ESP3D-WEBUI/extensions_samples/download.html

<script type="text/javascript">
  function sendMessage(msg){ 
      window.parent.postMessage(msg, '*');
  }

  function processMessage(eventMsg){
      if (eventMsg.data.type  && (!eventMsg.data.id||eventMsg.data.id=="downloadpanel")){
          if (eventMsg.data.type=="download"){
            const line =  eventMsg.data.content
            if (line.status=="success"){
                //it is text so we can read it
                var reader = new FileReader();

                reader.onload = function() {
                    //it is text display it
                    const resultPanel = document.getElementById("output");
                    resultPanel.innerHTML += "<br>" + reader.result + "<hr>"
                }
                reader.readAsText(eventMsg.data.content.response);
				//we ca also save it localy
                if (window.navigator.msSaveOrOpenBlob)
                // IE10+
                window.navigator.msSaveOrOpenBlob(eventMsg.data.content.response, eventMsg.data.content.initiato.url);
                else {
                // Others
                const a = document.createElement("a");
                const url = URL.createObjectURL(eventMsg.data.content.response);
                a.href = url;
                a.download = eventMsg.data.content.initiator.url;
                a.onload = () => {
                    URL.revokeObjectURL(a.href);
                };
                document.body.appendChild(a);
                a.click();
                setTimeout(function () {
                    document.body.removeChild(a);
                    window.URL.revokeObjectURL(url);
                }, 0);
                }
                      
            }
            if (line.status=="error"){
              //TBD
            }
          }
      }
  }

  window.onload = (event) => {
    window.addEventListener("message", processMessage, false);
  };
    
</script>

<div class="container">
    <button class="btn m-1" onclick="sendMessage({type:'download', target:'webui',id:'downloadpanel',url:'preferences.json'});">Download preferences.json</button>
    <div class="container m-2" id="output">
    </div>
</div>