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Runtime behavior - Bun

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Runtime behavior - BunSkip to main contentBun home pageSearch...⌘KInstall BunSearch...NavigationTest ExecutionRuntime behaviorRuntimePackage ManagerBundlerTest RunnerGuidesReferenceBlogFeedbackdiv:first-child]:!hidden peer-[.is-custom]:[&>div:first-child]:sm:!hidden peer-[.is-custom]:[&>div:first-child]:md:!hidden peer-[.is-custom]:[&>div:first-child]:lg:!hidden peer-[.is-custom]:[&>div:first-child]:xl:!hidden">Getting StartedTest runnerWriting testsTest configurationTest ExecutionRuntime behaviorFinding testsTest FeaturesLifecycle hooksMocksSnapshotsDates and timesSpecialized TestingDOM testingReportingCode coverageTest ReportersOn this pageEnvironment VariablesNODE_ENVTZ (Timezone)Test TimeoutsGlobal TimeoutPer-Test TimeoutInfinite TimeoutError HandlingUnhandled ErrorsPromise RejectionsCustom Error HandlingCLI Flags IntegrationMemory UsageDebuggingModule LoadingInstallation-related FlagsWatch and Hot ReloadingWatch ModeHot ReloadingGlobal VariablesProcess IntegrationExit CodesSignal HandlingEnvironment DetectionPerformance ConsiderationsSingle ProcessMemory ManagementTest IsolationTest ExecutionRuntime behaviorCopy pagespan]:line-clamp-1 overflow-hidden group flex items-center py-0.5 gap-1 text-sm text-gray-950/50 dark:text-white/50 group-hover:text-gray-950/70 dark:group-hover:text-white/70 rounded-none rounded-r-xl border px-3 border-gray-200 aspect-square dark:border-white/[0.07] bg-background-light dark:bg-background-dark hover:bg-gray-600/5 dark:hover:bg-gray-200/5" aria-label="More actions" type="button" id="radix-_R_2shjinpfd9rqaabsnpfdb_" aria-haspopup="menu" aria-expanded="false" data-state="closed">*]:[overflow-wrap:anywhere]">Learn about Bun test’s runtime integration, environment variables, timeouts, and error handling

