jQuery - Load json in a XML load Event

$.getJSON is asynchronous because AJAX is asynchronous. You could access the results of of an AJAX call only inside the success callback. In your example you are attempting to assign the return value of the $.getJSON call to the data[i].post variable. But this cannot possibly work because the value could be available much later, only after the AJAX call succeeds.

So you could invert your logic a little bit:

$.get('xmlfile.xml', function(xml) {
    var oXML = $(xml);

    oxml.find('user').each(function(i) {
        var index = i;
        $.getJSON($(this).attr('post'), function(res) {
            data[index].name = $(this).attr('name');
            data[index].pic = $(this).attr('pic');
            data[index].post = res;
        });
    });
});

The main problem is that getJSON is asyncronous

Your post properties in data are null because there are two things you're doing wrong here:

1. getJSON is asyncronous so it gets your data after your .each is already executed
2. getJSON doesn't return data but rather jqXHR object

Try changing it this way:

var data = []; // you likely want an array and not an object
var counter = 0;

$.get('xmlfile.xml', function(xml) {

    var oXML = $(xml);

    oxml.find('user').each(function(i) {
        var me = $(this);

        data.push({
            name: me.attr('name'),
            pic: me.attr('pic'),
            post: null
        });

        // increment counter
        counter++;

        $.getJSON(me.attr('post'), (function(index) {
            return function(data) {
                data[index].post = data;
                if (!--counter) // when it reaches ZERO
                {
                   setTimeout(function() {
                       // all JSONs have loaded now continue with your stuff
                   }, 0);
                }
            };
        })(i));
    });

});

Above code also detects when all JOSN objects have loaded so you can continue processing.

But apart from that it works this way:

1. it gets the XML
2. when XML is received it iterates through user elements and prepares data objects and pushes them into array
3. with each object it issues an Ajax call to get JSON data as well
4. each successful call to JSON then populates correct data object's post property and decreases counter
5. when counter reaches zero an additional function is being queued where you can put code that should execute after everything's loaded and done.

If your JSON requests can also fail, then I suggest to use $.ajax instead of getJSON so you can also provide error function inside of which you'd also decrement counter. Otherwise it wouldn't reach zero if only successes are being counted.