jQuery event target element attribute retrieval

Question

I'm having some trouble pulling attributes from an event target element.

I am using php to access a mysql database. From the query I pull out some image file names and their respective id's. I then display these images in a table with the following line:

print '<td><img id="img_' . $row['paint_id'] . '" class="thumb_target" src="img/paint/thumb/' . $row['paint_thumb'] .'"></td>';

As you can see, this line gives each image the id 'img_xx' where xx is the images numeric id in the sql database. I also give each image the class 'thumb_target'. In document ready I set a .click event for the thumb_target elements. This makes an AJAX call which should pass the img_xx id as data. I got this to work in chrome using

data: 'imgID=' + event.target.id

However, several hours later I decided to check something else in firefox and found that it doesn't work for all browsers. Using jQuery's method:

var id = $(this).attr('id');

I can't get id to be anything but undefined. Am I targeting a different element than the element I think I am?

Here's the pertinent javascript:

function runClick() {
  var id = $(this).attr('id');
  alert(id);
  $.ajax({
    url: 'overlay.php',
    //~ data: 'imgID=' + event.target.id,
    //~ data: ({ imgID: id }),
    data: 'imgID=' + id,
    dataType: 'json',
    success: function(data, textStatus, jqXHR) {
        processJSON(data);
    },
    error: function(jqXHR, textStatus, errorThrown){
        alert("Failed to perform AJAX call! textStatus: (" + textStatus +
              ") and errorThrown: (" + errorThrown + ")");
    }
  });
}

$(document).ready( function() {
  $('.thumb_target').click( function() {
    runClick();
  });

  $('#overlay').hide();
});

Here's a link to the test page: http://www.carlthomasedwards.com/painting2.php

Answer 1

runClick is executed in global scope, so this refers to the global object( window ).

Use that instead:

$('.thumb_target').click( function(event) {
    runClick.apply(this);
  });

or even more simple:

$('.thumb_target').click( runClick);

Answer 2

When the browser executes runClick , the this context isn't preserved. If you pause in the Chrome debugger , you can see that this is actually Window when runClick is called.

The way to bypass the problem is to pass the element into runClick :

function runClick(elem) {
  alert($(elem).attr('id'));
  ...
}

and

$('.thumb_target').click( function() {
  runClick(this);
});