bun-test-mocking

Bun Test Mocking

Bun provides Jest-compatible mocking with mock(), spyOn(), and module mocking.

Mock Functions

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

// Create mock function
const fn = mock(() => "original");

test("mock function", () => {
  fn("arg1", "arg2");

  expect(fn).toHaveBeenCalled();
  expect(fn).toHaveBeenCalledTimes(1);
  expect(fn).toHaveBeenCalledWith("arg1", "arg2");
});

jest.fn() Compatibility

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

const fn = jest.fn(() => "value");

test("jest.fn works", () => {
  const result = fn();
  expect(result).toBe("value");
  expect(fn).toHaveBeenCalled();
});

Mock Return Values

const fn = mock();

// Return value once
fn.mockReturnValueOnce("first");
fn.mockReturnValueOnce("second");

// Permanent return value
fn.mockReturnValue("default");

// Promise returns
fn.mockResolvedValue("resolved");
fn.mockResolvedValueOnce("once");
fn.mockRejectedValue(new Error("fail"));
fn.mockRejectedValueOnce(new Error("once"));

Mock Implementations

const fn = mock();

// Set implementation
fn.mockImplementation((x) => x * 2);

// One-time implementation
fn.mockImplementationOnce((x) => x * 10);

// Chain implementations
fn
  .mockImplementationOnce(() => "first")
  .mockImplementationOnce(() => "second")
  .mockImplementation(() => "default");

Spy on Methods

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

const obj = {
  method: () => "original",
};

test("spy on method", () => {
  const spy = spyOn(obj, "method");

  obj.method();

  expect(spy).toHaveBeenCalled();
  expect(obj.method()).toBe("original"); // Still works

  // Override implementation
  spy.mockImplementation(() => "mocked");
  expect(obj.method()).toBe("mocked");

  // Restore
  spy.mockRestore();
  expect(obj.method()).toBe("original");
});

Mock Modules

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

// Mock entire module
mock.module("./utils", () => ({
  add: mock(() => 999),
  subtract: mock(() => 0),
}));

// Now imports use mocked version
import { add } from "./utils";

test("mocked module", () => {
  expect(add(1, 2)).toBe(999);
});