test262

The good part about a language runtime is there's a massive corpus of tests that are in that language and agnostic to the implementation language. For a JS engine that's Test 262: https://github.com/tc39/test262 For node that's its unit tests which are mostly JS: https://github.com/nodejs/node/tree/main/test Node also runs the web platform tests too: https://web-platform-tests.org/ Bun has a similar large corpus of JS/TS tests: https://github.com/oven-sh/bun/tree/main/test You're right about general purpose rewrites, but language runtimes are a lot easier.

we're going to have to get a lot better at building test suites. for example every js exploit found in browsers should also be added to https://github.com/tc39/test262.

The primary JS test suite is maintained by the authors of the specification itself: https://github.com/tc39/test262

"about to be fixed" is pretty clickbaity with no substance. From the July TC39 meeting [0]: > After 7 years, Temporal is close to done. Our only focus is making sure in-progress implementations are successful, by fixing corner-case bugs found by implementors, and making editorial changes to improve clarity and simplicity. It's not (yet) on the October TC39 agenda [1]. The Chromium implementation issue hasn't been updated since 2022. [2] Webkit's implementation issue was updated once this year... to add a blocker [3] V8 is passing 75% of test262's tests, and JavascriptCore is passing 41%. Spidermonkey is a no-show. [4] Note that test262 is incomplete and hasn't seen activity this year [5], so those percentages may decrease over time. "about to be fixed" ehhhh. If it lands this year I'd be surprised. [0] https://ptomato.name/talks/tc39-2

It's awesome to see how more JS runtimes try to approach Wasm. This project reminds me to Static Hermes (the JS engine from Facebook to improve the speed of React Native projects on iOS and Android). I've spent a bit of time trying to review each, so hopefully this analysis will be useful for some readers. What are the main commonalities and differences between Static Hermes and Porffor? * They both aim for JS test262 conformance [1] * Porffor supports both Native and Wasm outputs while Static Hermes is mainly focused on Native outputs for now * Porffor is self-hosted (Porffor is written in pure JS and can compile itself), while Static Hermes relies on LLVM * Porffor currently doesn't support async/promise/await while Static Hermes does (with some limitations) * Static Hermes is written in C++ while Porffor is mainly JS * They

Indeed. These may be even more important... https://github.com/tc39/test262

I would be interested in some spec test suite coverage figures. I have used a few js engines in the past (nashorn, rhino, otto) and all had problems with missing edge cases in the APIs or really weird quirks in the interop with the host environment. https://github.com/tc39/test262 is the ecma test suite. rhino for example has extensive gaps: https://github.com/mozilla/rhino/blob/master/testsrc/test262... It is really frustrating that in the end, there aren't really that many actually compliant implementations. Especially implementing the exact regex behaviour. EDIT: apparently this exists, at least partially: https://kangax.github.io/compat-table/es6/ The smaller engines are missing however.

An example of a collaborative effort that achieves this is TC39's test262 [0], which is a full test suite for JavaScript conformance across engines and vendors. [0] https://github.com/tc39/test262

> You can implement JavaScript from the ES spec, but NOT regexp. The grammar doesn't really make sense [2]. The Chakra team found the same thing [3]. I haven't tried to implement the RegExp section, so I can't speak to that, but this is largely why we have Test262[1]. Also that Chakra blogpost is from 10 years ago, talking about ES5. Things have improved considerably since then, though of course we have to live with historical baggage. > With Firefox using irregexp, it further cements this unfortunate reality. Is this a specific unfortunate reality? I don't spot any RegExp entry in the last spec Incompatibilities section[2] that seems like a bad change or even having to do with regex grammar at all. [1] https://github.com/tc39/test262/tree/master/test/built-ins/R... [2] https://www.ecma-international.org/ecma-262/#sec-additions-a...

It uses the tc39 testsuite for verification: https://github.com/tc39/test262

Actually, that's a fascinating part of the story. We run the entire test262 ECMAScript Conformance Suite (https://github.com/tc39/test262) against our JS-Interpreter fork. We don't pass the whole thing yet, but it ensures we only ratchet toward conformance to the language spec. We checked, and this particular bug was not caught by the test262 suite - the broken sort worked for enough cases to slip past the Array.prototype.sort tests there. We've added a regression test for this issue in the layer that consumes the interpreter, but I've thought about proposing more aggressive validation of sort behavior to the test262 project. It was a great reminder to me that while tests may be necessary for quality, they are not always sufficient! Brad, Code.org Engineering