Iframe.readyState does not work in chrome

You can use the onload to signaling the load of the iframe

here is a simple example that working

var iframe = document.createElement("iframe");
iframe.style.display = "none";
// this function will called when the iframe loaded
iframe.onload = function (){
  iframe.style.display = "block";    
  alert("loaded");
};
// set the src last.
iframe.src ='http://www.test.com';

// add it to the page.
document.getElementById("one").appendChild(iframe);

Tested here:
http://jsfiddle.net/48MQW/5/
With src loaded last.
http://jsfiddle.net/48MQW/24/

The downloadable file content doesn't trigger the readystatechange event handler or the onload event handler. This couse you can set a cookie in server side together the file content, and client side check this cookie periodically. For example:

server

response.cookie('fileDownloaded','true');
response.header('attachment','your-file-name.any');
//...write bytes to response...

client

var checker = setInterval(()=>{
    if(document.cookie.indexOf('fileDownloaded')>-1){
        alert('done');
        clearInterval(checker);
    }
},100);

Of course, you can use your framework to check the cookie value correctly, this is just a poc, not a safe cookie parser.