Testing

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

When no tests match the filter, bun test exits with code 1.

To run a specific file in the test runner, make sure the path starts with ./ or / to distinguish it from a filter name.

bun test ./test/specific-file.test.ts

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.

CI/CD integration

bun test supports a variety of CI/CD integrations.

GitHub Actions

bun test automatically detects if it's running inside GitHub Actions and will emit GitHub Actions annotations to the console directly.

No configuration is needed, other than installing bun in the workflow and running bun test.

How to install bun in a GitHub Actions workflow

To use bun test in a GitHub Actions workflow, add the following step:

jobs:
  build:
    name: build-app
    runs-on: ubuntu-latest
    steps:
      - name: Checkout
        uses: actions/checkout@v4
      - name: Install bun
        uses: oven-sh/setup-bun@v2
      - name: Install dependencies # (assuming your project has dependencies)
        run: bun install # You can use npm/yarn/pnpm instead if you prefer
      - name: Run tests
        run: bun test

From there, you'll get GitHub Actions annotations.

JUnit XML reports (GitLab, etc.)

To use bun test with a JUnit XML reporter, you can use the --reporter=junit in combination with --reporter-outfile.

bun test --reporter=junit --reporter-outfile=./bun.xml

This will continue to output to stdout/stderr as usual, and also write a JUnit XML report to the given path at the very end of the test run.

JUnit XML is a popular format for reporting test results in CI/CD pipelines.

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

Concurrent test execution

By default, Bun runs all tests sequentially within each test file. You can enable concurrent execution to run async tests in parallel, significantly speeding up test suites with independent tests.

--concurrent flag

Use the --concurrent flag to run all tests concurrently within their respective files:

bun test --concurrent

When this flag is enabled, all tests will run in parallel unless explicitly marked with test.serial.

--max-concurrency flag

Control the maximum number of tests running simultaneously with the --max-concurrency flag:

# Limit to 4 concurrent tests

bun test --concurrent --max-concurrency 4

# Default: 20

bun test --concurrent

This helps prevent resource exhaustion when running many concurrent tests. The default value is 20.

test.concurrent

Mark individual tests to run concurrently, even when the --concurrent flag is not used:

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

// These tests run in parallel with each other
test.concurrent("concurrent test 1", async () => {
  await fetch("/api/endpoint1");
  expect(true).toBe(true);
});

test.concurrent("concurrent test 2", async () => {
  await fetch("/api/endpoint2");
  expect(true).toBe(true);
});

// This test runs sequentially
test("sequential test", () => {
  expect(1 + 1).toBe(2);
});

test.serial

Force tests to run sequentially, even when the --concurrent flag is enabled:

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

let sharedState = 0;

// These tests must run in order
test.serial("first serial test", () => {
  sharedState = 1;
  expect(sharedState).toBe(1);
});

test.serial("second serial test", () => {
  // Depends on the previous test
  expect(sharedState).toBe(1);
  sharedState = 2;
});

// This test can run concurrently if --concurrent is enabled
test("independent test", () => {
  expect(true).toBe(true);
});

// Chaining test qualifiers
test.failing.each([1, 2, 3])("chained qualifiers %d", input => {
  expect(input).toBe(0); // This test is expected to fail for each input
});

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

Randomize test execution order

Use the --randomize flag to run tests in a random order. This helps detect tests that depend on shared state or execution order.

bun test --randomize

When using --randomize, the seed used for randomization will be displayed in the test summary:

bun test --randomize

# ... test output ...
 --seed=12345
 2 pass
 8 fail
Ran 10 tests across 2 files. [50.00ms]

Reproducible random order with --seed

Use the --seed flag to specify a seed for the randomization. This allows you to reproduce the same test order when debugging order-dependent failures.

# Reproduce a previous randomized run

bun test --seed 123456

The --seed flag implies --randomize, so you don't need to specify both. Using the same seed value will always produce the same test execution order, making it easier to debug intermittent failures caused by test interdependencies.

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 defined 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.

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

test("random", () => {
  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", () => {
  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.

AI Agent Integration

When using Bun's test runner with AI coding assistants, you can enable quieter output to improve readability and reduce context noise. This feature minimizes test output verbosity while preserving essential failure information.

Environment Variables

Set any of the following environment variables to enable AI-friendly output:

• CLAUDECODE=1 - For Claude Code
• REPL_ID=1 - For Replit
• AGENT=1 - Generic AI agent flag

Behavior

When an AI agent environment is detected:

• Only test failures are displayed in detail
• Passing, skipped, and todo test indicators are hidden
• Summary statistics remain intact

# Example: Enable quiet output for Claude Code

CLAUDECODE=1 bun test

# Still shows failures and summary, but hides verbose passing test output

This feature is particularly useful in AI-assisted development workflows where reduced output verbosity improves context efficiency while maintaining visibility into test failures.