.vitest.experimental.ignoreFsModuleCache` to `true`:
 
 ```js [vitest.config.js]
 import { defineConfig } from 'vitest/config'
@@ -92,7 +92,7 @@ export default defineConfig({
 })
 ```
 
-Note that you can still define the cache key generator even the plugin opt-out of module caching.
+Note that you can still define the cache key generator even if the plugin opts out of module caching.
 
 ## experimental.fsModuleCachePath <Version type="experimental">4.0.11</Version> {#experimental-fsmodulecachepath}
 
@@ -108,7 +108,7 @@ At the moment, Vitest ignores the [test.cache.dir](/config/cache) or [cacheDir](
 ## experimental.openTelemetry <Version type="experimental">4.0.11</Version> {#experimental-opentelemetry}
 
 ::: tip FEEDBACK
-Please, leave feedback regarding this feature in a [GitHub Discussion](https://github.com/vitest-dev/vitest/discussions/9222).
+Please leave feedback regarding this feature in a [GitHub Discussion](https://github.com/vitest-dev/vitest/discussions/9222).
 :::
 
 - **Type:**
@@ -179,7 +179,7 @@ It's important that Node can process `sdkPath` content because it is not transfo
 ## experimental.printImportBreakdown <Version type="experimental">4.0.15</Version> {#experimental-printimportbreakdown}
 
 ::: tip FEEDBACK
-Please, leave feedback regarding this feature in a [GitHub Discussion](https://github.com/vitest-dev/vitest/discussions/9224).
+Please leave feedback regarding this feature in a [GitHub Discussion](https://github.com/vitest-dev/vitest/discussions/9224).
 :::
 
 - **Type:** `boolean`
@@ -197,3 +197,143 @@ 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">4.1.0</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/) 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](/config/pool).
+:::
+
+### 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.
+
+### Known 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 test files and setup 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.
+
+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](https://nodejs.org/api/module.html#customization-hooks) via [`execArgv`](/config/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
+
+::: warning Coverage Support
+At the momemnt Vitest supports coverage via `v8` provider as long as files can be transformed into JavaScript. To transform TypeScript, Vitest uses [`module.stripTypeScriptTypes`](https://nodejs.org/api/module.html#modulestriptypescripttypescode-options) which is available in Node.js since v22.13. If you are using a custom [module loader](https://nodejs.org/api/module.html#customization-hooks), 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:
+
+```ts
+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:
+
+```ts
+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:
+
+```ts
+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 simillar to how Vitest works by default.
+
+### 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
+```
+
+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 "${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.nodeLoader <Version type="experimental">4.1.0</Version> {#experimental-nodeloader}
+
+- **Type:** `boolean`
+- **Default:** `true`
+
+If module runner is disabled, Vitest uses a native [Node.js module loader](https://nodejs.org/api/module.html#customization-hooks) 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.

docs/guide/mocking/modules.md

@@ -317,6 +317,8 @@ The module mocking plugins are available in the [`@vitest/mocker` package](https
 
 When you run your tests in an emulated environment, Vitest creates a [module runner](https://vite.dev/guide/api-environment-runtimes.html#modulerunner) that can consume Vite code. The module runner is designed in such a way that Vitest can hook into the module evaluation and replace it with the mock, if it was registered. This means that Vitest runs your code in an ESM-like environment, but it doesn't use native ESM mechanism directly. This allows the test runner to bend the rules around ES Modules immutability, allowing users to call `vi.spyOn` on a seemingly ES Module.
 
