ts-testing

Choose bun test for greenfield Bun projects, fast local TDD, and zero-config TS/JSX. Choose Vitest for large suites needing multi-worker CI parallelism, mature V8/Istanbul coverage with thresholds, Jest migration, or real-browser component testing via Playwright.

Install vitest and @vitest/coverage-v8, then configure test.coverage in vitest.config.ts with provider v8 and thresholds for lines, functions, branches, and statements. CI fails automatically when coverage drops below the configured targets.

No, never mix bun:test and vitest imports in one package. Also run bun run test instead of bun test when Vitest is the configured runner, otherwise Bun executes its own runner instead of Vitest.

Bun test supports coverage via the --coverage flag with text and lcov reporters, configurable in bunfig.toml. However, coverage is experimental and numbers can shift across releases, so it should not be the sole quality gate on large suites.

Test bodies using describe, it, expect, snapshots, and lifecycle hooks rarely change since both runners are Jest-compatible. The migration cost is config, coverage wiring, and mock APIs: mock.module maps to vi.mock, and jest.fn maps to vi.fn.

Bun's mock.module may not intercept re-exports and dynamic import() calls as expected, so verify mocks actually apply. Bun also runs tests in a single process by default, limiting worker-level isolation for heavy suites compared to Vitest's forks or threads pools.