js无刷新实现textbox值相加，ଠ前面三个textbox值相加后显示在第四个textbox中

window.addEventListener('load',function(){
    
    var el = function(id){ return document.getElementById(id);};
    var changeHandle = function(){  
        el('tbPackLayerAll').value = el('tbPackLayer1').value+el('tbPackLayer2').value+el('tbPackLayer3').value;
     };
    
    el('tbPackLayer1').addEventListener('change',changeHandle);
    el('tbPackLayer2').addEventListener('change',changeHandle);
    el('tbPackLayer3').addEventListener('change',changeHandle);
    
});

function sum(){
    var max = document.getElementById("tbPackLayer3").value;
    var zhong= document.getElementById("tbPackLayer3").value;
    var min = document.getElementById("tbPackLayer3").value;
    document.getElementById("tbPackLayer3").innerHTML= parseInt(max) + parseInt(zhong) + parseInt(min) ;
}