+If module runner is [disabled](/config/experimental#experimental-vitemodulerunner) and [node loader](/config/experimental#experimental-nodeloader) is not explicitly disabled, Vitest will [register a loader hook](https://nodejs.org/api/module.html#customization-hooks) that transforms original modules into mocked ones. In this mode users cannot call `vi.spyOn` on an ES Module because Vitest uses a native loader mechanism with all its guard rails. In addition to that, Vitest also has to inject a `mock` query into every mocked module which is visible in the stack trace.
+
 ### Browser Mode
 
 Vitest uses native ESM in the Browser Mode. This means that we cannot replace the module so easily. Instead, Vitest intercepts the fetch request (via playwright's `page.route` or a Vite plugin API if using `preview` or `webdriverio`) and serves transformed code, if the module was mocked.

packages/coverage-v8/src/browser.ts

@@ -71,5 +71,13 @@ function filterResult(coverage: ScriptCoverage['result'][number]): boolean {
     return false
   }
 
+  if (coverage.url.includes('/@id/@vitest/')) {
+    return false
+  }
+
+  if (coverage.url.includes('/@vite/client')) {
+    return false
+  }
+
   return true
 }

packages/coverage-v8/src/index.ts

@@ -69,5 +69,13 @@ function filterResult(coverage: Profiler.ScriptCoverage): boolean {
     return false
   }
 
+  if (coverage.url.includes('/@id/@vitest/')) {
+    return false
+  }
+
+  if (coverage.url.includes('/@vite/client')) {
+    return false
+  }
+
   return true
 }

packages/coverage-v8/src/provider.ts

@@ -334,7 +334,12 @@ export class V8CoverageProvider extends BaseCoverageProvider<ResolvedCoverageOpt
     code: string
     map?: Vite.Rollup.SourceMap
   }> {
-    const transformResult = await onTransform(removeStartsWith(url, FILE_PROTOCOL)).catch(() => undefined)
+    // TODO: need to standardize file urls before this call somehow, this is messy
+    const filepath = url.match(/^file:\/\/\/\w:\//)
+      ? url.slice(8)
+      : removeStartsWith(url, FILE_PROTOCOL)
+    // TODO: do we still need to "catch" here? why would it fail?
+    const transformResult = await onTransform(filepath).catch(() => null)
 
     const map = transformResult?.map as Vite.Rollup.SourceMap | undefined
     const code = transformResult?.code
@@ -377,15 +382,12 @@ export class V8CoverageProvider extends BaseCoverageProvider<ResolvedCoverageOpt
       throw new Error(`Cannot access browser module graph because it was torn down.`)
     }
 
-    async function onTransform(filepath: string) {
-      if (environment === '__browser__' && project.browser) {
-        const result = await project.browser.vite.transformRequest(removeStartsWith(filepath, project.config.root))
-
-        if (result) {
-          return { ...result, code: `${result.code}// <inline-source-map>` }
-        }
+    const onTransform = async (filepath: string) => {
+      const result = await this.transformFile(filepath, project, environment)
+      if (result && environment === '__browser__' && project.browser) {
+        return { ...result, code: `${result.code}// <inline-source-map>` }
       }
-      return project.vite.environments[environment].transformRequest(filepath)
+      return result
     }
 
     const scriptCoverages = []

packages/mocker/package.json

@@ -44,7 +44,11 @@
       "types": "./dist/register.d.ts",
       "default": "./dist/register.js"
     },
-    "./*": "./*"
+    "./transforms": {
+      "types": "./dist/transforms.d.ts",
+      "default": "./dist/transforms.js"
+    },
+    "./package.json": "./package.json"
   },
   "main": "./dist/index.js",
   "module": "./dist/index.js",
@@ -79,6 +83,8 @@
     "@vitest/spy": "workspace:*",
     "@vitest/utils": "workspace:*",
     "acorn-walk": "catalog:",
+    "cjs-module-lexer": "^2.1.1",
+    "es-module-lexer": "^2.0.0",
     "msw": "catalog:",
     "pathe": "catalog:",
     "vite": "^6.3.5"

packages/mocker/rollup.config.js

@@ -17,6 +17,7 @@ const entries = {
   'browser': 'src/browser/index.ts',
   'register': 'src/browser/register.ts',
   'auto-register': 'src/browser/auto-register.ts',
+  'transforms': 'src/node/transforms.ts',
 }
 
 const external = [