Minimum function to get local IP address.

get-local-ip.js

function getLocalIP() {
  let ip;
  return new Promise(async (resolve, reject) => {
    const pc = new RTCPeerConnection({});
    pc.createDataChannel('dummy');
    pc.addEventListener('icecandidate', ev => {
      if (!ev.candidate) {
        pc.close();
        ip ? resolve(ip) : reject(new Error('IP not found'));
      }
      else {
        const match = /\d{1,3}\.\d{1,3}\.\d{1,3}\.\d{1,3}/.exec(ev.candidate.candidate);
        match && (ip = match[0]);
      }    
    }, false);
  
    const offer = await pc.createOffer();
    await pc.setLocalDescription(offer);
  });
};

main.js

getLocalIP().then(console.log);