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Testing philosophy ​

Correctness is defined by Node, never by the compiler's own expectations. A test never asserts "the compiler printed what we think it should"; it asserts "the binary printed what Node printed".

Counted from the repository when this page was built: 433 programs checked against Node at -O0 and -O2 (test fixtures, examples/ and every sample on this site), 157 programs that must be rejected with a specific code, 5 seeded fuzzers, 49 documented error codes, and 1 known bugs recorded as tests (listed on the limitations page).

  • Differential suite (tests/fixtures/run/, plus examples/ and every sample on this site): each program runs under Node and as a native binary at -O0 and at -O2. stdout and the exit code must match exactly; a crash or hang is always a failure. The emitted IR must also pass opt -passes=verify. An -O0/-O2 mismatch means undefined behavior in the generated code.
  • Rejection suite (tests/fixtures/reject/): each file names the CS#### code it must fail with. Every validator rule has at least one.
  • Known bugs are fixtures marked @known-bug: they must keep diverging from Node, so a fix cannot land unnoticed and a bug cannot be forgotten.
  • Seeded fuzzers generate programs inside the subset and diff them against Node: expressions and control flow, structural subtyping (reordered and extra fields across interfaces, literals and classes), unions, closures with generics, and console.log formatting.
  • Sanitized lane: the whole suite again with the runtime and every program built under AddressSanitizer and UndefinedBehaviorSanitizer.
  • GC stress lane: the differential suite with a collection forced every few allocations, so a missed root or a wrong layout shows up as a divergence.
  • Architecture tests: only src/lower may import TypeScript, the runtime's struct layouts match the IR builder's view of them, file-size ratchets, and the generated docs match their sources.

The fast lane (bun run test) stays under ten seconds; the slow lanes run in the background and in CI on Linux and macOS.

Found a divergence? ​

A program that compiles but behaves differently from Node is treated as a P0 bug, never as a known quirk. Please report it with the program, Node's output and the binary's output. It becomes a @known-bug fixture first, then the fix.

Next: Benchmarks.