Snapshots

Learn how to use snapshot testing in Bun to save and compare output between test runs

Snapshot testing saves the output of a value and compares it against future test runs. Use it for UI components, complex objects, or any output that needs to remain consistent.

Basic Snapshots

Snapshot tests are written using the .toMatchSnapshot() matcher:

test.ts

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

test("snap", () => {
  expect("foo").toMatchSnapshot();
});

The first time this test runs, Bun serializes the argument to expect and writes it to a snapshot file in a __snapshots__ directory alongside the test file.

Snapshot Files

After the first run, Bun creates:

directory structure

your-project/
├── snap.test.ts
└── __snapshots__/
    └── snap.test.ts.snap

The snapshot file contains:

__snapshots__/snap.test.ts.snap

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

exports[`snap 1`] = `"foo"`;

On future runs, Bun compares the argument against the snapshot on disk.

Updating Snapshots

Regenerate snapshots with:

terminal

$ bun test --update-snapshots

Do this when you've intentionally changed the output or added new snapshot tests.

Inline Snapshots

For smaller values, use .toMatchInlineSnapshot(). Inline snapshots are stored directly in your test file:

test.ts

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

test("inline snapshot", () => {
  // First run: snapshot will be inserted automatically
  expect({ hello: "world" }).toMatchInlineSnapshot();
});

After the first run, Bun automatically updates your test file:

test.ts

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

test("inline snapshot", () => {
  expect({ hello: "world" }).toMatchInlineSnapshot(`
{
  "hello": "world",
}
`);
});

Using Inline Snapshots

Write your test with .toMatchInlineSnapshot()
Run the test once
Bun automatically updates your test file with the snapshot
On subsequent runs, Bun compares the value against the inline snapshot

Error Snapshots

You can also snapshot error messages with .toThrowErrorMatchingSnapshot() and .toThrowErrorMatchingInlineSnapshot():

test.ts

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

test("error snapshot", () => {
  expect(() => {
    throw new Error("Something went wrong");
  }).toThrowErrorMatchingSnapshot();

  expect(() => {
    throw new Error("Another error");
  }).toThrowErrorMatchingInlineSnapshot();
});

After running, the inline version becomes:

test.ts

test("error snapshot", () => {
  expect(() => {
    throw new Error("Something went wrong");
  }).toThrowErrorMatchingSnapshot();

  expect(() => {
    throw new Error("Another error");
  }).toThrowErrorMatchingInlineSnapshot(`"Another error"`);
});

Advanced Snapshot Usage

Complex Objects

Snapshots work well with complex nested objects:

test.ts

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

test("complex object snapshot", () => {
  const user = {
    id: 1,
    name: "John Doe",
    email: "john@example.com",
    profile: {
      age: 30,
      preferences: {
        theme: "dark",
        notifications: true,
      },
    },
    tags: ["developer", "javascript", "bun"],
  };

  expect(user).toMatchSnapshot();
});

Array Snapshots

Arrays are also well-suited for snapshot testing:

test.ts

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

test("array snapshot", () => {
  const numbers = [1, 2, 3, 4, 5].map(n => n * 2);
  expect(numbers).toMatchSnapshot();
});

Function Output Snapshots

Snapshot the output of functions:

test.ts

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

function generateReport(data: any[]) {
  return {
    total: data.length,
    summary: data.map(item => ({ id: item.id, name: item.name })),
    timestamp: "2024-01-01", // Fixed for testing
  };
}

test("report generation", () => {
  const data = [
    { id: 1, name: "Alice", age: 30 },
    { id: 2, name: "Bob", age: 25 },
  ];

  expect(generateReport(data)).toMatchSnapshot();
});

React Component Snapshots

Snapshots work well for React components:

test.ts

import { test, expect } from "bun:test";
import { render } from "@testing-library/react";

function Button({ children, variant = "primary" }) {
  return <button className={`btn btn-${variant}`}>{children}</button>;
}

test("Button component snapshots", () => {
  const { container: primary } = render(<Button>Click me</Button>);
  const { container: secondary } = render(<Button variant="secondary">Cancel</Button>);

  expect(primary.innerHTML).toMatchSnapshot();
  expect(secondary.innerHTML).toMatchSnapshot();
});

