JSFiddle - React, Tailwind, and code Playground

HTML

Golang base64 Session Description: <textarea id="remoteSessionDescription"></textarea> <br/>
<button onclick="window.startSession()"> Start Session </button> <br />
Browser base64 Session Description <textarea id="localSessionDescription" readonly="true"></textarea> <br />

<br />

Message: <textarea id="message">This is my DataChannel message!</textarea> <br/>
<button onclick="window.sendMessage()"> Send Message </button> <br />

<div id="logs"></div>

JavaScript

/* eslint-env browser */

let pc = new RTCPeerConnection()
let log = msg => {
  document.getElementById('logs').innerHTML += msg + '<br>'
}

pc.onsignalingstatechange = e => log(pc.signalingState)
pc.oniceconnectionstatechange = e => log(pc.iceConnectionState)
pc.onicecandidate = event => {
  if (event.candidate === null) {
    document.getElementById('localSessionDescription').value = btoa(pc.localDescription.sdp)
  }
}

pc.ondatachannel = e => {
  let dc = e.channel
  log('New DataChannel ' + dc.label)
  dc.onclose = () => console.log('dc has closed')
  dc.onopen = () => console.log('dc has opened')
  dc.onmessage = e => log(`Message from DataChannel '${dc.label}' payload '${e.data}'`)
  window.sendMessage = () => {
    let message = document.getElementById('message').value
    if (message === '') {
      return alert('Message must not be empty')
    }

    dc.send(message)
  }
}

window.startSession = () => {
  let sd = document.getElementById('remoteSessionDescription').value
  if (sd === '') {
    return alert('Session Description must not be empty')
  }

  pc.setRemoteDescription(new RTCSessionDescription({type: 'offer', sdp: atob(sd)})).catch(log)
  pc.createAnswer().then(d => pc.setLocalDescription(d)).catch(log)
}