[ Nodejs | Cluster ]: How to wait answer of all my workers?

Question

I have a clustered node app, and I'm looking for a way to get state of all my workers from one request.

I use express to ask the masten but it's not express related. with one request, i would like to have something like this:

[
     {
       pid: 'xxxxxx',
       state: 'I\'m fine'
     },
     {
       pid: 'xxxxxx',
       state: 'I\'m sick'
     },
     {
       pid: 'xxxxxx',
       state: 'I\'m fine'
     },
]

To send express answer, I need to wait all worker answers, and concat them. But the question is: How to wait for a spécific answer from a worker instead of a simple listenner?

Here is a code sample of the clustered app. I'm trying to get the previous Json in request of http://localhost:8000/

import * as cluster from 'cluster';
import * as express from 'express';

if (cluster.isMaster) {
    // Here is master
    // we start by creating few workers
    for (let i = 0; i < 4; i++) {
        cluster.fork();
    }

    // for each worker, if we receive a message, we print it
    for (const id in cluster.workers) {
        if (cluster.workers.hasOwnProperty(id)) {
            cluster.workers[id].on('message', (msg) => {
                console.log(`[${cluster.workers[id].process.pid}] ${msg}`);
            });
        }
    }

    // create express server
    const app = express();
    app.get('/', (req, res) => {
        // request from client
        console.log('\nHow are you all?');
        // for each workers
        for (const id in cluster.workers) {
            if (cluster.workers.hasOwnProperty(id)) {
                // ask state
                cluster.workers[id].send('howareyou');
                // how to wait for answer?
            }
        }

        // probably concat all answers here
        // send result to client
        res.setHeader('Content-Type', 'application/json');
        res.json({question: 'How can I have state of all my workers?'});
    });
    app.listen(8000);
} else {
    // Here is worker.
    process.on('message', (message) => {
    // When master ask state, send back random answer
        if (message === 'howareyou') {
            process.send(`I'm ${Math.random() * 100 < 75 ? 'fine' : 'sick'}`);
        }
    });
}

Thanks in advance :)