bun-types/docs/guides/test/spy-on.mdx

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

---
title: Spy on methods in `bun test`
sidebarTitle: Spy on methods
mode: center
---

Use the `spyOn` utility to track method calls with Bun's test runner.

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

const leo = {
  name: "Leonardo",
  sayHi(thing: string) {
    console.log(`Sup I'm ${this.name} and I like ${thing}`);
  },
};

const spy = spyOn(leo, "sayHi");
```

---

Once the spy is created, it can be used to write `expect` assertions relating to method calls.

{/* prettier-ignore */}
```ts
import { test, expect, spyOn } from "bun:test";

const leo = {
  name: "Leonardo",
  sayHi(thing: string) {
    console.log(`Sup I'm ${this.name} and I like ${thing}`);
  },
};

const spy = spyOn(leo, "sayHi");

test("turtles", () => { // [!code ++]
  expect(spy).toHaveBeenCalledTimes(0); // [!code ++]
  leo.sayHi("pizza"); // [!code ++]
  expect(spy).toHaveBeenCalledTimes(1); // [!code ++]
  expect(spy.mock.calls).toEqual([["pizza"]]); // [!code ++]
}); // [!code ++]
```

---

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