Find local IP using WebRTC

find-local-ip.js

const findLocalIP = () => {
  return new Promise((resolve, reject) => {
  // compatibility for firefox and chrome
    const myPeerConnection = window.RTCPeerConnection ||
                             window.mozRTCPeerConnection ||
                             window.webkitRTCPeerConnection
    const pc = new myPeerConnection({ iceServers: [] })
    const noop = () => {}
    const localIPs = {}
    const ipRegex = /([0-9]{1,3}(\.[0-9]{1,3}){3}|[a-f0-9]{1,4}(:[a-f0-9]{1,4}){7})/g

    const ipIterate = (ip) => {
      if (!localIPs[ip]) resolve(ip)
      localIPs[ip] = true
    }
    pc.createDataChannel('') // create a bogus data channel
    pc.createOffer(sdp => {
      sdp.sdp.split('\n').forEach(line => {
        if (line.indexOf('candidate') < 0) return
        line.match(ipRegex).forEach(ipIterate)
      })
      pc.setLocalDescription(sdp, noop, noop)
    }, noop) // create offer and set local description
    pc.onicecandidate = (ice) => { // listen for candidate events
      if (
        !ice ||
        !ice.candidate ||
        !ice.candidate.candidate ||
        !ice.candidate.candidate.match(ipRegex)
      ) return
      ice.candidate.candidate.match(ipRegex).forEach(ipIterate)
    }
  });
}

// example usage
findLocalIP()
  .then(ip => {
    console.log(ip);
  });