bun-types/docs/guides/test/mock-functions.md

Type definitions and documentation for Bun, an incredibly fast JavaScript runtime

---
name: Mock functions in `bun test`
---

Create mocks with the `mock` function from `bun:test`.

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

const random = mock(() => Math.random());
```

---

The mock function can accept arguments.

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

const random = mock((multiplier: number) => multiplier * Math.random());
```

---

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

```ts
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 }
//  ]
```

---

These extra properties make it possible to write `expect` assertions about usage of the mock function, including how many times it was called, the arguments, and the return values.

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

const random = mock((multiplier: number) => multiplier * Math.random());

test("random", async () => {
  const a = random(1);
  const b = random(2);
  const c = random(3);

  expect(random).toHaveBeenCalled();
  expect(random).toHaveBeenCalledTimes(3);
  expect(random.mock.args).toEqual([[1], [2], [3]]);
  expect(random.mock.results[0]).toEqual({ type: "return", value: a });
});
```

---

See [Docs > Test Runner > Mocks](https://bun.sh/docs/test/mocks) for complete documentation on mocking with the Bun test runner.