How to check & uncheck all the checkboxes in a form

  The below function is used to check all the checkboxes & uncheck them in a form

checked = false;
function checkedAll () {
    if (checked == false){checked = true}else{checked = false}
             for (var i = 0; i < document.getElementById('myform').elements.length; i++) {
                   document.getElementById('myform').elements[i].checked = checked;
                  }
}