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

Bun aims for compatibility with Jest, but not everything is implemented. To track compatibility, see this tracking issue.

Run tests

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bun test

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

https://mintcdn.com/bun-1dd33a4e/Hq64iapoQXHbYMEN/icons/typescript.svg?fit=max&auto=format&n=Hq64iapoQXHbYMEN&q=85&s=c6cceedec8f82d2cc803d7c6ec82b240

math.test.ts

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.

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bun test <filter> <filter> ...

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

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

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

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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:

.github/workflows/test.yml

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.

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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.

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# 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:

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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:

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# 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:

math.test.ts

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:

math.test.ts

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.

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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.

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bun test --randomize

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

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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.

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# 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.

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# 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.

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bun test --watch

Lifecycle hooks

Bun supports the following lifecycle hooks:

Hook	Description
`beforeAll`	Runs once before all tests.
`beforeEach`	Runs before each test.
`afterEach`	Runs after each test.
`afterAll`	Runs once after all tests.

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

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bun test --preload ./setup.ts

See Test > Lifecycle for complete documentation.

Mocks

Create mock functions with the mock function.

math.test.ts

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.

math.test.ts

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.

math.test.ts

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

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

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

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bun test --update-snapshots

See Test > Snapshots for complete documentation.

UI & DOM testing

Bun is compatible with popular UI testing libraries:

See Test > DOM Testing for complete documentation.

Performance

Bun’s test runner is fast.

Running 266 React SSR tests faster than Jest can print its version number.

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

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# 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.

CLI Usage

bun test <patterns>

Execution Control

--timeout

number

default:"5000"

Set the per-test timeout in milliseconds (default 5000)

--rerun-each

number

Re-run each test file NUMBER times, helps catch certain bugs

--concurrent

boolean

Treat all tests as test.concurrent() tests

--randomize

boolean

Run tests in random order

--seed

number

Set the random seed for test randomization

--bail

number

default:"1"

Exit the test suite after NUMBER failures. If you do not specify a number, it defaults to 1.

--max-concurrency

number

default:"20"

Maximum number of concurrent tests to execute at once (default 20)

Test Filtering

--todo

boolean

Include tests that are marked with test.todo()

--test-name-pattern

string

Run only tests with a name that matches the given regex. Alias: -t

Reporting

--reporter

string

Test output reporter format. Available: junit (requires —reporter-outfile), dots. Default: console output.

--reporter-outfile

string

Output file path for the reporter format (required with —reporter)

--dots

boolean

Enable dots reporter. Shorthand for —reporter=dots

Coverage

--coverage

boolean

Generate a coverage profile

--coverage-reporter

string

default:"text"

Report coverage in text and/or lcov. Defaults to text

--coverage-dir

string

default:"coverage"

Directory for coverage files. Defaults to coverage

Snapshots

--update-snapshots

boolean

Update snapshot files. Alias: -u

Examples

Run all test files:

terminal

bun test

Run all test files with “foo” or “bar” in the file name:

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bun test foo bar

Run all test files, only including tests whose names includes “baz”:

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bun test --test-name-pattern baz

Getting Started

Test Execution

Test Features

Specialized Testing

Reporting

Test runner

Run tests

CI/CD integration

GitHub Actions

How to install `bun` in a GitHub Actions workflow

JUnit XML reports (GitLab, etc.)

Timeouts

Concurrent test execution

`--concurrent` flag

`--max-concurrency` flag

`test.concurrent`

`test.serial`

Rerun tests

Randomize test execution order

Reproducible random order with `--seed`

Bail out with `--bail`

Watch mode

Lifecycle hooks

Mocks

Snapshot testing

UI & DOM testing

Performance

AI Agent Integration

Environment Variables

Behavior

CLI Usage

Execution Control

Test Filtering

Reporting

Coverage

Snapshots

Examples

Getting Started

Test Execution

Test Features

Specialized Testing

Reporting

​Run tests

​CI/CD integration

​GitHub Actions

​How to install bun in a GitHub Actions workflow

​JUnit XML reports (GitLab, etc.)

​Timeouts

​Concurrent test execution

​--concurrent flag

​--max-concurrency flag

​test.concurrent

​test.serial

​Rerun tests

​Randomize test execution order

​Reproducible random order with --seed

​Bail out with --bail

​Watch mode

​Lifecycle hooks

​Mocks

​Snapshot testing

​UI & DOM testing

​Performance

​AI Agent Integration

​Environment Variables

​Behavior

​CLI Usage

​Execution Control

​Test Filtering

​Reporting

​Coverage

​Snapshots

​Examples

Run tests

CI/CD integration

GitHub Actions

How to install `bun` in a GitHub Actions workflow

JUnit XML reports (GitLab, etc.)

Timeouts

Concurrent test execution

`--concurrent` flag

`--max-concurrency` flag

`test.concurrent`

`test.serial`

Rerun tests

Randomize test execution order

Reproducible random order with `--seed`

Bail out with `--bail`

Watch mode

Lifecycle hooks

Mocks

Snapshot testing

UI & DOM testing

Performance

AI Agent Integration

Environment Variables

Behavior

CLI Usage

Execution Control

Test Filtering

Reporting

Coverage

Snapshots

Examples