Property Matchers

For values that change between test runs (like timestamps or IDs), use property matchers:

test.ts

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

test("snapshot with dynamic values", () => {
  const user = {
    id: Math.random(), // This changes every run
    name: "John",
    createdAt: new Date().toISOString(), // This also changes
  };

  expect(user).toMatchSnapshot({
    id: expect.any(Number),
    createdAt: expect.any(String),
  });
});

The snapshot file stores:

snapshot file

exports[`snapshot with dynamic values 1`] = `
{
  "createdAt": Any<String>,
  "id": Any<Number>,
  "name": "John",
}
`;

Best Practices

Keep Snapshots Small

test.ts

// Good: Focused snapshots
test("user name formatting", () => {
  const formatted = formatUserName("john", "doe");
  expect(formatted).toMatchInlineSnapshot(`"John Doe"`);
});

// Avoid: Huge snapshots that are hard to review
test("entire page render", () => {
  const page = renderEntirePage();
  expect(page).toMatchSnapshot(); // This could be thousands of lines
});

Use Descriptive Test Names

test.ts

// Good: Clear what the snapshot represents
test("formats currency with USD symbol", () => {
  expect(formatCurrency(99.99)).toMatchInlineSnapshot(`"$99.99"`);
});

// Avoid: Unclear what's being tested
test("format test", () => {
  expect(format(99.99)).toMatchInlineSnapshot(`"$99.99"`);
});

test.ts

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

describe("Button component", () => {
  test("primary variant", () => {
    expect(render(<Button variant="primary">Click</Button>))
      .toMatchSnapshot();
  });

  test("secondary variant", () => {
    expect(render(<Button variant="secondary">Cancel</Button>))
      .toMatchSnapshot();
  });

  test("disabled state", () => {
    expect(render(<Button disabled>Disabled</Button>))
      .toMatchSnapshot();
  });
});

Handle Dynamic Data

test.ts

// Good: Normalize dynamic data
test("API response format", () => {
  const response = {
    data: { id: 1, name: "Test" },
    timestamp: Date.now(),
    requestId: generateId(),
  };

  expect({
    ...response,
    timestamp: "TIMESTAMP",
    requestId: "REQUEST_ID",
  }).toMatchSnapshot();
});

// Or use property matchers
test("API response with matchers", () => {
  const response = getApiResponse();

  expect(response).toMatchSnapshot({
    timestamp: expect.any(Number),
    requestId: expect.any(String),
  });
});

Managing Snapshots

Reviewing Snapshot Changes

When snapshots change, carefully review them:

terminal

# See what changed
$ git diff __snapshots__/

# Update if changes are intentional
$ bun test --update-snapshots

# Commit the updated snapshots
$ git add __snapshots__/
$ git commit -m "Update snapshots after UI changes"

Organizing Large Snapshot Files

For large projects, consider organizing tests to keep snapshot files manageable:

directory structure

tests/
├── components/
│   ├── Button.test.tsx
│   └── __snapshots__/
│       └── Button.test.tsx.snap
├── utils/
│   ├── formatters.test.ts
│   └── __snapshots__/
│       └── formatters.test.ts.snap

Troubleshooting

Snapshot Failures

When snapshots fail, you'll see a diff:

diff

- Expected
+ Received

  Object {
-   "name": "John",
+   "name": "Jane",
  }

Common causes:

Intentional changes (update with --update-snapshots)
Unintentional changes (fix the code)
Dynamic data (use property matchers)
Environment differences (normalize the data)

Platform Differences

Be aware of platform-specific differences:

test.ts

// Paths might differ between Windows/Unix
test("file operations", () => {
  const result = processFile("./test.txt");

  expect({
    ...result,
    path: result.path.replace(/\\/g, "/"), // Normalize paths
  }).toMatchSnapshot();
});

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