Member-only story
How to Assert a Custom Section in a WASM (in JavaScript)
Read and assert a WebAssembly module metadata using Vitest, a quick recipe.
Not sure if this blog post will ever be read or useful to anyone — feels like one of those super ultra-niche things. And to be honest, it’s nothing groundbreaking either. If you check MDN or ask your favorite AI, you’ll probably figure it out pretty quickly.
But hey, I felt like sharing anyway.
On Juno, every container — or smart contract, or idenpendent microservice (call them what you want) — is a WebAssembly module running on the Internet Computer. While I was building support for serverless functions written in TypeScript, I ran into a need, keeping track of the version of each deployed function written by the developers.
After a few iterations, I landed on an approach where each module gets a pseudo-package.json embedded as a custom section in the WASM binary. This way, any part of the ecosystem — whether tooling or the platform itself — can read the module's version and dependencies.
Once the feature was working, I knew I needed a test for it. Embedding and exposing that metadata is pretty critical. So I wrote a Vitest assertion to validate the metadata, and figured I’d share the solution with you here.
