---
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, 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](/docs/test/mocks) for complete documentation on mocking with the Bun test runner.
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Per
Per Lindgren
Jorge Aparicio
nils-grepit
Emil Fresk