micro-javascript

I had that experience back in January! I used it for a joke in a talk recently... did anyone really need a half-finished buggy slow implementation of a JavaScript interpreter in Python? They did not. https://github.com/simonw/micro-javascript It's taken me the best part of this year to readjust and find a pace and level of ambition that fits.

I wouldn't call these flawless but here you go: - https://github.com/simonw/denobox is a new Python library that gives you the ability to run arbitrary JavaScript and WASM in a sandbox provided by Deno, because it turns out a Python library can depend on deno these days. I built that on my phone in bed yesterday morning. - https://github.com/simonw/pwasm is a WebAssembly runtime written in pure Python with no dependencies, built by feeding Claude Code the official WASM specification along with its conformance test suite and having it hack away at that (again via my phone) to get as many of the tests to pass as possible. It's pretty slow and not really useful yet but it's certainly interesting. - https://github.com/datasette/datasette-transactions is a Datasette plugin which provides a JSON API for starting a SQLite transaction, running mu

> Why is it still hard to take an existing fully concrete specification, and an existing test suite, and dump out a working feature-complete port of huge, old, and popular projects? Lots of stuff like this will even be in the training We have a smaller version of that ability already: - https://simonwillison.net/2025/Dec/15/porting-justhtml/ See also https://www.dbreunig.com/2026/01/08/a-software-library-with-... I need to write these up properly, but I pulled a similar trick with an existing JavaScript test suite for https://github.com/simonw/micro-javascript and the official WebAssembly test suite for https://github.com/simonw/pwasm

>A JavaScript interpreter written in Python? I'm assuming this refers to the python port of Bellard's MQJS [1]? It's impressive and very useful, but leaving out the "based on mqjs" part is misleading. [1] https://github.com/simonw/micro-javascript?

Have you spent much time with Codex 5.1 or 5.2 in OpenAI Codex or a Claude Opus 4".5 in Claude code over the last ~6 weeks? I think they represent a meaningful step change in what models can build. For me they are the moment we went from building relatively trivial things unassisted to building quite large and complex system that take multiple hours, often still triggered by a single prompt. Some personal examples from the past few weeks. - A spec-compliant HTML5 parsing library by Codex 5.2: https://simonwillison.net/2025/Dec/15/porting-justhtml/ - A CLI-based transcript export and publishing tool by Opus 4.5: https://simonwillison.net/2025/Dec/25/claude-code-transcript... - A full JavaScript interpreter in dependency/free Python (!) https://github.com/simonw/micro-javascript - and here's that transcript published using the above-mention

The hardest problem in computer science in 2025 is showing an AI cynic an example of LLM usage that they find impressive. How about this one? I had Claude Code run from my phone build a dependency-free JavaScript interpreter in Python, using MicroQuickJS as initial inspiration but later diverging from it on the road to passing its test suite: https://static.simonwillison.net/static/2025/claude-code-mic... Here's the latest version of that project, which I released as an alpha because I haven't yet built anything real on top of it: https://github.com/simonw/micro-javascript Again, I built this on my phone, while engaging with all sorts of other pleasant holiday activities.

https://pypi.org/project/micro-javascript/ - https://github.com/simonw/micro-javascript Here's the transcript showing how I built it: https://static.simonwillison.net/static/2025/claude-code-mic...