2015-12-11 17:04
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JavaScript XMLHttpRequest覆盖以前的responseText

I need to do two squential XMLHttpRequests via JavaScript, for data update on two different JavaScript charts, displayed on this php page:

<script type="text/javascript" src="canvasjs.min.js"></script>
<script type="text/javascript" src="AJAXrestFunc.js"></script>
<script type="text/javascript">
        window.onload = function () {
            var updateInterval = 1000;  // milliseconds
            var xhttp1, xhttp2
            function getXHR() {
                if (window.XMLHttpRequest) { // code for IE7+, Firefox, Chrome, Opera, Safari
                    return new XMLHttpRequest();
                else { // code for IE6, IE5
                    return new ActiveXObject("Microsoft.XMLHTTP");
            xhttp1 = getXHR();
            setInterval(function(){updateChart("started", xhttp1)}, updateInterval);
            xhttp2 = getXHR();
            setInterval(function(){updateChart("finished", xhttp2)}, updateInterval);
    <div id="started" style="height:300px; width:100%;"></div>
    <div id="finished" style="height:300px; width:100%;"></div>

I tried to create two different XMLHttpRequest with the function getXHR. The paramater "started"\"finished" in the function updateChart, defines the REST call that I need for the update task.

Here is the updateChart:

function updateChart (func, xhttp) {
     jsonresponse = restCall(func, xhttp);
     parsed = JSON.parse(jsonresponse);
     dps = [];
     count = 0;
     var sum = 0;
     for (var key in parsed) {
         dps[count] = {label: "TaskExecutor"+key, y: parsed[key]};
         sum = sum + parsed[key];

     var totalTask = "Total Tasks "+func+" "+sum;

     var chart = new CanvasJS.Chart(func,{
          theme: "theme4",
          title:{text: "Task "+func},
          axisY: {title: func},
              verticalAlign: "top",
              horizontalAlign: "centre",
              fontSize: 18
          data : [{
              type: "column",
              showInLegend: true,
              legendMarkerType: "none",
              legendText: totalTask,
              indexLabel: "{y}",
              dataPoints: dps



which calls the function restCall(), here defined:

 function restCall(func, xhttp) {
     xhttp.onreadystatechange = function() {
         if (xhttp.readyState == 4 && xhttp.status == 200) {
             content = xhttp.responseText; //here I get the responseText
     };"GET", "/testingREST/monitor/all_"+func, true);
     return content;

I don't know why, but the chart gets and display the last responseText from the xhttp object (like It's overwritten) of the two restcall (even if I reverse the order request, the last call wins).

Infact, when I log the response (in the first three rows updateChart()), I got something like:

"{"1":2,"2":3,"3":10,"5":7,"12":4}" AJAXrestFunc.js:14:4
"started" AJAXrestFunc.js:15:4
"{"1":2,"2":3,"3":10,"5":7,"12":4}" AJAXrestFunc.js:14:4
"finished" AJAXrestFunc.js:15:4
"{"1":2,"2":3,"3":10,"5":7,"12":4}" AJAXrestFunc.js:14:4
"started" AJAXrestFunc.js:15:4
"{"1":2,"2":3,"3":10,"5":7,"12":4}" AJAXrestFunc.js:14:4
"finished" AJAXrestFunc.js:15:4
"{"1":2,"2":3,"3":10,"5":7,"12":4}" AJAXrestFunc.js:14:4
"started" AJAXrestFunc.js:15:4
"{"1":2,"2":3,"3":10,"5":7,"12":4}" AJAXrestFunc.js:14:4

"{"1":2,"2":3,"3":10,"5":7,"12":4}" is the JSON response which I got when I call the rest function "finished". But there isn't the JSON "started" response. Maybe it's overwritten? Someone knows why?

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1条回答 默认 最新

  • douzi8112 2015-12-14 12:46

    As other users pointed out, the problem was the setting of a global variable (content) which was overwritten at each call.

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