echo'd PHP encode JSON called via AJAX returns what exactly?

I think I'm missing something here:

Using AjAX I get some data from a database and send it back in JSON format $jsondata = array();

while ($Row = mysql_fetch_array($params))
{

    $jsondata[]= array('cat_id'=>$Row["cat_id"], 
                          'category'=>$Row["category"], 
                     'category_desc'=>$Row["category_desc"],
                     'cat_bgd_col'=>$Row["cat_bgd_col"]);
};

echo("{\"Categories\": ".json_encode($jsondata)."};");

No problem so far I think.

On the cleint side I receive back the above into

ajaxRequest.responseText

and if I do this

var categoriesObject = ajaxRequest.responseText; 
alert(categoriesObject);

I see what I expect to see ie the entire array in the alert.

Where it all goes wrong is trying to access the response. The error I get is that the "categoriesObject" is not an object - if not what is it? what's bugginh me is that I can't even access it like this:

document.开发者_JAVA百科write(categoriesObject.Categories[0].category);

so what am I doing wrong?

1. You should not create JSON manually. Use:

   echo json_encode(array('Categories' => $jsondata));
   

   or just

   echo json_encode($jsondata);
   

   I don't see a reason to add Categories.

2. You have to decode the JSON on the client side, using JSON.parse (available in most browsers, but also available as script):

   var data = JSON.parse(ajaxRequest.responseText);
   

3. If you want to be very correct, add

   header('Content-type: application/json');
   

   to your PHP script.

Are you acutally parsing the JSON? It won't work without.

var categoriesObject = JSON.parse(ajaxRequest.responseText);