Check if checkbox is checked with jQuery

$('#checkbox').is(':checked'); 

The above code returns true if the checkbox is checked or false if not.

All following methods are useful:

$('#checkbox').is(":checked")

$('#checkbox').prop('checked')

$('#checkbox')[0].checked

$('#checkbox').get(0).checked

It is recommended that DOMelement or inline "this.checked" should be avoided instead jQuery on method should be used event listener.

IDs must be unique in your document, meaning that you shouldn't do this:

<input type="checkbox" name="chk[]" id="chk[]" value="Apples" />
<input type="checkbox" name="chk[]" id="chk[]" value="Bananas" />

Instead, drop the ID, and then select them by name, or by a containing element:

<fieldset id="checkArray">
    <input type="checkbox" name="chk[]" value="Apples" />

    <input type="checkbox" name="chk[]" value="Bananas" />
</fieldset>

And now the jQuery:

var atLeastOneIsChecked = $('#checkArray:checkbox:checked').length > 0;
//there should be no space between identifier and selector

// or, without the container:

var atLeastOneIsChecked = $('input[name="chk[]"]:checked').length > 0;

$('#' + id).is(":checked")

That gets if the checkbox is checked.

For an array of checkboxes with the same name you can get the list of checked ones by:

var $boxes = $('input[name=thename]:checked');

Then to loop through them and see what's checked you can do:

$boxes.each(function(){
    // Do stuff here with this
});

To find how many are checked you can do:

$boxes.length;