Mocks | Bun Docs

Mocking replaces a dependency with a controlled implementation. Bun supports function mocks, spies, and module mocks.

Basic Function Mocks#

Create mocks with the mock function.

test.ts

import { test, expect, mock } from "bun:test";

const random = mock(() => Math.random());

test("random", () => {
  const val = random();
  expect(val).toBeGreaterThan(0);
  expect(random).toHaveBeenCalled();
  expect(random).toHaveBeenCalledTimes(1);
});

Jest Compatibility#

You can also use jest.fn(), as in Jest. It behaves identically.

test.ts

import { test, expect, jest } from "bun:test";

const random = jest.fn(() => Math.random());

test("random", () => {
  const val = random();
  expect(val).toBeGreaterThan(0);
  expect(random).toHaveBeenCalled();
  expect(random).toHaveBeenCalledTimes(1);
});

Mock Function Properties#

mock() returns a new function decorated with additional properties.

test.ts

import { mock } from "bun:test";

const random = mock((multiplier: number) => multiplier * Math.random());

random(2);
random(10);

random.mock.calls;
// [[ 2 ], [ 10 ]]

random.mock.results;
//  [
//    { type: "return", value: 0.6533907460954099 },
//    { type: "return", value: 0.6452713933037312 }
//  ]

Available Properties and Methods#

Mock functions implement the following properties and methods:

Property/Method	Description
`mockFn.getMockName()`	Returns the mock name
`mockFn.mock.calls`	Array of call arguments for each invocation
`mockFn.mock.results`	Array of return values for each invocation
`mockFn.mock.instances`	Array of instances created with `new`
`mockFn.mock.contexts`	Array of `this` contexts for each invocation
`mockFn.mock.lastCall`	Arguments of the most recent call
`mockFn.mockClear()`	Clears call history
`mockFn.mockReset()`	Clears call history and removes implementation
`mockFn.mockRestore()`	Restores original implementation
`mockFn.mockImplementation(fn)`	Sets a new implementation
`mockFn.mockImplementationOnce(fn)`	Sets implementation for next call only
`mockFn.mockName(name)`	Sets the mock name
`mockFn.mockReturnThis()`	Sets the return value to `this`
`mockFn.mockReturnValue(value)`	Sets a return value
`mockFn.mockReturnValueOnce(value)`	Sets return value for next call only
`mockFn.mockResolvedValue(value)`	Sets a resolved Promise value
`mockFn.mockResolvedValueOnce(value)`	Sets resolved Promise for next call only
`mockFn.mockRejectedValue(value)`	Sets a rejected Promise value
`mockFn.mockRejectedValueOnce(value)`	Sets rejected Promise for next call only
`mockFn.withImplementation(fn, callback)`	Temporarily changes implementation

Practical Examples#

Basic Mock Usage#

test.ts

import { test, expect, mock } from "bun:test";

test("mock function behavior", () => {
  const mockFn = mock((x: number) => x * 2);

  // Call the mock
  const result1 = mockFn(5);
  const result2 = mockFn(10);

  // Verify calls
  expect(mockFn).toHaveBeenCalledTimes(2);
  expect(mockFn).toHaveBeenCalledWith(5);
  expect(mockFn).toHaveBeenLastCalledWith(10);

  // Check results
  expect(result1).toBe(10);
  expect(result2).toBe(20);

  // Inspect call history
  expect(mockFn.mock.calls).toEqual([[5], [10]]);
  expect(mockFn.mock.results).toEqual([
    { type: "return", value: 10 },
    { type: "return", value: 20 },
  ]);
});

Dynamic Mock Implementations#

test.ts

import { test, expect, mock } from "bun:test";

test("dynamic mock implementations", () => {
  const mockFn = mock();

  // Set different implementations
  mockFn.mockImplementationOnce(() => "first");
  mockFn.mockImplementationOnce(() => "second");
  mockFn.mockImplementation(() => "default");

  expect(mockFn()).toBe("first");
  expect(mockFn()).toBe("second");
  expect(mockFn()).toBe("default");
  expect(mockFn()).toBe("default"); // Uses default implementation
});

Async Mocks#

test.ts

import { test, expect, mock } from "bun:test";

test("async mock functions", async () => {
  const asyncMock = mock();

