Question

Here is a program that calls ajax to read a value from the header of a http response and set a variable called _ecpop and use that for later operations…

So I defined a variable globally called _ecpop as

var _ecpop="";

and then here is the ajax call code:

if (typeof XMLHttpRequest != "undefined") {
    _ec_req = new XMLHttpRequest();
    } else if (window.ActiveXObject) {
    _ec_req = new ActiveXObject("Microsoft.XMLHTTP");
    }
    // make the request
    _ec_req.open("GET", requestString, true);
    // function to deal with response
    _ec_req.onreadystatechange = function()  {
    if (_ec_req.readyState==4)
        {
        if (_ec_req.status==200)
            {
            var respHeaders = _ec_req.getAllResponseHeaders();
            // strip out unnecessary headers
            respHeaders = respHeaders.replace (/\n/g,"|X|");
            var beg = respHeaders.indexOf("ECAcc (");
            _ecpop = respHeaders.substring(beg+7,beg+10);
            //make an if statement here...
            console.log(_ecpop);

            }

        }
    }
    console.log(_ecpop);

and the rest of the code uses _ecpop.

The problem is the browser executes the rest of the code with _ecpop=””. i.e. first the second console.log(_ecpop); returns empty and then the first console.log(_ecpop); returns the correct value.

Is there anybody who knows how I can get around it? it is so irritating …

Thanks,

Amir.

Answer 1

AJAX (an acronym for Asyncronous Javascript And XML) is inherently asynchronous. That means that it begins executing when the Javascript engine encounters it in a script, but the rest of the script continues processing without waiting for the AJAX call to complete.

You can circumvent this behavior by instructing your AJAX call to operate synchronously:

_ec_req.open("GET", requestString, false);

Setting false as third parameter of open() will provide your requested behavior.