bun.sh/docs/test/mocks.md

Create mocks with the mock function.

import { test, expect, mock } from "bun:test";
const random = mock(() => Math.random());

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

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

{% /callout %}

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 }
//  ]

The following properties and methods are implemented on mock functions.

• mockFn.getMockName()
• mockFn.mock.calls
• mockFn.mock.results
• mockFn.mock.instances
• mockFn.mock.contexts
• mockFn.mock.lastCall
• mockFn.mockClear()
• mockFn.mockReset()
• mockFn.mockRestore()
• mockFn.mockImplementation(fn)
• mockFn.mockImplementationOnce(fn)
• mockFn.mockName(name)
• mockFn.mockReturnThis()
• mockFn.mockReturnValue(value)
• mockFn.mockReturnValueOnce(value)
• mockFn.mockResolvedValue(value)
• mockFn.mockResolvedValueOnce(value)
• mockFn.mockRejectedValue(value)
• mockFn.mockRejectedValueOnce(value)
• mockFn.withImplementation(fn, callback)

.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);
});