Catch checked change event of a checkbox

<input type="checkbox" id="something" />

$("#something").click( function(){
   if( $(this).is(':checked') ) alert("checked");
});

Edit: Doing this will not catch when the checkbox changes for other reasons than a click, like using the keyboard. To avoid this problem, listen to changeinstead of click.

For checking/unchecking programmatically, take a look at Why isn't my checkbox change event triggered?

The click will affect a label if we have one attached to the input checkbox?

I think that is better to use the .change() function

<input type="checkbox" id="something" />

$("#something").change( function(){
  alert("state changed");
});

Use the :checked selector to determine the checkbox's state:

$('input[type=checkbox]').click(function() {
    if($(this).is(':checked')) {
        ...
    } else {
        ...
    }
});