This is a proof-of-concept of storing a JavaScript library on StarkNet. You can see the result here. It's a rendering of the StarkNet logo in a <canvas> tag using p5.js. The interesting thing about it is that both the rendering library and the JS code used to draw the logo are both stored on chain as smart contracts.
There's two parts of the system. First, transforming the JS code into Cairo and making it available on chain. Second, calling a StarkNet function and decoding the result into executable JS code. Let's have a look at how it works.
Putting JS code on chain is a bit convoluted but still pretty easy to follow. The code is ready and base64 encoded into a string. This string is prepended with data:text/javascript;base64, data URL scheme, then gzipped and base64 encoded again. This output string is split into chunks of 31 characters and encoded as Cairo short strings. Finally, this array of short strings is "wrapped" in a StarkNet function.
The js_to_cairo.py script takes care of automating this pipeline:
./js_to_cairo.py --code-file snlogo.js --func-name get_snlogojs --output-file snlogojs.cairoTo see the before and after, check the snlogo.js JS code and the resulting snlogojs.cairo Cairo code.
Turning Cairo short strings into executable JS is just the process above in reverse. After calling the function returning an array of short strings, use decodeShortString from starknet.js on each element, combine them into a single string, decompress (e.g. using fflate) and put the end result in a <script> tag. See page.tsx for details.
The p5.js lib has been deployed to 0x4cd1b9b78b42d9f4b30abab92e8616ca34596f10ae5131f6cd3d913767c71e5. You can call get_p5js on it.
The code to draw the StarkNet logo in p5 has been deployed to 0x4c8c96e96ccf04a0fcadad1bc259e52bd5c220c0cbbc9cc9c17109d4e5dd312. Use get_snlogojs to fetch the code.