Custom Package

What you’ll build: A JS package that adds a custom hello bin to fishbowl.

Prerequisites: Hello World →

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What is a package?

A fishbowl package is an ESM bundle that exports an Extension — a plain object describing bins, fileservers, mounts, and init services to wire into a system image when the package is installed.

Write the extension

Extensions are isomorphic — the same code runs in Node.js and the browser.

src/index.ts

import type { Extension } from '@fishnet/core'

export default function myPackage(): Extension {
  return {
    bins: {
      hello: async (proc) => {
        const name = proc.argv[1] ?? 'world'
        await proc.stdout.write(`Hello, ${name}!\n`)
        return 0
      },
    },
  }
}

Use it directly

import { Unix } from '@fishnet/core'
import { stdSystem } from '@fishnet/core/presets'
import { nodeRuntime } from '@fishnet/runtime/platform/node'
import myPackage from './src/index.ts'

const image = Unix().use(stdSystem()).use(myPackage()).build()
const instance = await image.boot(nodeRuntime())
// hello Alice → "Hello, Alice!"

Publish to a registry

Coming soon — the registry tooling is in active development. For now, packages can be embedded directly or served from a static HTTP server using the registry-tools package.

Further reading

• Extension interface → — Full Extension type reference
• Packages architecture → — Registry, resolver, evaluator internals
• ProcContext → — The interface bins receive