JSFiddle - React, Tailwind, and code Playground

HTML

Browser base64 Session Description <textarea id="localSessionDescription" readonly="true"></textarea> <br />
Golang base64 Session Description: <textarea id="remoteSessionDescription"></textarea> <br/>
<button onclick="window.startSession()"> Start Session </button> <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({
  iceServers: [
    {
      urls: "stun:stun.l.google.com:19302"
    }
  ]
})

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

let sendChannel = pc.createDataChannel('foo')
sendChannel.onclose = () => console.log('sendChannel has closed')
sendChannel.onopen = () => console.log('sendChannel has opened')
sendChannel.onmessage = e => log(`sendChannel got '${e.data}'`)

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

pc.onnegotiationneeded = e =>
  pc.createOffer().then(d => pc.setLocalDescription(d)).catch(log)

window.sendMessage = () => {
  let message = document.getElementById('message').value
  if (message === '') {
    return alert('Message must not be empty')
  }

  sendChannel.send(message)
}

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

  try {
    pc.setRemoteDescription(new RTCSessionDescription({type: 'answer', sdp: atob(sd)}))
  } catch (e) {
    alert(e)
  }
}