bun-test

Cursor/Claude Rules for Bun Test Mocking

Rule 1: Use mock.module() for Module-Level Mocking

When mocking entire modules, use mock.module(path, callback) (1) :

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") ;
}) ;

Rule 2: Use spyOn() for Individual Function Mocking

To mock specific functions from imported modules, use spyOn() with mockImplementation() , (2) :

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) ;
}) ;

Rule 3: Mock Return Values Using Mock Methods

For mocking return values, use the mock function methods (1) :

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 }
// ]

Rule 4: Use Jest-Compatible Syntax

Bun supports Jest-compatible mocking syntax :

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) ;
}) ;

Rule 5: Mock Fetch Using spyOn (Community Solution)

For mocking fetch, use spyOn on the global object (3) :