cannot read property "appendChild" of null

 window.onload = function(){

            var allA = document.getElementsByTagName("a");

            for(var i=0;i<allA.length;i++){

                 allA[i].onclick = function(){

                     var tr = this.parentNode.parentNode;

                     var name = tr.getElementsByTagName("td")[0].innerHTML;

                     var flag = confirm("你确定删除"+name+"吗？"); 

                     if(flag){

                        tr.parentNode.removeChild(tr);

                     }

                 }

            }

            var addEmpButton =document.getElementById("addEmpButton");

            addEmpButton.onclick = function(){

                var name = document.getElementById("empName").value;

                var email = document.getElementById("email").value;

                var salary = document.getElementById("salary").value;

                var tr = document.createElement("tr");

                var nameTd= document.createElement("td");

                var emailTd= document.createElement("td");

                var salaryTd= document.createElement("td");

                var aTd= document.createElement("td");

                var a = document.createElement("a");

                var nameText = document.createTextNode(empName);

                var emailText = document.createTextNode(email);

                var salaryText = document.createTextNode(salary);

                var delText = document.createTextNode("Delete");

                nameTd.appendChild(nameText);

                emailTd.appendChild(emailText);

                salaryTd.appendChild(salaryText);

                a.appendChild(delText);

                aTd.appendChild(a);

                tr.appendChild(nameTd);

                tr.appendChild(emailTd);

                tr.appendChild(salaryTd);

                tr.appendChild(aTd);

                var employeeTable = document.getElementById(employeeTable);

                employeeTable.appendChild(tr);

                var employeeTable = document.getElementById("employeeTable");
 

（）中是字符串，要加引号

你这个employeeTable哪来的，应该是document.getElementById(employeeTable);返回null。