how to get a JavaScript array as result from jQuery.ajax()?

I'm using something like this,

jQuery(function($) {
    function updateData()
    {

        var html = $.ajax({
            url: 'ajaxMap.php?id='+lastId,
            success: function(data) {
                            alert(data);
            }
        });
    }

    setInterval(function(){
        updateData()
    }, 2000);
});

I expect to get a JavaScript array after querying the PHP file. As shown in the code, 'data' is returned after querying the url (The PHP fi开发者_如何转开发le simply outputs a JavaScript array). When I use alert(data), it shows me the same array. But I cannot use this as an array. For example, I can't use array functions like .length, etc on this. JavaScript treats this as a simple string. What am I doing wrong here? Let me know if I need to provide more information.

Use this JS (not tested)

$(document).load(function(){
  setInterval(function() {
    $.getJSON('ajaxMap.php?id='+lastId,function(data) {
      alert(data);
    });
  });
});

And json_encode($array) on server side ('application/json' MIME header would be great but it is not needed)

You need to parse Json string like this or this