Most efficient way to re-use jQuery-selected elements

Question

I can imagine the correct answer to this based on theory, but I'm just looking for some confirmation. I'm wondering what the most efficient way to re-use a jQuery-selected element is. For example:

$('#my_div').css('background','red');
//some other code
$('#my_div').attr('name','Red Div');

vs.

myDiv = $('#my_div');
myDiv.css('background','red');
//some other code
myDiv.attr('name','Red Div');

I assume the second example is more efficient because the element #my_div doesn't have to get found more than once. Is that correct?

Similarly, is it more efficient to first save $(this) in a varaible, such as 'obj', and then reuse 'obj' rather than using $(this) over and over? In this case, jQuery isn't being forced to find an element over and over again, but it IS being forced to convert this to a jQuery object [$(this)]. So as a general rule of thumb, should a jQuery object ALWAYS be stored in a variable if it will be used more than once?

Answer 1

You should write your code such that you limit the number of DOM traversals.

When you write something like this:

$('#my_div').css('background','red');
//some other code
$('#my_div').attr('name','Red Div');

You are finding #my_div twice, which is inefficient.

You can improve this either by assigning the result of a selector (i.e. var x = $('.something')) and manipulate the variable x, or you can chain your method calls like this:

$('#my_div').css('background','red').attr('name','Red Div');

You'll see the above code used a lot, because you're finding the element once. The css() method will apply a CSS style and return the actual result of $('#my_div'), so you can invoke another method, in this case attr().

My preferred way of handling the re-use of selectors is to store them as a variable, and wrap my stuff in a closure.