So basically I am building an express server and I recently looked into the topic of scaling NodeJS apps. One of the first things I saw was the built in cluster module to take advantage of all the threads of the machine's processor. Here is my code of the implementation I've made:

import cluster from "cluster";
import { cpus } from "os";

import dotenv from "dotenv";
dotenv.config();

import express, { Express } from "express";
import log from "./logger";
import database from "./database";
import router from "./router";

const app: Express = express();

// Middleware
app.use(express.json());
app.use(express.urlencoded({ extended: false }));

const port = <number>(<unknown>process.env.PORT);
const host = <string>process.env.HOST;

const numCPU = cpus().length;
if (cluster.isPrimary) {
   for (let i = 0; i < numCPU; i++) {
      cluster.fork();
   }

   cluster.on("listening", (worker) => {
      // Not printing anything
      console.log(worker.process.pid);
   });

   cluster.on("exit", (worker, code, signal) => {
      log.error(`Worker ${worker.process.pid} died. Starting a new worker...`);
      cluster.fork();
   });
} else {
   app.listen(port, host, () => {
      // Only prints this once.
      log.info(`Server ${process.pid} listening at http://${host}:${port}`);
      database();
      router(app);
   });
}

The problem is that the cluster.isPrimary block never runs meaning of course that no other processes are forked by the cluster. However when I use cluster.isMaster, everything works exactly as intended. But since isMaster is deprecated and is replaced by isPrimary, thats what I'm trying to use but it just doesn't work.

Solutions I've tried that don't work:

1. Kill all node processes on my machine.
2. Remove everything from this main file leaving just the barebone app and cluster configuration.

What am I doing wrong?