JavaScript中的XMLHttpRequest

Question

I have done Laravel/PHP web server and run it on http://120.77.44.4/ Now I need to calculate Network Speed in JavaScript. To implement this, I tried to send a HTTP request to the server and calculate the response data size and time. When I divide the response data size by time, I can get the download speed of the client. I tried it myself, but I can not get as I need.

This is the JavaScript function in php code.

var xmlhttp = new XMLHttpRequest();
xmlhttp.open("GET", "http://120.77.44.4/RequestSpeed", true);

xmlhttp.onreadystatechange = function() {
  if (xmlhttp.readyState == XMLHttpRequest.DONE ) {
    if (xmlhttp.status == 200) {
      alert('OK, 200');
    } else if (xmlhttp.status == 400) {
      alert('There was an error 400');
    } else {
      alert('something else other than 200 was returned');
    }
  }
};

xmlhttp.send();

This is the code in server.

public function RequestSpeed() {
  file_get_contents('./test.txt', true);
  return response();
}

The server was done by using Laravel/MySql.