Javascript async call inside for loop

Question

I am making an async API call from inside a for loop in JavaScript and I am stuck on how to understand the result. This code is making the right calls to the API (I know this from looking at fiddler).

However when the reslts come back and I print the results at console.log(translation), I only see the result from the last API call.

I know this is an async problem but how best should I address the incoming response from xmlhttp2?

        for (var j = 0; j <= current; j++) {
        //some preprocessing                

            var xmlhttp2=new XMLHttpRequest();
            xmlhttp2.onreadystatechange=function()
            {
                if (xmlhttp2.readyState==4 && xmlhttp2.status==200)
                {
                    var translation = xmlhttp2.responseText;
                    console.log(translation);
                }
            }                               

            var sendparam = "http://api.foo.do.stuff.etc";
            xmlhttp2.open("GET",sendparam,true);
            xmlhttp2.send();

        }
    }

Use this.responseText instead of xmlhttp2.responseText, which has the loop scope problem — Bergi
– Bergi, Commented Feb 26, 2015 at 22:23

tsurti · Accepted Answer · 2015-02-26 22:21:46Z

1

Figured it out. Closures FTW

    for (var j = 0; j <= current; j++) {
    //some preprocessing                
        (function (j) {
        var xmlhttp2=new XMLHttpRequest();
        xmlhttp2.onreadystatechange=function()
        {
            if (xmlhttp2.readyState==4 && xmlhttp2.status==200)
            {
                var translation = xmlhttp2.responseText;
                console.log(translation);
            }
        }                               

        var sendparam = "http://api.foo.do.stuff.etc";
        xmlhttp2.open("GET",sendparam,true);
        xmlhttp2.send();
        })(j);
    }

answered Feb 26, 2015 at 22:21

tsurti

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