  // Mock resolved values
  asyncMock.mockResolvedValueOnce("first result");
  asyncMock.mockResolvedValue("default result");

  expect(await asyncMock()).toBe("first result");
  expect(await asyncMock()).toBe("default result");

  // Mock rejected values
  const rejectMock = mock();
  rejectMock.mockRejectedValue(new Error("Mock error"));

  await expect(rejectMock()).rejects.toThrow("Mock error");
});

Spies with spyOn()#

Use spyOn() to track calls to a function without replacing it with a mock. You can pass spies to .toHaveBeenCalled() and .toHaveBeenCalledTimes().

test.ts

import { test, expect, spyOn } from "bun:test";

const ringo = {
  name: "Ringo",
  sayHi() {
    console.log(`Hello I'm ${this.name}`);
  },
};

const spy = spyOn(ringo, "sayHi");

test("spyon", () => {
  expect(spy).toHaveBeenCalledTimes(0);
  ringo.sayHi();
  expect(spy).toHaveBeenCalledTimes(1);
});

Advanced Spy Usage#

test.ts

import { test, expect, spyOn, afterEach, jest } from "bun:test";

class UserService {
  async getUser(id: string) {
    // Original implementation
    return { id, name: `User ${id}` };
  }

  async saveUser(user: any) {
    // Original implementation
    return { ...user, saved: true };
  }
}

const userService = new UserService();

afterEach(() => {
  // Restore all spies after each test
  jest.restoreAllMocks();
});

test("spy on service methods", async () => {
  // Spy without changing implementation
  const getUserSpy = spyOn(userService, "getUser");
  const saveUserSpy = spyOn(userService, "saveUser");

  // Use the service normally
  const user = await userService.getUser("123");
  await userService.saveUser(user);

  // Verify calls
  expect(getUserSpy).toHaveBeenCalledWith("123");
  expect(saveUserSpy).toHaveBeenCalledWith(user);
});

test("spy with mock implementation", async () => {
  // Spy and override implementation
  const getUserSpy = spyOn(userService, "getUser").mockResolvedValue({
    id: "123",
    name: "Mocked User",
  });

  const result = await userService.getUser("123");

  expect(result.name).toBe("Mocked User");
  expect(getUserSpy).toHaveBeenCalledWith("123");
});

Module Mocks with mock.module()#

Use mock.module(path: string, callback: () => Object) to override the behavior of a module.

test.ts

import { test, expect, mock } from "bun:test";

mock.module("./module", () => {
  return {
    foo: "bar",
  };
});

test("mock.module", async () => {
  const esm = await import("./module");
  expect(esm.foo).toBe("bar");

  const cjs = require("./module");
  expect(cjs.foo).toBe("bar");
});

Like the rest of Bun, module mocks support both import and require.

Overriding Already Imported Modules#

Calling mock.module() overrides the module even if it has already been imported.

test.ts

import { test, expect, mock } from "bun:test";

// The module we're going to mock is here:
import { foo } from "./module";

test("mock.module", async () => {
  const cjs = require("./module");
  expect(foo).toBe("bar");
  expect(cjs.foo).toBe("bar");

  // We update it here:
  mock.module("./module", () => {
    return {
      foo: "baz",
    };
  });

  // And the live bindings are updated.
  expect(foo).toBe("baz");

  // The module is also updated for CJS.
  expect(cjs.foo).toBe("baz");
});

Hoisting & Preloading#

To make sure a module is mocked before it's imported, use --preload to load your mocks before your tests run.

my-preload.ts

import { mock } from "bun:test";

mock.module("./module", () => {
  return {
    foo: "bar",
  };
});

terminal

bun test --preload ./my-preload

To avoid typing --preload every time you run tests, add it to your bunfig.toml:

bunfig.toml

[test]
# Load these modules before running tests.
preload = ["./my-preload"]

Module Mock Best Practices#

When to Use Preload#

Mocking a module that's already been imported updates the module cache, so anything that imports it gets the mocked version. Bun has already evaluated the original module, though, so its side effects have already happened.

To prevent the original module from being evaluated at all, use --preload to load your mocks before your tests run.

