server/apps/user_migrate/js/export.js

$(document).ready(function(){
    // Do the export
	$('#exportbtn').click(function(){
		// Show loader
		$('.loading').show();
		$.getJSON(
			OC.filePath('user_migrate','ajax','export.php'),
			{operation:'create'},
			function(result){
				if(result.status == 'success'){
					// Download the file
					window.location = OC.linkTo('user_migrate','ajax/export.php') + '?operation=download';
					$('.loading').hide();
					$('#exportbtn').val(t('user_migrate', 'Export'));
				} else {
					// Cancel loading
					$('#exportbtn').html('Failed');
					// Show Dialog	
					OC.dialogs.alert(t('user_migrate', 'Something went wrong while the export file was being generated'), t('user_migrate', 'An error has occurred'), function(){ 
						$('#exportbtn').html(t('user_migrate', 'Export')+'<img style="display: none;" class="loading" src="'+OC.filePath('core','img','loading.gif')+'" />'); 
					});
				}
			}
		// End ajax
		);
	});
});