为什么XMLHttpRequest不起作用？

我是javascript和AJAX新手，已经在此问题上花费了近8个小时，我知道它很简单，只是找不到我做错什么了。我的网站上有一个图片，带有on-click=SendCommand() 。这是我的js代码：

function SendCommand(){
        alert("BingoBango!");

        var xmlhttp = new XMLHttpRequest();
        xmlhttp.open("GET","https://www.justanexample.com/API/MobileAPP/SendCommand.php?apikey=7785adf3a5d3a3adsf555nb5v55bsaer5v&mac=b827eb6ffa19&command=2",true);
        xmlhttp.send();
};

我会收到警报消息，但使用Firebug或chrome javascript控制台就不会有错误。而且，该页面未执行，但我可以将确切的URL复制并粘贴到浏览器中，并且可以成功执行。

This is how I would do it

JS

function SendCommand()
{
var xmlhttp;
if (window.XMLHttpRequest)
  {// code for IE7+, Firefox, Chrome, Opera, Safari
  xmlhttp=new XMLHttpRequest();
  }
else
  {// code for IE6, IE5
  xmlhttp=new ActiveXObject("Microsoft.XMLHTTP");
  }
xmlhttp.onreadystatechange=function()
  {
  if (xmlhttp.readyState==4 && xmlhttp.status==200)
    {
    document.getElementById("myDiv").innerHTML=xmlhttp.responseText;
    }
}
xmlhttp.open("GET","https://www.justanexample.com/API/MobileAPP/SendCommand.php?apikey=7785adf3a5d3a3adsf555nb5v55bsaer5v&mac=b827eb6ffa19&command=2",true);
xmlhttp.send();
}

HTML

<button type="button" onclick="SendCommand()">My button</button>
<div id="myDiv"></div>

The page it is calling is calling a python script as well as updating a mysql record. Is there something i can do in my request to ask it to not use the browser cache and actually hit my server?

You should not use a GET request for things that execute actions on the server. Use POST instead, which should not be cached.

If that doesn't help, adapt your HTTP cache headers or, as a last resort, append random strings to the url.