how to know checkbox is checked or not code example

Example 1: How to check whether a checkbox is checked in jQuery?

//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
}

Example 2: jquery check if checkbox is not checked

if (!$("#checkboxID").is(":checked")) {
    // do something if the checkbox is NOT checked
}

Example 3: submit form when checkbox is checked

onChange="this.form.submit()"

Example 4: checkbox is not getting checked

<input type="checkbox" name="free" value="mon09" onClick="this.checked=!this.checked;">

Example 5: jquery check if checkbox is checked

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