sveltejs/svelte

Testing

Active contributors: Rich Harris, Simon H, Dominic Gannaway

Test runner

Vitest configured in vitest.config.js and vitest-xhtml-environment.ts. The top-level pnpm test runs vitest run against everything; per-suite filters are how most contributors operate.

Where tests live

All Svelte tests are under packages/svelte/tests/. The directory is organised by what the test exercises rather than by the source layout:

Directory	What it covers
`tests/runtime-runes/`	Component runtime tests in runes mode (Svelte 5)
`tests/runtime-legacy/`	Component runtime tests in legacy mode (Svelte 4 compatibility)
`tests/runtime-browser/`	Browser-only runtime tests (run via Playwright Chromium)
`tests/runtime-production/`	Tests asserting production-mode behaviour (no DEV-only code paths)
`tests/runtime-xhtml/`	XHTML quirks
`tests/server-side-rendering/`	SSR output tests against the server runtime
`tests/hydration/`	Hydration correctness on top of SSR'd HTML
`tests/parser-modern/`, `tests/parser-legacy/`	Parser snapshot tests (compare against `.expected` JSON)
`tests/validator/`	Compile-time error and warning emission tests
`tests/css/`, `tests/css-parse.test.ts`	CSS pruning, encapsulation, parser
`tests/snapshot/`	Code-generation snapshots (compiler output diffed against `.expected.js` files)
`tests/sourcemaps/`	Source-map correctness
`tests/preprocess/`	`svelte/compiler#preprocess`
`tests/migrate/`	The Svelte 4 → 5 codemod
`tests/types/`	TypeScript-level checks (`tsc` is run against this dir as part of `pnpm check`)
`tests/store/`, `tests/motion/`, `tests/signals/`	Standalone unit tests for those modules
`tests/manual/`	Manual-only repros
`tests/print/`	`svelte/compiler#print` round-tripping
`tests/compiler-errors/`	Compiler errors via the public `compile()` entry point

The shared harness lives in tests/suite.ts and tests/helpers.js. tests/animation-helpers.js provides timer/transition stubs for runtime tests.

Running a focused test

# Whole suite
pnpm test runtime-runes

# Single test name (matches via vitest's -t)
pnpm test validator -t a11y-alt-text

# Faster iteration: drop the other tests entirely
FILTER=a11y-alt-text pnpm test validator

Snapshot updating

Snapshot-based suites (parser, snapshot, hydration, ssr) write the expected output to disk and compare. To regenerate after an intentional change:

UPDATE_SNAPSHOTS=true pnpm test

Inspect the resulting diff carefully — overly broad updates are a common review nit.

Browser tests

tests/runtime-browser/ uses Playwright (@playwright/test) running headless Chromium. Install with pnpm playwright install chromium (CI does this every run).

Async-aware effects live behind a runtime flag in packages/svelte/src/internal/flags/async.js. CI runs runtime-runes twice — once with async mode on (default) and once with SVELTE_NO_ASYNC=true (the TestNoAsync job). When you add a runtime test that doesn't depend on async features, both jobs should pass.

Benchmarks

Benchmarks (benchmarking/) are not part of the test suite, but CI runs pnpm bench to detect catastrophic regressions. Use pnpm bench:compare locally to compare your branch against another commit.

Writing a test

Most test files are .test.ts next to their helpers, or _config.js + main.svelte pairs under a sample folder. Look at neighbouring tests in the suite you're modifying — they're highly regular.