code · pull · Jan 23, 2026 · Jan 23, 2026 · Jan 23, 2026 · Jan 23, 2026
diff --git a/docs/api/mock.md b/docs/api/mock.md
@@ -50,6 +50,47 @@ fn.length // == 2
 The custom function implementation in the types below is marked with a generic `<T>`.
 :::
 
+::: warning Class Support {#class-support}
+Shorthand methods like `mockReturnValue`, `mockReturnValueOnce`, `mockResolvedValue` and others cannot be used on a mocked class. Class constructors have [unintuitive behaviour](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Classes/constructor) regarding the return value:
+
+```ts {2,7}
+const CorrectDogClass = vi.fn(class {
+  constructor(public name: string) {}
+})
+
+const IncorrectDogClass = vi.fn(class {
+  constructor(public name: string) {
+    return { name }
+  }
+})
+
+const Marti = new CorrectDogClass('Marti')
+const Newt = new IncorrectDogClass('Newt')
+
+Marti instanceof CorrectDogClass // ✅ true
+Newt instanceof IncorrectDogClass // ❌ false!
+```
+
+Even though the shapes are the same, the _return value_ from the constructor is assigned to `Newt`, which is a plain object, not an instance of a mock. Vitest guards you against this behaviour in shorthand methods (but not in `mockImplementation`!) and throws an error instead.
+
+If you need to mock constructed instance of a class, consider using the `class` syntax with `mockImplementation` instead:
+
+```ts
+mock.mockReturnValue({ hello: () => 'world' }) // [!code --]
+mock.mockImplementation(class { hello = () => 'world' }) // [!code ++]
+```
+
+If you need to test the behaviour where this is a valid use case, you can use `mockImplementation` with a `constructor`:
+
+```ts
+mock.mockImplementation(class {
+  constructor(name: string) {
+    return { name }
+  }
+})
+```
+:::
+
 ## getMockImplementation
 
 ```ts

diff --git a/docs/package.json b/docs/package.json
@@ -27,7 +27,7 @@
     "@vite-pwa/assets-generator": "^1.0.2",
     "@vite-pwa/vitepress": "^1.1.0",
     "@vitejs/plugin-vue": "catalog:",
-    "@voidzero-dev/vitepress-theme": "^4.1.1",
+    "@voidzero-dev/vitepress-theme": "^4.3.0",
     "https-localhost": "^4.7.1",
     "tinyglobby": "catalog:",
     "unocss": "catalog:",

diff --git a/eslint.config.js b/eslint.config.js
@@ -107,6 +107,7 @@ export default antfu(
       `**/*.md/${GLOB_SRC}`,
     ],
     rules: {
+      'prefer-arrow-callback': 'off',
       'perfectionist/sort-imports': 'off',
       'style/max-statements-per-line': 'off',
       'import/newline-after-import': 'off',

diff --git a/examples/fastify/package.json b/examples/fastify/package.json
@@ -12,7 +12,7 @@
   },
   "devDependencies": {
     "@vitest/ui": "latest",
-    "fastify": "^5.6.2",
+    "fastify": "^5.7.1",
     "supertest": "^7.2.2",
     "tsx": "^4.21.0",
     "vite": "latest",

diff --git a/examples/lit/package.json b/examples/lit/package.json
@@ -19,7 +19,7 @@
   "devDependencies": {
     "@vitest/browser-playwright": "latest",
     "jsdom": "latest",
-    "playwright": "^1.57.0",
+    "playwright": "^1.58.0",
     "vite": "latest",
     "vitest": "latest"
   },

diff --git a/examples/opentelemetry/package.json b/examples/opentelemetry/package.json
@@ -9,11 +9,11 @@
   },
   "devDependencies": {
     "@opentelemetry/api": "^1.9.0",
-    "@opentelemetry/context-zone": "^2.3.0",
-    "@opentelemetry/exporter-trace-otlp-proto": "^0.209.0",
-    "@opentelemetry/resources": "^2.3.0",
-    "@opentelemetry/sdk-node": "^0.209.0",
-    "@opentelemetry/sdk-trace-web": "^2.3.0",
+    "@opentelemetry/context-zone": "^2.5.0",
+    "@opentelemetry/exporter-trace-otlp-proto": "^0.211.0",
+    "@opentelemetry/resources": "^2.5.0",
+    "@opentelemetry/sdk-node": "^0.211.0",
+    "@opentelemetry/sdk-trace-web": "^2.5.0",
     "@vitest/browser-playwright": "latest",
     "vite": "latest",
     "vitest": "latest"

diff --git a/examples/projects/package.json b/examples/projects/package.json
@@ -10,12 +10,12 @@
   },
