Running Test262

running-test262

The fleet has multiple parsers + runtimes that conform to ECMA262 or to a TC39 proposal:

• ultrathink/packages/acorn/: the JS parser, multiple lang ports (cpp/go/rust/typescript).
• ultrathink/packages/test262-parser-runner/: the canonical shared runner package.
• socket-btm/packages/temporal-infra/: Temporal-proposal C++ port.

Every one of them ships its own scripts/test262-*.mts runner + an unsupported-features config. Running test262 by hand (downloading the suite, scanning the metadata blocks, running each test) is the wrong shape. The runners already encode the suite-traversal, the per-feature skip logic, the harness setup, and the result-aggregation. Always reach for the existing runner.

Test262 submodule pin

The fleet pins to a shared tc39/test262 SHA. As of 2026-05-21 both ultrathink + socket-btm pin 7e115f46a. When bumping in one repo, bump in the other so cross-fleet comparison stays apples-to-apples.

Annotation lives in each repo's .gitmodules with the pattern # test262-YYYY.MM.DD (commit-date of the pinned SHA, enforced by the gitmodules-comment-guard hook).

🚨 Strict allowlist policy

An allowlist entry is ONLY for non-parser test fails. Anything a parser should handle MUST NOT be allowlisted; it must be fixed in the parser. This is strict; the runners enforce it via design choices below.

What counts as "non-parser":

• Unimplemented TC39 feature: the proposal is at Stage 3+ but we haven't ported the grammar yet (decorators, source-phase imports). Goes in test262-config/test262.unsupported-features keyed on the TC39 feature name (NOT a test path).
• Runner / harness bug: the test runner itself produces a false signal (e.g. async-throws semantics, error-name matching). Fix the runner, don't allow-list the symptom.
• Runtime-only test: the test exercises a runtime API (Reflect.*, Temporal.*) that the parser-conformance run can't evaluate. The runners skip these by classification, not per-path allowlist.

What does NOT count and must be fixed in the parser:

• "Parser rejects valid input." Fix the parser.
• "Parser accepts invalid input." Fix the parser.
• "Parser produces wrong AST shape." Fix the parser.
• "Cross-impl divergence: Rust + TS pass, Go fails." Fix Go.

If you feel tempted to add a per-test-path allowlist entry, the answer is almost always "the parser needs fixing." The unsupported-features file is the only escape valve and it's feature-name-keyed by design. You can't sneak a parser bug past it.

Canonical runners per repo

Invocation patterns

Multi-lane (recommended for cross-lane parity checks)

cd packages/acorn

# All 4 lanes, full suite
node test/test262-compare.mts

# Subset of lanes
node test/test262-compare.mts --lane rust,go

# All lanes, filtered to a single category
node test/test262-compare.mts --include 'language/expressions/await'

# Single test path, all lanes
node test/test262-compare.mts test/language/statements/class/private-method.js

Lanes: rust, go, cpp, typescript. Flags forward to each per-lane runner.

Single-lane

# Per-lane direct invocation
cd packages/acorn/lang/rust && node scripts/test262.mts
cd packages/acorn/lang/go && node scripts/test262.mts
cd packages/acorn/lang/cpp && node scripts/test262.mts
cd packages/acorn/lang/typescript && node scripts/test262.mts

# socket-btm temporal-infra
cd socket-btm/packages/temporal-infra && node test/scripts/test262-temporal-runner.mts

Single-case debug

Pass the test path positionally:

# Single lane
node scripts/test262.mts test/language/expressions/await/await-in-nested-function.js

# All lanes
node test/test262-compare.mts test/language/expressions/await/await-in-nested-function.js

Targeted filtering

node scripts/test262.mts --include 'export'          # regex on path
node scripts/test262.mts --exclude 'surrogate'       # regex on path
node scripts/test262.mts --category module           # named feature group
node scripts/test262.mts --include 'class' --exclude 'async'

Vitest-integrated mode

Each repo also wires a vitest test that wraps the runner. Useful for CI integration and selective re-runs:

pnpm exec vitest run test/unit/test262.test.mts             # ultrathink acorn
pnpm exec vitest run test/unit/test262-temporal.test.mts    # socket-btm temporal

Common failure modes

• Submodule missing. The test262 suite is a git submodule. If the runner errors with "test262 suite not found", run git submodule update --init --recursive.
• Feature classification drift. The runner uses each test's metadata block (/*--- features: [...] ---*/) to decide whether to run or skip. If a new TC39 feature is added upstream, classify it in the unsupported-features config first; do not let the runner silently pass tests for features the parser doesn't implement.
• "Allowlist drift": does NOT apply here. The acorn lanes don't carry a per-test-path allowlist. If a test starts passing or failing, that's the parser's behavior; either the parser is correct and the test is correct (good), or one of them is wrong and that's a bug.
• Cross-fleet drift. ultrathink and socket-btm should pin the same tc39/test262 SHA. If you're investigating a flaky test, double-check both .gitmodules files first.

Never write a homebrew runner

The existing runners encode dozens of edge cases (strict-mode harness wrapping, async-throws semantics, error-name matching, the negative.phase distinction between parse vs early errors). Recreating that surface from scratch reliably misses cases. If you find yourself wanting to "just run a few test262 files by hand," reach for the runner with a filter arg instead.

Reference

• TC39 test262 spec: https://github.com/tc39/test262
• Each runner's source is the source of truth for invocation flags and exit-code conventions; cat the runner first if the invocation is unclear.
• Strict allowlist policy + multi-lane behavior + tc39/test262 pin date all encoded in this skill. Read this skill before touching either system.