Javascript passing arrays between nested Functions()

Question

I keep getting undefined in the alertbox popup...am I missing something? And there is data on nodeArray[0], because if I put it inside the for loop it alerts correctly.

function grabNodes(env, poolName){
    var env = "dev";
    var nodeArray = new Array();

    var ajaxRequest; //initialize ajax object

    var browser = navigator.appName; //find the browser name
    if(browser == "Microsoft Internet Explorer"){
        /* Create the object using MSIE's method */
        ajaxRequest = new ActiveXObject("Microsoft.XMLHTTP");
    }
    else{
        /* Create the object using other browser's method */
        ajaxRequest = new XMLHttpRequest();
    }

    // Create a function that will receive data sent from the server
    ajaxRequest.onreadystatechange = function(){
        if(ajaxRequest.readyState == 4){
        // Get the data from the server's response
        //what on this page is changing
            var xmlRes=ajaxRequest.responseXML.documentElement;
            try {
                for(var i=0; i<xmlRes.getElementsByTagName('node').length;i++){
                    nodeArray[i] = xmlRes.getElementsByTagName('node')[i].firstChild.nodeValue;
                }//end for loop
            }
            catch (err){
            }
             alert(nodeArray[0]);
        }

    }

//return nodeArray;
}

function is called by:

function other(){
    oNodeArray = grabNodes(env, poolName);
}

Felix Kling · Accepted Answer · 2011-02-22 21:06:21Z

3

Ajax calls are asynchronous. alert(nodeArray[0]); is executed before the function you assigned to ajaxRequest.onreadystatechange.

Put the alert in the callback and it will work (provided you get a valid response). Every code that has to operate on the result has to be called from the callback.

Update: You cannot return data. You have to restructure your code to use callbacks.

If your current code is like

oNodeArray = grabNodes(env, poolName);

// work on `oNodearray`

you have to change it to:

grabNodes(env, poolName, function(oNodearray) {
     // work on `oNodearray`
});

and the Ajax callback has to call the function you pass and pass the array to it:

function grabNodes(env, poolName, callback){
    //....
    ajaxRequest.onreadystatechange = function(){
        // stuff
        callback(nodeArray);
    }
}

Because Ajax calls are asynchronous, they don't block the execution of the code. You need to pass a function that will be executed once the data is received.

Further comments:

Use var nodeArray = []; to initialise an array.
var env = "dev"; will shadow the parameter env. If you want to assign a default value, do
```
env = env || 'dev';
```

edited Feb 22, 2011 at 21:06

answered Feb 22, 2011 at 20:47

Felix Kling

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3 Comments

Jim Over a year ago

so the i moved the alert and it gives me the data so I know it is working. But I am not understanding the return statement...if I put it there where does it return to? I need it to return to the oNodeArray which is in another function that calls grabNodes()

Felix Kling Over a year ago

@Jim: You don't need to put a return statement in the callback, it will have no effect. Please see my updated answer.

Matt Molnar Over a year ago

grabNodes() has already returned by the time you set the array in the ajax callback. You need to re-formulate your code such that you can call a given function with nodeArray as a parameter from your ajax callback. You're trying to program this in a synchronous manner but ajax is by definition not synchronous.

amit_g · Accepted Answer · 2011-02-22 20:49:02Z

0

The ajax call is most likely not yet complete when the alert is invoked. This should be placed inside the onreadystatechange handler.

answered Feb 22, 2011 at 20:49

amit_g

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