# Source: https://bun.com/docs/guides/test/mock-functions.md

> ## Documentation Index
> Fetch the complete documentation index at: https://bun.com/docs/llms.txt
> Use this file to discover all available pages before exploring further.

# Mock functions in `bun test`

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

```ts test.ts icon="https://mintcdn.com/bun-1dd33a4e/Hq64iapoQXHbYMEN/icons/typescript.svg?fit=max&auto=format&n=Hq64iapoQXHbYMEN&q=85&s=c6cceedec8f82d2cc803d7c6ec82b240" theme={"theme":{"light":"github-light","dark":"dracula"}}
import { test, expect, mock } from "bun:test";

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

***

The mock function can accept arguments.

```ts test.ts icon="https://mintcdn.com/bun-1dd33a4e/Hq64iapoQXHbYMEN/icons/typescript.svg?fit=max&auto=format&n=Hq64iapoQXHbYMEN&q=85&s=c6cceedec8f82d2cc803d7c6ec82b240" theme={"theme":{"light":"github-light","dark":"dracula"}}
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 test.ts icon="https://mintcdn.com/bun-1dd33a4e/Hq64iapoQXHbYMEN/icons/typescript.svg?fit=max&auto=format&n=Hq64iapoQXHbYMEN&q=85&s=c6cceedec8f82d2cc803d7c6ec82b240" theme={"theme":{"light":"github-light","dark":"dracula"}}
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 test.ts icon="https://mintcdn.com/bun-1dd33a4e/Hq64iapoQXHbYMEN/icons/typescript.svg?fit=max&auto=format&n=Hq64iapoQXHbYMEN&q=85&s=c6cceedec8f82d2cc803d7c6ec82b240" theme={"theme":{"light":"github-light","dark":"dracula"}}
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.calls).toEqual([[1], [2], [3]]);
  expect(random.mock.results[0]).toEqual({ type: "return", value: a });
});
```

***

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