Bytecode Alliance

Jco (@bytecodealliance/jco on NPM)is a “multi-tool for the JS WebAssembly ecosystem.” At the 2026 Bytecode Alliance Plumbers Summit, Technical Steering Committee member Bailey Hayes put it another way: Jco is “like five projects in one.” It’s certainly a project with many facets—five big ones, arguably! Recognizing what those facets are, and how they fit together, is the key to understanding why Jco matters beyond the JavaScript ecosystem. In this blog series, we’ll draw on Victor Adossi’s Plumbers Summit presentation to take an in-depth look at Jco from five different perspectives, in order to better grasp how you can use (and contribute to!) Jco today. There’s a lot to unpack here, so in this first post, we’ll try to get to grips with Jco as a layered architecture that brings together many pieces of the Wasm and JS ecosystem.

The Bytecode Alliance is pleased to invite you to the next installment in our ongoing Plumbers Summits event series, each designed to bring our members and community together to collectively contribute to the strategic planning for the upcoming year. Our next event will be held Wednesday and Thursday, February 25 and 26, 2026. This will be an all online event, supporting full remote participation for anyone anywhere, with sessions recorded so they can be watched anytime afterward via our YouTube channel

In April of 2015, Luke Wagner made the first commits to a new repository called WebAssembly/design, adding a high-level design document for a “binary format to serve as a web compilation target.”