js2: Instructions file for Claude Code

js2wasm

TypeScript-to-WebAssembly compiler using WasmGC.

⚠ Direction: standalone only — the JS-host lane is being sunset (project-lead decision, 2026-10-10)

Applies to every session and agent. All new work targets the two no-host targets only:

• --target standalone: pure WasmGC, no JS host.
• The Linear/WASI native backend: src/codegen-linear/ and the IR Linear backend.

Consequences:

• No new JS-host work. Do not add JS-host IR paths, new host imports, or host-only fast paths. Do not prioritise test262 host-lane (gc) results over standalone.
• Existing JS-host code stays until it is retired. Do not delete it opportunistically: removing it needs its own reviewed change. Until then, keep gc output byte-identical when you change shared code.
• The "One native semantic core" principle below replaces the older "Dual-mode: JS host optional" one (#6708).

⚠ IR path: code quality comes first (project-lead rule, 2026-10-22)

Applies to every change under src/ir/ and to the codegen seams the IR calls. Quality outranks coverage: never take a claim faster at the cost of messier code, because the IR is meant to replace the legacy hacks, not reproduce them.

What "clean" means here:

• Modular. One concern per module, following the IR layering (core, dialects, selection, lowering, backend). A new arm goes next to its siblings, in the same shape: same file layout, same naming, same helper pattern.
• No duplication. Before adding a predicate, wrapper, lowering arm or helper, look for an existing one and extend it. Two arms that differ by a name or a flag become one parameterised arm. Copy-paste is a defect, not a shortcut.
• No AI slop. That means:
  • no long files or functions (split by concern instead of growing them);
  • no inconsistent structure between sibling arms;
  • no defensive branches for cases that cannot occur;
  • no speculative abstractions with one user;
  • no dead parameters;
  • no comments that narrate the code instead of explaining why.
• Size budgets are a ceiling, not a target. A func-budget-allow: or loc-budget-allow: grant is the exception, and the review must agree that the growth cannot reasonably be split.