repl.js

'use strict';

const { EventEmitter } = require('events');
const { spawn } = require('child_process');
const WebSocket = require('ws');

class Session extends EventEmitter {
  constructor(url) {
    super();

    this.ws = new WebSocket(url);
    this.ws.on('message', (d) => {
      this.onMessage(d);
    });
    this.ws.on('open', () => {
      this.emit('open');
    });

    this.messageCounter = 0;
    this.messages = new Map();
  }

  onMessage(d) {
    const { id, result } = JSON.parse(d);
    const { resolve } = this.messages.get(id);
    this.messages.delete(id);
    resolve(result);
  }

  post(method, params) {
    return new Promise((resolve, reject) => {
      const id = this.messageCounter;
      this.messageCounter += 1;
      const message = {
        method,
        params,
        id,
      };
      this.messages.set(id, { resolve, reject });
      this.ws.send(JSON.stringify(message));
    });
  }
}

const child = spawn(process.execPath, ['--inspect', './stub.js'], {
  cwd: process.cwd(),
  windowsHide: true,
});

child.stdout.on('data', (data) => {
  process.stdout.write(data);
});

const re = /^Debugger listening on ([^ \n]+)/;
child.stderr.on('data', (data) => {
  const s = data.toString();
  const r = re.exec(s);
  if (r !== null) {
    const [, url] = r;
    start(url);
  }
});

function start(wsUrl) {
  const session = new Session(wsUrl);

  session.on('open', () => {
    // TIMEOUTS:
    // >9ms -> NEVER RESPONDS
    // 9ms  -> RESPONDS ~50% OF TIME
    // <9ms -> ALWAYS RESPONDS
    setTimeout(() => {
      session.post('Runtime.evaluate', {
        expression: '1 + 1',
        replMode: true,
      }).then(console.log);
    }, 100);
  });
}

stub.js

'use strict';

// keep inspect target alive
setTimeout(() => {}, 2147483647);