Use snapshot testing in `bun test`

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

snap.test.ts

import { test, expect } from "bun:test";

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

The first time this test runs, Bun evaluates the value passed into expect() and writes it to a __snapshots__ directory alongside the test file. (Note the snapshots: +1 added line in the output.)

terminal

bun test test/snap

Text

test/snap.test.ts:
✓ snapshot [1.48ms]

 1 pass
 0 fail
 snapshots: +1 added
 1 expect() calls
Ran 1 test across 1 file. [82.00ms]

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

File

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(). Bun pretty-prints the {foo: "bar"} object in Jest's snapshot format, which is not strict JSON (note the trailing comma).

snap.test.ts.snap

// Bun Snapshot v1, https://bun.sh/docs/test/snapshots

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

On later runs, Bun reads the snapshot file and compares it to the value passed into expect(). If they differ, the test fails.

terminal

bun test

Text

bun test v1.3.3 (9c68abdb)
test/snap.test.ts:
✓ snapshot [1.05ms]

 1 pass
 0 fail
 1 snapshots, 1 expect() calls
Ran 1 test across 1 file. [101.00ms]

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

terminal

bun test --update-snapshots

Text

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 test across 1 file. [102.00ms]

See Snapshots.