Bun Mocks

Mocks

Learn how to create and use mock functions, spies, and module mocks in Bun tests

Mocking is essential for testing by allowing you to replace dependencies with controlled implementations. Bun provides comprehensive mocking capabilities including function mocks, spies, and module mocks.

Basic Function Mocks

Create mocks with the mock function.

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

Alternatively, you can use the jest.fn() function, as in Jest. It behaves identically.

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

The result of mock() is a new function that's been decorated with some additional properties.

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

The following properties and methods are implemented on mock functions:

Practical Examples

Basic Mock Usage

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

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

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()

It's possible to track calls to a function without replacing it with a mock. Use spyOn() to create a spy; these spies can be passed to .toHaveBeenCalled() and .toHaveBeenCalledTimes().

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

import { test, expect, spyOn, afterEach } 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()

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

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

If you need to override a module that's already been imported, there's nothing special you need to do. Just call mock.module() and the module will be overridden.

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

If you need to ensure a module is mocked before it's imported, you should use --preload to load your mocks before your tests run.

import { mock } from "bun:test";

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

bun test --preload ./my-preload

To make your life easier, you can put preload in your bunfig.toml:

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

Module Mock Best Practices

When to Use Preload

What happens if I mock a module that's already been imported?

If you mock a module that's already been imported, the module will be updated in the module cache. This means that any modules that import the module will get the mocked version, BUT the original module will still have been evaluated. That means that any side effects from the original module will still have happened.

If you want to prevent the original module from being evaluated, you should use --preload to load your mocks before your tests run.

Practical Module Mock Examples

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

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

Reset all mock function state (calls, results, etc.) without restoring their original implementation:

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");
});

This resets the .mock.calls, .mock.instances, .mock.contexts, and .mock.results properties of all mocks, but unlike mock.restore(), it does not restore the original implementation.

Restore All Mocks

Instead of manually restoring each mock individually with mockFn.mockRestore(), restore all mocks with one command by calling mock.restore(). Doing so does not reset the value of modules overridden with mock.module().

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");
});

Using mock.restore() can reduce the amount of code in your tests by adding it to afterEach blocks in each test file or even in your test preload code.

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:

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.clearAllMocks
});

This makes it easier to port tests from Vitest to Bun without having to rewrite all your mocks.

Implementation Details

Understanding how mock.module() works helps you use it more effectively:

Cache Interaction

Module mocks interact with both ESM and CommonJS module caches.

Lazy Evaluation

The mock factory callback is only evaluated when the module is actually imported or required.

Path Resolution

Bun automatically resolves the module specifier as though you were doing 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, using --preload is recommended for mocks that need to prevent side effects.

Live Bindings

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

Advanced Patterns

Factory Functions

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

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

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

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

afterEach(() => {
  // Clean up all mocks
  mock.restore();
  mock.clearAllMocks();
});

Best Practices

Keep Mocks Simple

// 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("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

__mocks__ directory and auto-mocking are not supported yet. If this is blocking you from switching to Bun, please file an issue.

ESM vs CommonJS

Module mocks have different implementations for ESM and CommonJS modules. For ES Modules, Bun has added patches to JavaScriptCore that allow Bun to override export values at runtime and update live bindings recursively.