bun test

Bun ships with a fast, built-in, Jest-compatible test runner. Tests are executed with the Bun runtime, and support the following features.

• TypeScript and JSX
• Lifecycle hooks
• Snapshot testing
• UI & DOM testing
• Watch mode with --watch
• Script pre-loading with --preload

Run tests

bun test

Tests are written in JavaScript or TypeScript with a Jest-like API. Refer to Writing tests for full documentation.

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

test("2 + 2", () => {
  expect(2 + 2).toBe(4);
});

The runner recursively searches the working directory for files that match the following patterns:

• *.test.{js|jsx|ts|tsx}
• *_test.{js|jsx|ts|tsx}
• *.spec.{js|jsx|ts|tsx}
• *_spec.{js|jsx|ts|tsx}

You can filter the set of test files to run by passing additional positional arguments to bun test. Any test file with a path that matches one of the filters will run. Commonly, these filters will be file or directory names; glob patterns are not yet supported.

bun test <filter> <filter> ...

To filter by test name, use the -t/--test-name-pattern flag.

# run all tests or test suites with "addition" in the name

bun test --test-name-pattern addition

The test runner runs all tests in a single process. It loads all --preload scripts (see Lifecycle for details), then runs all tests. If a test fails, the test runner will exit with a non-zero exit code.

Timeouts

Use the --timeout flag to specify a per-test timeout in milliseconds. If a test times out, it will be marked as failed. The default value is 5000.

# default value is 5000

bun test --timeout 20

Rerun tests

Use the --rerun-each flag to run each test multiple times. This is useful for detecting flaky or non-deterministic test failures.

bun test --rerun-each 100

Bail out with --bail

Use the --bail flag to abort the test run early after a pre-determined number of test failures. By default Bun will run all tests and report all failures, but sometimes in CI environments it's preferable to terminate earlier to reduce CPU usage.

# bail after 1 failure

bun test --bail

# bail after 10 failure

bun test --bail 10

Watch mode

Similar to bun run, you can pass the --watch flag to bun test to watch for changes and re-run tests.

bun test --watch

Lifecycle hooks

Bun supports the following lifecycle hooks:

These hooks can be define inside test files, or in a separate file that is preloaded with the --preload flag.

$ bun test --preload ./setup.ts

See Test > Lifecycle for complete documentation.

Mocks

Create mock functions with the mock function. Mocks are automatically reset between tests.

import { test, expect, mock } from "bun:test";
const random = mock(() => Math.random());

test("random", async () => {
  const val = random();
  expect(val).toBeGreaterThan(0);
  expect(random).toHaveBeenCalled();
  expect(random).toHaveBeenCalledTimes(1);
});

Alternatively, you can use jest.fn(), it behaves identically.

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

const random = mock(() => Math.random());
const random = jest.fn(() => Math.random());

See Test > Mocks for complete documentation.

Snapshot testing

Snapshots are supported by bun test.

// example usage of toMatchSnapshot
import { test, expect } from "bun:test";

test("snapshot", async () => {
  expect({ a: 1 }).toMatchSnapshot();
});

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

bun test --update-snapshots

See Test > Snapshots for complete documentation.

UI & DOM testing

Bun is compatible with popular UI testing libraries:

• HappyDOM
• DOM Testing Library
• React Testing Library

See Test > DOM Testing for complete documentation.

Performance

Bun's test runner is fast.