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Compatibility

View as Markdown Edit on GitHub Docs at 5b53f69

Phasis is verified against three independent oracles:

  1. test262 — the official TC39 ECMAScript conformance suite. 100 % of the revision pinned in the repository.
  2. Web Platform Tests — the official WHATWG / W3C suite for non-ECMAScript Web APIs (fetch, XHR, WebSocket, Streams, Headers, Blob, encoding, URL, structured-clone, abort, hr-time, atob, crypto). 100 % on the imported corpus.
  3. Popular libraries — 204 widely-used npm packages (acorn, lodash, zod, ramda, immer, redux, date-fns, marked, fuse.js, semver, validator, …) execute byte-equal to Node.js.

vs other engines

Engine test262 Notes
Phasis 100 % Pure PHP. Targets ES2024+.
V8 (Node, Chrome) 99.8 % Reference C++ engine.
SpiderMonkey (Firefox) 99.6 % Reference C++ engine.
JavaScriptCore (Safari) 99.4 % Reference C++ engine.
QuickJS ~97 % Lightweight embeddable engine in C.
Hermes (React Native) ~95 % Embeddable engine optimized for mobile.

Numbers from test262.fyi. Phasis sits in the top tier alongside the production browser engines.

Pages

  • test262 coverage — what's tested, how the suite runs, why 100% matters.
  • Web APIs (WPT) — Web Platform Pack and Fetch Pack: URL, encoding, atob/btoa, structuredClone, performance, Headers, Blob, AbortSignal, EventTarget, Streams, plus crypto, WebSocket, and XMLHttpRequest.
  • Streams — full WHATWG Streams Standard: ReadableStream, WritableStream, TransformStream, BYOB, tee, pipeTo/pipeThrough, queuing strategies, async iteration.
  • Popular packages — CI-gated byte-equality with Node.js for 204 npm packages across parsing, templating, validation, date/time, math, hashing, search/diff, color, URL handling, and more.
  • Spec surface — every ECMAScript built-in, syntactic feature, and standard-library object that Phasis ships.
  • Known limitations — where Phasis intentionally diverges from V8: performance ceiling, multi-agent concurrency, Wasm, browser-only Web APIs.

How it's measured

  • test262 — bin/test262 runs the suite locally. The full matrix runs in the compat-matrix.yml workflow, which is started by hand rather than on every push; a successful run commits its snapshot to COMPAT.md, which records when it was taken and against which commit. The suite revision is the one pinned by the test262 git submodule.
  • Web Platform Tests — bin/wpt runs the imported fixtures. The wpt.yml workflow runs them on pushes to main and pull requests that change src/, the fixtures, or the Composer files.
  • Popular libraries — composer test runs PHPUnit including the popular-library byte-equality suite under tests/Popular/. The popular.yml workflow runs it under the same path filter and also regenerates every oracle under real Node, so a stale oracle is caught when a vendored library is bumped.

A WPT or popular-package regression fails the pull request that causes it. A test262 regression shows up the next time the matrix is run.