Verification

Supercov instruments source before the test command runs. Seven release gates check that the instrumented program matches the original program and that coverage calculations are correct. Any failed gate blocks publication.

1. Semantic differential execution

Original and instrumented programs are executed in isolated scopes and compared on three axes: return values, thrown errors, and the observable order of side effects.

The fixtures deliberately target the places where a naive transform breaks: getters, proxies, optional calls and this binding, computed logical assignments, parameter defaults, try/catch/finally, iterator closing, switch fallthrough, labelled loops, async functions and generators.

2. Deterministic generated corpus

A generated corpus exercises 160 nested combinations of short-circuiting, ternaries, coercion and thrown expressions on every run. It is deterministic, so a regression reproduces exactly rather than appearing once in CI and never again.

3. Property testing

Seeded fast-check properties generate a further 500 nested expressions and 300 control-flow executions per run, with shrinking and a reproducible seed printed on failure.

4. Coverage oracles

Behaviour equivalence is not enough — the numbers have to be right too. Separate oracles assert exact decision vectors, MC/DC witnesses and branch alternatives independently of what the program does.

5. An independent MC/DC implementation

The same three-condition masking-MC/DC golden cases must produce identical verdicts under Supercov and under Clang/LLVM source-based MC/DC: 100% for a complete witness set and 33.33% for an incomplete one.

This is the gate that matters most. MC/DC has enough subtlety — masking versus unique-cause, short-circuit evaluation, compound conditions — that agreement with an independently implemented, widely audited toolchain is far stronger evidence than any self-consistent test suite.

6. TC39 Test262

Release CI shards the pinned Test262 corpus across 16 workers, runs the official harness against original and instrumented sources, and rejects any scenario that passes originally but fails after transformation.

Some categories are excluded by construction, with reason counts printed for every shard:

The last category is handled in the product, not hidden: when application code directly coerces or observes a function’s source, Supercov leaves that body uninstrumented and records a visible semantic-safety completeness blocker. Dedicated differential fixtures cover the async and generator cases that Test262 cannot compare.

7. Performance budgets

Transform latency, transactional workspace preparation, output expansion and runtime probe overhead are each checked against explicit budgets. A change that makes instrumentation correct but unusably slow fails the same way a wrong answer does.

Cross-platform and compatibility gates

Alongside the seven correctness gates, the compatibility workflow runs Node 22, 24 and 25, Playwright 1.55 and current, Vite 5 and current, Vitest 2 and current, Chromium, Firefox and WebKit, and modern JavaScript, JSX, TypeScript and TSX syntax fixtures.

Filesystem publication, symlink handling, copy fallback, ENOSPC, failed rename and forced-termination recovery are all exercised on Ubuntu, macOS and Windows.

A clean-room gate packs the npm tarball, invokes it through npx in a project with no build step, and asserts that not a single source or configuration file changed.

Running the gates yourself

npm test
npm run test:clang-mcdc
npm run benchmark:check
TEST262_DIR=/path/to/test262 npm run test:test262

The Clang/LLVM oracle requires clang and llvm to be installed. The Test262 gate requires a checkout of the pinned corpus.