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Let's prototype a JavaScript JIT compiler!

Andreas Kling · 49.4K views · 2023-10-11 · 1:21:25

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Overview

Andreas Kling live-codes a prototype JavaScript JIT compiler for the LibJS engine used in SerenityOS and Ladybird. He builds a standalone bytecode VM, a simple interpreter, and then a simple JIT in about 80 minutes.

On Hacker News

HN commenters cite it as a concrete example of how a skilled developer uses Copilot, mostly as fancy autocomplete for codebase-specific work.

Discussed on HN; no positive recommendation is recorded in this export · HN mentions 2023–2025

Topics

Compilers, interpreters & program analysisSoftware practice, open source & careers
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jitbytecode vmlibjsladybirdlive codingc++

What people said

shakna · 2025-08 · Reference

Ladybird is 425k LOC. Chromium is 3.5m LOC. Starting a new browser, using LLMs... Is not going to maintain enough context. Whilst Andreas does use Copilot a fair bit [0], he tends to do a line at a time, frequently disagree and rewrites his own, before prompting again. That is... He basically uses it as a fancy autocomplete. Not much else. [0] [VIDEO]

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Kiro · 2023-11 · Reference

> The code is usually the same code I'd get a few web searches away My impression is that people normally don't use Copilot as a substitute for finding solutions (ChatGPT is much better for that), but as a way to help with otherwise tedious tasks that are really specific to your codebase. Check out 6:05 and 6:25 in this Andreas Kling video for a good example: [VIDEO] Regarding your second point Copilot helps me when I least expects it. I think the video illustrates what I mean with that as well.

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Nullabillity · 2023-11 · Criticism

> Check out 6:05 and 6:25 in this Andreas Kling video for a good example: [VIDEO] Oof. So not only was it a poor replacement for a macro, it didn't even generate the correct code!

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Sources

Video by Andreas Kling, played through YouTube.

Data and selection notes

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