空XmlHttpRequest.response，但是XHR包含我的数据

Question

I'm new to JS and i'm using XHR in order to connect a mongodb hosted on mongolab.

I use REST because mongo don't have a js driver (they got only node.js driver and I can't use it for my project). So, I use a GET request on my DB, then I don't know how to retrieve the response. Here is my GET function :

function getRequest(callback){
  var xhr = getXMLHttpRequest();
  xhr.onreadystatechange = function() {
        if (xhr.readyState == 4 && (xhr.status == 200 || xhr.status == 0)) {
           callback(xhr.responseText);
        }
    };

  xhr.open("GET",restURI+apiKey, true);
  xhr.send(null);
  console.log(xhr);
  console.log(xhr.responseText); 
}
function readData(sData) {
   XHR reading
    if (sData == "OK") {
        alert("All clear");
    } else {
        alert("Some issues");
    }
}

I got an empty string when I try :

console.log(xhr.response);

So I use to suppose that I failed in my REST request but no, when logging the xhr object i got this :

So my questions are :

1. Why response and responseText are empty on the first line of the xhr object ?
2. How do I retrieve my data ?

Answer 1

Here you are, xmlhttprequest with json:

<div id="id01"></div>

<script>
var xmlhttp = new XMLHttpRequest();
var url = "myTutorials.txt";

xmlhttp.onreadystatechange = function() {
    if (xmlhttp.readyState == 4 && xmlhttp.status == 200) {
        var myArr = JSON.parse(xmlhttp.responseText);
        myFunction(myArr);
    }
}
xmlhttp.open("GET", url, true);
xmlhttp.send();

function myFunction(arr) {
    var out = "";
    var i;
    for(i = 0; i < arr.length; i++) {
        out += '<a href="' + arr[i].url + '">' + 
        arr[i].display + '</a><br>';
    }
    document.getElementById("id01").innerHTML = out;
}
</script>

Refer: http://www.w3schools.com/json/json_http.asp

Hope this help.