Demonstrate event loop hang if "Runtime.runIfWaitingForDebugger" is not called after connecting to Deno inspector. Forked from https://gist.github.com/evanwashere/04cfa233f613343e1c9d6fb72cfd628c

inspector.js

class Emitter {
  #on = Object.create(null);
  #once = Object.create(null);

  on(n, fn) { (this.#on[n] ?? (this.#on[n] = [])).push(fn); }
  once(n, fn) { (this.#once[n] ?? (this.#once[n] = [])).push(fn); }
  emit(n, ...args) { this.#on[n]?.forEach(_ => _(...args)); if (this.#once[n]) this.#once[n] = (this.#once[n].forEach(_ => _(...args)), undefined); }
  off(n, fn) { if (!fn) (delete this.#on[n], delete this.#once[n]); else { let o = this.#on[n]?.indexOf(fn) ?? -1; if (o !== -1) this.#on[n].splice(o, 1); o = this.#once[n]?.indexOf(fn) ?? -1; if (o !== -1) this.#once[n].splice(o, 1); } }
}

class Inspector extends Emitter {
  #seq = 0;
  #socket = null;
  #promises = Object.create(null);

  async call(method, params) {
    if (!this.#socket) throw new Error('not connected');

    return await new Promise((ok, err) => {
      this.#promises[this.#seq] = { ok, err };
      this.#socket.send(JSON.stringify({ method, params, id: this.#seq++ }));
    });
  }

  async connect(url) {
    return await new Promise((ok, err) => {
      this.#socket = new WebSocket(url);

      this.#socket.onopen = () => ok();
      this.#socket.onerror = _ => err(_.error);

      this.#socket.onclose = () => {
        this.#seq = 0;
        this.#socket = null;
        const e = new Error('connection closed');
        for (const _ in this.#promises) this.#promises[_].err(e);

        this.#promises = Object.create(null);
      };

      this.#socket.onmessage = _ => {
        const { id, error, method, params, result } = JSON.parse(_.data);
        if (method === 'Runtime.executionContextDestroyed') this.#socket.close();

        else if (method) this.emit(method, params);

        else {
          if (!error) this.#promises[id].ok(result);

          else {
            const e = new Error(error.message);
            this.#promises[id].err((e.code = error.code, e));
          }

          delete this.#promises[id];
        }
      };
    });
  }
}

const inspectorUrl = Deno.args[0];

if (!inspectorUrl.startsWith("ws://")) {
  throw new Error("Invalid inspector URL");
}

const inspector = new Inspector;
await inspector.connect(inspectorUrl);
const context = new Promise(r => inspector.once('Runtime.executionContextCreated', _ => r(_.context)));

console.log('connected');
await inspector.call('Runtime.enable');
// Remove this comment to unhang event loop
// await inspector.call('Runtime.runIfWaitingForDebugger');

async function evl(expr) {
  const { result } = await inspector.call('Runtime.evaluate', {
    replMode: true,
    expression: expr,
    awaitPromise: true,
    objectGroup: 'repl',
    generatePreview: true,
    contextId: (await context).id,
  });

  await inspector.call('Runtime.releaseObjectGroup', { objectGroup: 'repl' });

  return result;
}

while (true) {
  console.log(await evl(prompt()));
}

await inspector.call('Runtime.disable');

To run:

$ deno repl --inspect
Debugger listening on <url>
....

$ deno run --allow-net inspector.js <url>