//using plane javascript
if(document.getElementById('on_or_off_checkbox').checked) { //I am checked
}
//using jQuery
if($('#on_or_off_checkbox').is(':checked')){ //I am checked
}

$(".checkbox").change(function() { if(this.checked) { //Do stuff }
});

$('input.checkbox').is(':checked')

if($('#on_or_off_checkbox').is(':checked')){ //I am checked
}

if($("#rule").is(":checked")){ $("#form").submit(); }

if($('#checkboxID').is(':checked')){ return true; }

$('#isAgeSelected').click(function() { $("#txtAge").toggle(this.checked);
});
<script src="https://ajax.googleapis.com/ajax/libs/jquery/2.1.1/jquery.min.js"> </script>
<input type="checkbox" id="isAgeSelected"/>
<div id="txtAge" style="display:none">Age is something</div>

if ($('#myCheckbox').is(':checked')) { // Checkbox is checked console.log("Checkbox is checked");
} else { // Checkbox is not checked console.log("Checkbox is not checked");
}

// HTML
<input type="checkbox" id="myCheckbox">
// jQuery
if ($('#myCheckbox').is(':checked')) { console.log('Checkbox is checked');
} else { console.log('Checkbox is not checked');
}

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