   "devDependencies": {
     "@testing-library/jest-dom": "^6.9.1",
-    "@testing-library/react": "^16.3.1",
+    "@testing-library/react": "^16.3.2",
     "@testing-library/user-event": "^14.6.1",
-    "@types/react": "^19.2.8",
+    "@types/react": "^19.2.9",
     "@vitejs/plugin-react": "^5.1.2",
     "@vitest/ui": "latest",
-    "fastify": "^5.6.2",
+    "fastify": "^5.7.1",
     "jsdom": "^27.4.0",
     "react": "^19.2.3",
     "supertest": "^7.2.2",

diff --git a/examples/typecheck/package.json b/examples/typecheck/package.json
@@ -10,7 +10,7 @@
     "test:run": "vitest run"
   },
   "devDependencies": {
-    "@types/node": "^24.10.7",
+    "@types/node": "^24.10.9",
     "@vitest/ui": "latest",
     "typescript": "^5.9.3",
     "vite": "latest",

diff --git a/package.json b/package.json
@@ -3,7 +3,7 @@
   "type": "module",
   "version": "4.1.0-beta.2",
   "private": true,
-  "packageManager": "[email protected].0",
+  "packageManager": "[email protected].1",
   "description": "Next generation testing framework powered by Vite",
   "engines": {
     "node": "^20.0.0 || ^22.0.0 || >=24.0.0"
@@ -38,13 +38,13 @@
     "test:browser:playwright": "pnpm -C test/browser run test:playwright"
   },
   "devDependencies": {
-    "@antfu/eslint-config": "^6.7.3",
-    "@antfu/ni": "^28.1.0",
-    "@playwright/test": "^1.57.0",
+    "@antfu/eslint-config": "^7.2.0",
+    "@antfu/ni": "^28.2.0",
+    "@playwright/test": "^1.58.0",
     "@rollup/plugin-commonjs": "^29.0.0",
     "@rollup/plugin-json": "^6.1.0",
     "@rollup/plugin-node-resolve": "^16.0.3",
-    "@types/node": "24.10.7",
+    "@types/node": "24.10.9",
     "@types/ws": "catalog:",
     "@vitest/browser": "workspace:*",
     "@vitest/coverage-istanbul": "workspace:*",
@@ -57,7 +57,7 @@
     "magic-string": "^0.30.21",
     "pathe": "^2.0.3",
     "premove": "^4.0.0",
-    "rollup": "^4.55.1",
+    "rollup": "^4.56.0",
     "rollup-plugin-dts": "^6.3.0",
     "rollup-plugin-license": "^3.6.0",
     "tinyglobby": "catalog:",

diff --git a/packages/browser-playwright/package.json b/packages/browser-playwright/package.json
@@ -59,7 +59,7 @@
     "tinyrainbow": "catalog:"
   },
   "devDependencies": {
-    "playwright": "^1.57.0",
+    "playwright": "^1.58.0",
     "vitest": "workspace:*"
   }
 }
diff --git a/packages/browser-webdriverio/package.json b/packages/browser-webdriverio/package.json
@@ -58,7 +58,7 @@
     "@vitest/browser": "workspace:*"
   },
   "devDependencies": {
-    "@wdio/types": "^9.20.0",
+    "@wdio/types": "^9.23.2",
     "vitest": "workspace:*",
     "webdriverio": "^9.20.0"
   }

diff --git a/packages/coverage-istanbul/package.json b/packages/coverage-istanbul/package.json
@@ -44,7 +44,7 @@
     "vitest": "workspace:*"
   },
   "dependencies": {
-    "@babel/core": "^7.23.9",
+    "@babel/core": "^7.28.6",
     "@istanbuljs/schema": "^0.1.3",
     "@jridgewell/gen-mapping": "^0.3.13",
     "@jridgewell/trace-mapping": "catalog:",

diff --git a/packages/mocker/package.json b/packages/mocker/package.json
@@ -83,7 +83,7 @@
     "@vitest/spy": "workspace:*",
     "@vitest/utils": "workspace:*",
     "acorn-walk": "catalog:",
-    "cjs-module-lexer": "^2.1.1",
+    "cjs-module-lexer": "^2.2.0",
     "es-module-lexer": "^2.0.0",
     "msw": "catalog:",
     "pathe": "catalog:",

diff --git a/packages/spy/src/index.ts b/packages/spy/src/index.ts
@@ -121,27 +121,63 @@ export function createMockInstance(options: MockInstanceOption = {}): Mock<Proce
   }
 
   mock.mockReturnValue = function mockReturnValue(value) {
-    return mock.mockImplementation(() => value)
+    return mock.mockImplementation(function () {
+      if (new.target) {
+        throwConstructorError('mockReturnValue')
+      }
+
+      return value
+    })
   }
 
   mock.mockReturnValueOnce = function mockReturnValueOnce(value) {
-    return mock.mockImplementationOnce(() => value)
+    return mock.mockImplementationOnce(function () {
