(function (){
    var jval = $.fn.val
    function superval(value){
        if (value === undefined){
            if (this.is("input") && this.attr("type") == "checkbox"){
                if (this.attr("value")===undefined){
                    return this[0].checked
                }
            }
            return jval.call(this)
        } else {
            return jval.call(this, value)
        }
    }
    $.fn.val = superval
})()

console.log($('<input type="checkbox"/>').val()); // false
console.log($('<input type="checkbox" checked="whatever"/>').val()); // true
console.log($('<input type="checkbox" checked="whatever" value="yes"/>').val()); // yes
console.log($('<input type="text" value="heya"/>').val()); // heya

<script src="https://cdnjs.cloudflare.com/ajax/libs/jquery/3.3.1/jquery.min.js"></script>