javascript pass array data with push, outside ajax success call

Question

I want to read the (images) files name inside a specific folder and then to preview images in Krajee Bootstrap FileInput (FileInput demo link) .

I have a problem with this code in javascript:

var initialPreviewArray = [];
var j = [];

$.ajax({
        type: "POST",
        url: "my_gallery_ajax.php?",
        async: "FALSE",           
        contentType: "application/json; charset=utf-8",
        dataType: "json",
        success: function (data) {            
            // alert('ok');
            $.each(data, function(i,filename) {               
              j[i] = filename;  
              initialPreviewArray.push("<img id='"+i+"' src='user_1/'"+j[i]+"'>"); 
           });
           alert(j[2]); // test - is working - value: '11.jpg'
        }
    });

   alert(j[2]); // test - not working - value: 'undefined'

My goal is to fill 'initialPreviewArray' with data from AJAX, inside or outside.

Inside 'AJAX call' initialPreviewArray.push is not working, even with specific value (outside AJAX is working):

initialPreviewArray.push("<img style='height: 50px' id='1' src='user_1/1.jpg'>");

Conclusion: I am not able to get data outside AJAX.