Practical Module Mock Examples#

api-client.test.ts

import { test, expect, mock, beforeEach } from "bun:test";

// Mock the API client module
mock.module("./api-client", () => ({
  fetchUser: mock(async (id: string) => ({ id, name: `User ${id}` })),
  createUser: mock(async (user: any) => ({ ...user, id: "new-id" })),
  updateUser: mock(async (id: string, user: any) => ({ ...user, id })),
}));

test("user service with mocked API", async () => {
  const { fetchUser } = await import("./api-client");
  const { UserService } = await import("./user-service");

  const userService = new UserService();
  const user = await userService.getUser("123");

  expect(fetchUser).toHaveBeenCalledWith("123");
  expect(user.name).toBe("User 123");
});

Mocking External Dependencies#

database.test.ts

import { test, expect, mock } from "bun:test";

// Mock external database library
mock.module("pg", () => ({
  Client: mock(function () {
    return {
      connect: mock(async () => {}),
      query: mock(async (sql: string) => ({
        rows: [{ id: 1, name: "Test User" }],
      })),
      end: mock(async () => {}),
    };
  }),
}));

test("database operations", async () => {
  const { Database } = await import("./database");
  const db = new Database();

  const users = await db.getUsers();
  expect(users).toHaveLength(1);
  expect(users[0].name).toBe("Test User");
});

Global Mock Functions#

Clear All Mocks#

mock.clearAllMocks() resets the .mock.calls, .mock.instances, .mock.contexts, and .mock.results properties of every mock. Unlike mock.restore(), it does not restore the original implementation:

test.ts

import { expect, mock, test } from "bun:test";

const random1 = mock(() => Math.random());
const random2 = mock(() => Math.random());

test("clearing all mocks", () => {
  random1();
  random2();

  expect(random1).toHaveBeenCalledTimes(1);
  expect(random2).toHaveBeenCalledTimes(1);

  mock.clearAllMocks();

  expect(random1).toHaveBeenCalledTimes(0);
  expect(random2).toHaveBeenCalledTimes(0);

  // Note: implementations are preserved
  expect(typeof random1()).toBe("number");
  expect(typeof random2()).toBe("number");
});

Reset All Mocks#

jest.resetAllMocks() (and its vi.resetAllMocks() alias) calls mockFn.mockReset() on every mock. On top of what clearAllMocks() does, it drops the implementations set by mockImplementation(), mockReturnValue() and friends. It does not restore the original implementation of a spy:

test.ts

import { expect, jest, test } from "bun:test";

const random = jest.fn(() => Math.random());

test("resetting all mocks", () => {
  random();
  expect(random).toHaveBeenCalledTimes(1);

  jest.resetAllMocks();

  expect(random).toHaveBeenCalledTimes(0);
  // unlike clearAllMocks(), the implementation is gone
  expect(random()).toBeUndefined();
});

Restore All Mocks#

mock.restore() restores every mock at once, instead of calling mockFn.mockRestore() on each one. It does not reset modules overridden with mock.module().

test.ts

import { expect, mock, spyOn, test } from "bun:test";

import * as fooModule from "./foo.ts";
import * as barModule from "./bar.ts";
import * as bazModule from "./baz.ts";

test("foo, bar, baz", () => {
  const fooSpy = spyOn(fooModule, "foo");
  const barSpy = spyOn(barModule, "bar");
  const bazSpy = spyOn(bazModule, "baz");

  // Original implementations still work
  expect(fooModule.foo()).toBe("foo");
  expect(barModule.bar()).toBe("bar");
  expect(bazModule.baz()).toBe("baz");

  // Mock implementations
  fooSpy.mockImplementation(() => 42);
  barSpy.mockImplementation(() => 43);
  bazSpy.mockImplementation(() => 44);

  expect(fooModule.foo()).toBe(42);
  expect(barModule.bar()).toBe(43);
  expect(bazModule.baz()).toBe(44);

  // Restore all
  mock.restore();

  expect(fooModule.foo()).toBe("foo");
  expect(barModule.bar()).toBe("bar");
  expect(bazModule.baz()).toBe("baz");
});

Call mock.restore() in an afterEach block, or in your test preload script, instead of repeating cleanup in every test.

