test262 coverage

test262 is the official conformance test suite maintained by TC39 and the canonical way to compare JavaScript engines.

Phasis passes the full suite at 100 % across every category — language semantics, the standard library, Intl, annexB legacy web features, the staging directory of incubating tests, and the harness self-tests. No feature-flag bypasses, no host-gap blocklist entries, no per-shard exclusions.

The suite is pinned: the test262 git submodule fixes the revision Phasis is measured against, and new upstream tests count only after the submodule is bumped. The full-suite snapshot in COMPAT.md is regenerated by the compat-matrix.yml workflow, which is started by hand rather than on every push; COMPAT.md records when the snapshot was taken and against which commit.

For comparison, test262.fyi tracks the same suite against every production JS engine:

Those engines each carry a small residual gap (newly-landed proposals, rare edge cases under construction). Phasis closes the gap by virtue of being a single-purpose conformance-first implementation.

Running it locally

The runner needs the test262 submodule and the repository's test classes, neither of which ships in the Composer package, so run it from a clone:

git clone https://github.com/inline0/phasis
cd phasis
git submodule update --init test262
composer install
./bin/test262 --jobs 4

A full run on a recent Mac takes around 20 minutes; the CI matrix shards in parallel and finishes in single-digit minutes.

To run just one category while debugging:

./bin/test262 --category built-ins/Array
./bin/test262 --category language/expressions/yield --failures

See the CLI reference for the full flag set.

Why this matters

JavaScript has subtle semantics — NaN !== NaN, -0 === 0, [1] + 1 === "11", the exact order of property enumeration, the timing of Promise microtasks. A library that runs JS user code needs every one of those right, every time. test262 is the bar for "right"; passing it at 100 % means a JS program that runs on V8 produces byte-identical output on Phasis (modulo browser-only APIs).