Copy pagespan]:line-clamp-1 overflow-hidden group flex items-center py-0.5 gap-1 text-sm text-gray-950/50 dark:text-white/50 group-hover:text-gray-950/70 dark:group-hover:text-white/70 rounded-none rounded-r-xl border px-3 border-gray-200 aspect-square dark:border-white/[0.07] bg-background-light dark:bg-background-dark hover:bg-gray-600/5 dark:hover:bg-gray-200/5" aria-label="More actions" type="button" id="radix-_R_5hjinpfd9rqaabsnpfdb_" aria-haspopup="menu" aria-expanded="false" data-state="closed">bun test is deeply integrated with Bun’s runtime. This is part of what makes bun test fast and simple to use. ​Environment Variables ​NODE_ENV bun test automatically sets $NODE_ENV to "test" unless it’s already set in the environment or via .env files. This is standard behavior for most test runners and helps ensure consistent test behavior. test.tsCopyimport { test, expect } from "bun:test"; test("NODE_ENV is set to test", () => { expect(process.env.NODE_ENV).toBe("test"); }); You can override this by setting NODE_ENV explicitly: terminalCopyNODE_ENV=development bun test ​TZ (Timezone) By default, all bun test runs use UTC (Etc/UTC) as the time zone unless overridden by the TZ environment variable. This ensures consistent date and time behavior across different development environments. test.tsCopyimport { test, expect } from "bun:test"; test("timezone is UTC by default", () => { const date = new Date(); expect(date.getTimezoneOffset()).toBe(0); }); To test with a specific timezone: terminalCopyTZ=America/New_York bun test ​Test Timeouts Each test has a default timeout of 5000ms (5 seconds) if not explicitly overridden. Tests that exceed this timeout will fail. ​Global Timeout Change the timeout globally with the --timeout flag: terminalCopybun test --timeout 10000 # 10 seconds ​Per-Test Timeout Set timeout per test as the third parameter to the test function: test.tsCopyimport { test, expect } from "bun:test"; test("fast test", () => { expect(1 + 1).toBe(2); }, 1000); // 1 second timeout test("slow test", async () => { await new Promise(resolve => setTimeout(resolve, 8000)); }, 10000); // 10 second timeout ​Infinite Timeout Use 0 or Infinity to disable timeout: test.tsCopytest("test without timeout", async () => { // This test can run indefinitely await someVeryLongOperation(); }, 0); ​Error Handling ​Unhandled Errors bun test tracks unhandled promise rejections and errors that occur between tests. If such errors occur, the final exit code will be non-zero (specifically, the count of such errors), even if all tests pass. This helps catch errors in asynchronous code that might otherwise go unnoticed: test.tsCopyimport { test } from "bun:test"; test("test 1", () => { // This test passes expect(true).toBe(true); }); // This error happens outside any test setTimeout(() => { throw new Error("Unhandled error"); }, 0); test("test 2", () => { // This test also passes expect(true).toBe(true); }); // The test run will still fail with a non-zero exit code // because of the unhandled error ​Promise Rejections Unhandled promise rejections are also caught: test.tsCopyimport { test } from "bun:test"; test("passing test", () => { expect(1).toBe(1); }); // This will cause the test run to fail Promise.reject(new Error("Unhandled rejection")); ​Custom Error Handling You can set up custom error handlers in your test setup: test-setup.tsCopyprocess.on("uncaughtException", error => { console.error("Uncaught Exception:", error); process.exit(1); }); process.on("unhandledRejection", (reason, promise) => { console.error("Unhandled Rejection at:", promise, "reason:", reason); process.exit(1); }); ​CLI Flags Integration Several Bun CLI flags can be used with bun test to modify its behavior: ​Memory Usage terminalCopy# Reduces memory usage for the test runner VM bun test --smol ​Debugging terminalCopy# Attaches the debugger to the test runner process bun test --inspect bun test --inspect-brk ​Module Loading terminalCopy# Runs scripts before test files (useful for global setup/mocks) bun test --preload ./setup.ts # Sets compile-time constants bun test --define "process.env.API_URL='http://localhost:3000'" # Configures custom loaders bun test --loader .special:special-loader # Uses a different tsconfig bun test --tsconfig-override ./test-tsconfig.json # Sets package.json conditions for module resolution bun test --conditions development # Loads environment variables for tests bun test --env-file .env.test ​Installation-related Flags Copy# Affect any network requests or auto-installs during test execution bun test --prefer-offline bun test --frozen-lockfile ​Watch and Hot Reloading ​Watch Mode When running bun test with the --watch flag, the test runner will watch for file changes and re-run affected tests. terminalCopybun test --watch The test runner is smart about which tests to re-run: math.test.tsCopyimport { add } from "./math.js"; import { test, expect } from "bun:test"; test("addition", () => { expect(add(2, 3)).toBe(5); }); If you modify math.js, only math.test.ts will re-run, not all tests. ​Hot Reloading The --hot flag provides similar functionality but is more aggressive about trying to preserve state between runs: terminalCopybun test --hot For most test scenarios, --watch is the recommended option as it provides better isolation between test runs. ​Global Variables The following globals are automatically available in test files without importing (though they can be imported from bun:test if preferred): test.tsCopy// All of these are available globally test("global test function", () => { expect(true).toBe(true); }); describe("global describe", () => { beforeAll(() => { // global beforeAll }); it("global it function", () => { // it is an alias for test }); }); // Jest compatibility jest.fn(); // Vitest compatibility vi.fn(); You can also import them explicitly if you prefer: test.tsCopyimport { test, it, describe, expect, beforeAll, beforeEach, afterAll, afterEach, jest, vi } from "bun:test"; ​Process Integration ​Exit Codes bun test uses standard exit codes: 0: All tests passed, no unhandled errors 1: Test failures occurred >1: Number of unhandled errors (even if tests passed) ​Signal Handling The test runner properly handles common signals: terminalCopy# Gracefully stops test execution kill -SIGTERM test-process-pid> # Immediately stops test execution kill -SIGKILL test-process-pid> ​Environment Detection Bun automatically detects certain environments and adjusts behavior: test.tsCopy// GitHub Actions detection if (process.env.GITHUB_ACTIONS) { // Bun automatically emits GitHub Actions annotations } // CI detection if (process.env.CI) { // Certain behaviors may be adjusted for CI environments } ​Performance Considerations ​Single Process The test runner runs all tests in a single process by default. This provides: Faster startup - No need to spawn multiple processes Shared memory - Efficient resource usage Simple debugging - All tests in one process However, this means: Tests share global state (use lifecycle hooks to clean up) One test crash can affect others No true parallelization of individual tests ​Memory Management terminalCopy# Monitor memory usage bun test --smol # Reduces memory footprint # For large test suites, consider splitting files bun test src/unit/ bun test src/integration/ ​Test Isolation Since tests run in the same process, ensure proper cleanup: test.tsCopyimport { afterEach } from "bun:test"; afterEach(() => { // Clean up global state global.myGlobalVar = undefined; delete process.env.TEST_VAR; // Reset modules if needed jest.resetModules(); });

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