Use snapshot testing in `bun test`

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

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.)

$ bun test test/snap
bun test v1.3.5 (9c68abdb)

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.

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.

// 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.

$ bun test
bun test v1.3.5 (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.

$ bun test --update-snapshots
bun test v1.3.5 (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 for complete documentation on snapshots with the Bun test runner.