# Bun Use snapshot testing in `bun test`

> Use snapshot testing in `bun test`

- Skill: `jarle/bun-use-snapshot-testing-in-bun-test` (Agent Skill)
- Install (CLI): `npx skillmds@latest add jarle/bun-use-snapshot-testing-in-bun-test`
- Raw SKILL.md: https://api.skillmd.com/api/skills/jarle/bun-use-snapshot-testing-in-bun-test/raw
- Safety review: pending (external: skill-scanner PASS, skillspector PASS)
- Works with: Claude Code, Claude.ai, OpenAI Codex
- Category: Coding & Dev Tools
- Author: jarle (https://skillmd.com/u/jarle)
- Updated: 2026-09-21
- Page: https://skillmd.com/skills/jarle/bun-use-snapshot-testing-in-bun-test

---


# Use snapshot testing in `bun test`

Bun's test runner supports Jest-style snapshot testing via `.toMatchSnapshot()`.

```ts snap.test.ts icon="https://mintcdn.com/bun-1dd33a4e/nIz6GtMH5K-dfXeV/icons/typescript.svg?fit=max&auto=format&n=nIz6GtMH5K-dfXeV&q=85&s=5d73d76daf7eb7b158469d8c30d349b0" theme={"theme":{"light":"github-light","dark":"dracula"}}
import { test, expect } from "bun:test";

test("snapshot", () => {
  expect({ foo: "bar" }).toMatchSnapshot();
});
```

***

The first time this test is executed, Bun will evaluate the value passed into `expect()` and write it to disk in a directory called `__snapshots__` that lives alongside the test file. (Note the `snapshots: +1 added` line in the output.)

```sh terminal icon="terminal" theme={"theme":{"light":"github-light","dark":"dracula"}}
bun test test/snap
```

```txt  theme={"theme":{"light":"github-light","dark":"dracula"}}
test/snap.test.ts:
✓ snapshot [1.48ms]

 1 pass
 0 fail
 snapshots: +1 added
 1 expect() calls
Ran 1 tests across 1 files. [82.00ms]
```

***

The `__snapshots__` directory contains a `.snap` file for each test file in the directory.

```txt File Tree icon="folder-tree" theme={"theme":{"light":"github-light","dark":"dracula"}}
test
├── __snapshots__
│   └── snap.test.ts.snap
└── snap.test.ts
```

***

The `snap.test.ts.snap` file is a JavaScript file that exports a serialized version of the value passed into `expect()`. The `{foo: "bar"}` object has been serialized to JSON.

```js snap.test.ts.snap icon="file-code" theme={"theme":{"light":"github-light","dark":"dracula"}}
// Bun Snapshot v1, https://bun.com/docs/test/snapshots

exports[`snapshot 1`] = `
{
  "foo": "bar",
}
`;
```

***

Later, when this test file is executed again, Bun will read the snapshot file and compare it to the value passed into `expect()`. If the values are different, the test will fail.

```sh terminal icon="terminal" theme={"theme":{"light":"github-light","dark":"dracula"}}
bun test
```

```txt  theme={"theme":{"light":"github-light","dark":"dracula"}}
bun test v1.3.3 (9c68abdb)
test/snap.test.ts:
✓ snapshot [1.05ms]

 1 pass
 0 fail
 1 snapshots, 1 expect() calls
Ran 1 tests across 1 files. [101.00ms]
```

***

To update snapshots, use the `--update-snapshots` flag.

```sh terminal icon="terminal" theme={"theme":{"light":"github-light","dark":"dracula"}}
bun test --update-snapshots
```

```txt  theme={"theme":{"light":"github-light","dark":"dracula"}}
bun test v1.3.3 (9c68abdb)
test/snap.test.ts:
✓ snapshot [0.86ms]

 1 pass
 0 fail
 snapshots: +1 added  # the snapshot was regenerated
 1 expect() calls
Ran 1 tests across 1 files. [102.00ms]
```

***

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