jquery, selector for class within id

Question

Below, how should I select the elements that contain the class my_class within the element with id = "my_id"?

Note that the element may also have another class, which I am not selecting for.

<div id = "my_id">     <span class = "my_class hidden">hi</span>     <span class = "my_class">hello</span> </div> 

was trying

$("#my_id [class*=my_class ]") 

Answer 1

You can use the class selector along with descendant selector

$("#my_id .my_class") 

Answer 2

Just use the plain ol' class selector.

$('#my_id .my_class') 

It doesn't matter if the element also has other classes. It has the .my_class class, and it's somewhere inside #my_id, so it will match that selector.

Regarding performance

According to the jQuery selector performance documentation, it's faster to use the two selectors separately, like this:

$('#my_id').find('.my_class') 

Here's the relevant part of the documentation:

ID-Based Selectors

// Fast: $( "#container div.robotarm" );  // Super-fast: $( "#container" ).find( "div.robotarm" ); 

The .find() approach is faster because the first selection is handled without going through the Sizzle selector engine – ID-only selections are handled using document.getElementById(), which is extremely fast because it is native to the browser.

Selecting by ID or by class alone (among other things) invokes browser-supplied functions like document.getElementById() which are quite rapid, whereas using a descendent selector invokes the Sizzle engine as mentioned which, although fast, is slower than the suggested alternative.