Answers for "checkbox element value is checked but not"

javacript is checkbox checked

//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
}

check if not checked vanila js

<input type="checkbox" id="check">
<a href="#" onclick="check()">click</a>
<button onclick="check()">button</button>
<script>
 function check() {
    		if (document.getElementById('check').checked) {
            alert("checked");
        } else {
            alert("Not checked.");
        }
       
    }

</script>