Answers for "if checkbox is checked"

jquery if checkbox checked

$( "SELECTOR" ).attr( "checked" )  // Returns ‘true’ if present on the element, returns undefined if not present
$( "SELECTOR" ).prop( "checked" ) // Returns true if checked, false if unchecked.
$( "SELECTOR" ).is( ":checked" ) // Returns true if checked, false if unchecked.

if checkbox is checked

if ($('#id_input').is(':checked')) { }

checkbox is checked jquery

$(your_checkbox).is(':checked');

get the state of a checkbox

function checkAddress()
{
    var chkBox = document.getElementById('checkAddress');
    if (chkBox.checked)
    {
        // ..
    }
}