Check for internet connectivity using TCP

checkTCP.js

const { createConnection } = require("net");

function checkTCP(host = "1.1.1.1", port = 53) {
  return new Promise((resolve, reject) => {
    const client = createConnection({ host, port }, () => {
      console.log(`TCP connection established on port ${port}`);
      client.end();
      resolve();
    });

    client.setTimeout(3000);

    client.on("timeout", err => {
      console.error(`TCP connection on port ${port} timed out`);
      client.destroy();
      reject(err);
    });

    client.on("error", err => {
      console.error(`Error trying to connect via TCP on port ${port}`);
      reject(err);
    });
  });
}

Example, how to call that function:

let isOnline;

checkTCP()
  .then(() => (isOnline = true))
  .catch(() => (isOnline = false))
  .finally(() => console.log({ isOnline }));

Example, how to check for an SSH connection at 192.168.1.1 on port 22:

let isOnline;

checkTCP("192.168.1.1", 22)
  .then(() => (isOnline = true))
  .catch(() => (isOnline = false))
  .finally(() => console.log({ isOnline }));