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

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.

File Tree

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.

snap.test.ts.snap

// Bun Snapshot v1, https://bun.com/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

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.

terminal

bun test --update-snapshots

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 Snapshots.