Vitest Compatibility#

For added compatibility with tests written for Vitest, Bun provides the vi object as an alias for parts of the Jest mocking API:

test.ts

import { test, expect, vi } from "bun:test";

// Using the 'vi' alias similar to Vitest
test("vitest compatibility", () => {
  const mockFn = vi.fn(() => 42);

  mockFn();
  expect(mockFn).toHaveBeenCalled();

  // The following functions are available on the vi object:
  // vi.fn
  // vi.spyOn
  // vi.mock
  // vi.restoreAllMocks
  // vi.resetAllMocks
  // vi.clearAllMocks
});

You can port tests from Vitest without rewriting your mocks.

Implementation Details#

Cache Interaction#

Module mocks interact with both ESM and CommonJS module caches.

Lazy Evaluation#

Bun evaluates the mock factory callback only when the module is imported or required.

Path Resolution#

Bun resolves the module specifier the same way it resolves an import, supporting:

Relative paths ('./module')
Absolute paths ('/path/to/module')
Package names ('lodash')

Import Timing Effects#

When mocking before first import: No side effects from the original module occur
When mocking after import: The original module's side effects have already happened

For this reason, use --preload for mocks that need to prevent side effects.

Live Bindings#

Mocked ESM modules maintain live bindings, so changing the mock updates all existing imports.

Advanced Patterns#

Factory Functions#

test.ts

import { mock } from "bun:test";

function createMockUser(overrides = {}) {
  return {
    id: "mock-id",
    name: "Mock User",
    email: "mock@example.com",
    ...overrides,
  };
}

const mockUserService = {
  getUser: mock(async (id: string) => createMockUser({ id })),
  createUser: mock(async (data: any) => createMockUser(data)),
  updateUser: mock(async (id: string, data: any) => createMockUser({ id, ...data })),
};

Conditional Mocking#

test.ts

import { test, expect, mock } from "bun:test";

const shouldUseMockApi = process.env.NODE_ENV === "test";

if (shouldUseMockApi) {
  mock.module("./api", () => ({
    fetchData: mock(async () => ({ data: "mocked" })),
  }));
}

test("conditional API usage", async () => {
  const { fetchData } = await import("./api");
  const result = await fetchData();

  if (shouldUseMockApi) {
    expect(result.data).toBe("mocked");
  }
});

Mock Cleanup Patterns#

test.ts

import { afterEach, beforeEach, mock } from "bun:test";

beforeEach(() => {
  // Set up common mocks
  mock.module("./logger", () => ({
    log: mock(() => {}),
    error: mock(() => {}),
    warn: mock(() => {}),
  }));
});

afterEach(() => {
  // Restore spies and clear call history; neither call resets the mock.module() override
  mock.restore();
  mock.clearAllMocks();
});

Best Practices#

Keep Mocks Simple#

test.ts

// Good: Simple, focused mock
const mockUserApi = {
  getUser: mock(async id => ({ id, name: "Test User" })),
};

// Avoid: Overly complex mock behavior
const complexMock = mock(input => {
  if (input.type === "A") {
    return processTypeA(input);
  } else if (input.type === "B") {
    return processTypeB(input);
  }
  // ... lots of complex logic
});

Use Type-Safe Mocks#

interface UserService {
  getUser(id: string): Promise<User>;
  createUser(data: CreateUserData): Promise<User>;
}

const mockUserService: UserService = {
  getUser: mock(async (id: string) => ({ id, name: "Test User" })),
  createUser: mock(async data => ({ id: "new-id", ...data })),
};

Test Mock Behavior#

test.ts

test("service calls API correctly", async () => {
  const mockApi = { fetchUser: mock(async () => ({ id: "1" })) };

  const service = new UserService(mockApi);
  await service.getUser("123");

  // Verify the mock was called correctly
  expect(mockApi.fetchUser).toHaveBeenCalledWith("123");
  expect(mockApi.fetchUser).toHaveBeenCalledTimes(1);
});

Notes#

Auto-mocking#

Bun does not support the __mocks__ directory or auto-mocking. If this is blocking you from switching to Bun, file an issue.

ESM vs CommonJS#

Module mocks have different implementations for ESM and CommonJS modules. For ES modules, Bun patches JavaScriptCore so it can override export values at runtime and update live bindings recursively.