node:inspector

node:inspector (and node:inspector/promises) exposes the V8 inspector: a Session speaking the Chrome DevTools Protocol, the inspector console, and the experimental Network/DOMStorage broadcast namespaces. The inspector is host machinery – a WebSocket server and a protocol dispatcher wired into the running isolate – so WASI cannot express it. Like node:child_process, it is host-backed and denied by default.

Map it to the Node passthrough to grant it:

jco transpile component.wasm \
  --map 'jco:node/inspector@0.1.0=@bytecodealliance/jco-std/wasi/0.2.x/node/24.x.x/inspector/host/node'

The guest code is ordinary Node:

import { Session } from 'node:inspector/promises';

const session = new Session();
session.connect();
const { result } = await session.post('Runtime.evaluate', { expression: '6 * 7' });
result.value; // 42, evaluated in the host isolate

Argument validation, session state, the EventEmitter surface, and error reconstruction all run guest-side, so ERR_INSPECTOR_NOT_CONNECTED, ERR_INSPECTOR_ALREADY_CONNECTED, ERR_INVALID_ARG_TYPE, and the protocol’s ERR_INSPECTOR_COMMAND all match Node exactly. CDP payloads cross the boundary as JSON; the inspector console forwards its arguments as JSON too, which is best-effort for functions, symbols, and cycles.

The host calls back into the component

The inspector’s two callbacks – a post response and a session notification – run the other way, from host to guest. A component cannot implement a resource declared on an imported interface (its methods would run host-side), so the callbacks are a guest-exported interface, jco:node/inspector-callbacks@0.1.0, holding one resource per callback kind: a one-shot post-callback and a long-lived notification-listener. When bundled source imports node:inspector, Jco adds both the import jco:node/inspector@0.1.0; and the matching export jco:node/inspector-callbacks@0.1.0; to the selected world, and bundles the JS export alongside the entry – neither is written by hand.

The embedder wires the exported interface to the host adapter after instantiation:

import * as inspectorHost from '@bytecodealliance/jco-std/wasi/0.2.x/node/24.x.x/inspector/host/node';
import * as component from './transpiled/component.js';

inspectorHost.attachCallbacks(component.inspectorCallbacks);

Two timing rules follow from the component model, which forbids calling into a component while a task is already active in it:

• In-isolate post responses are synchronous. Runtime.evaluate, Debugger.*, and the other in-isolate methods resolve during the post call, so the host returns the response directly and an awaited Session.post never needs a re-entrant callback. This is the same behavior as Node, which also fires those callbacks synchronously.
• Notifications arrive between guest tasks, like node:cluster’s events: the host queues each notification and delivers it once no exported call is in flight, never into a suspended await.