+      if (new.target) {
+        throwConstructorError('mockReturnValueOnce')
+      }
+
+      return value
+    })
   }
 
   mock.mockResolvedValue = function mockResolvedValue(value) {
-    return mock.mockImplementation(() => Promise.resolve(value))
+    return mock.mockImplementation(function () {
+      if (new.target) {
+        throwConstructorError('mockResolvedValue')
+      }
+
+      return Promise.resolve(value)
+    })
   }
 
   mock.mockResolvedValueOnce = function mockResolvedValueOnce(value) {
-    return mock.mockImplementationOnce(() => Promise.resolve(value))
+    return mock.mockImplementationOnce(function () {
+      if (new.target) {
+        throwConstructorError('mockResolvedValueOnce')
+      }
+
+      return Promise.resolve(value)
+    })
   }
 
   mock.mockRejectedValue = function mockRejectedValue(value) {
-    return mock.mockImplementation(() => Promise.reject(value))
+    return mock.mockImplementation(function () {
+      if (new.target) {
+        throwConstructorError('mockRejectedValue')
+      }
+
+      return Promise.reject(value)
+    })
   }
 
   mock.mockRejectedValueOnce = function mockRejectedValueOnce(value) {
-    return mock.mockImplementationOnce(() => Promise.reject(value))
+    return mock.mockImplementationOnce(function () {
+      if (new.target) {
+        throwConstructorError('mockRejectedValueOnce')
+      }
+
+      return Promise.reject(value)
+    })
   }
 
   mock.mockClear = function mockClear() {
@@ -644,6 +680,12 @@ export function resetAllMocks(): void {
   REGISTERED_MOCKS.forEach(mock => mock.mockReset())
 }
 
+function throwConstructorError(shorthand: string): never {
+  throw new TypeError(
+    `Cannot use \`${shorthand}\` when called with \`new\`. Use \`mockImplementation\` with a \`class\` keyword instead. See https://vitest.dev/api/mock#class-support for more information.`,
+  )
+}
+
 export type {
   Constructable,
   MaybeMocked,

diff --git a/packages/spy/src/types.ts b/packages/spy/src/types.ts
@@ -47,7 +47,7 @@ export type MockParameters<T extends Procedure | Constructable> = T extends Cons
     ? Parameters<T> : never
 
 export type MockReturnType<T extends Procedure | Constructable> = T extends Constructable
-  ? void
+  ? InstanceType<T>
   : T extends Procedure
     ? ReturnType<T> : never
 

diff --git a/packages/ui/package.json b/packages/ui/package.json
@@ -78,16 +78,16 @@
     "d3-graph-controller": "^3.1.6",
     "floating-vue": "^5.2.2",
     "mime": "^4.1.0",
-    "rollup": "^4.55.1",
+    "rollup": "^4.56.0",
     "splitpanes": "^4.0.4",
     "typescript": "^5.9.3",
     "unocss": "catalog:",
     "vite": "^5.0.0",
     "vite-plugin-pages": "^0.33.2",
-    "vitest-browser-vue": "2.0.1",
+    "vitest-browser-vue": "2.0.2",
     "vue": "catalog:",
     "vue-router": "^4.6.4",
-    "vue-tsc": "^3.2.2",
+    "vue-tsc": "^3.2.3",
     "vue-virtual-scroller": "2.0.0-beta.8"
   }
 }
diff --git a/packages/vitest/package.json b/packages/vitest/package.json
@@ -193,7 +193,7 @@
   },
   "devDependencies": {
     "@antfu/install-pkg": "^1.1.0",
-    "@bomb.sh/tab": "^0.0.9",
+    "@bomb.sh/tab": "^0.0.11",
     "@edge-runtime/vm": "^5.0.0",
     "@jridgewell/trace-mapping": "catalog:",
     "@opentelemetry/api": "^1.9.0",
@@ -202,17 +202,17 @@
     "@types/istanbul-lib-coverage": "catalog:",
     "@types/istanbul-reports": "catalog:",
     "@types/jsdom": "^27.0.0",
-    "@types/node": "^24.10.7",
+    "@types/node": "^24.10.9",
     "@types/picomatch": "^4.0.2",
     "@types/prompts": "^2.4.9",
-    "@types/sinonjs__fake-timers": "^15.0.0",
+    "@types/sinonjs__fake-timers": "^15.0.1",
     "acorn": "8.11.3",
     "acorn-walk": "catalog:",
     "birpc": "catalog:",
     "cac": "catalog:",
     "empathic": "^2.0.0",
     "flatted": "catalog:",
-    "happy-dom": "^20.1.0",
+    "happy-dom": "^20.3.7",
     "jsdom": "^27.4.0",
     "local-pkg": "^1.1.2",
     "mime": "^4.1.0",