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);