Estoy creando una aplicación de contactos que actualiza una tabla con la entrada de los usuarios, pero parece que no puedo actualizar la tabla una vez que ingrese los datos, solo obtengo el error anterior. No estoy seguro de cómo he cambiado el método, he probado muchas funciones diferentes, etc., pero no he tenido suerte.

var nameField, addressField, emailField, postcodeField;

function twoDigit(v){
if(v.toString().length < 2){
    return "0"+v.toString();
} else {
    return v.toString();
}
}

var Contact = function(name, address, email, postcode){
this.name = name;
this.address = address;
this.email = email;
this.postcode = postcode;
this.completed = false;
};

var contacts = [];

Contact.prototype.toString = function(){
var s = this.name + '\n' + this.address + '\n' + this.email + + '/n'
  +  this.postcode.toString() + '\n';
if(this.completed){
    s += "Completed\n\n";
} else {
    s += "Not Completed\n\n";
}
return s;
};

var addContact = function(nameField, addressField, emailField, postcodeField){
a = new Contact(nameField.value, addressField.value, 
emailField.value, postcodeField.value);
contacts.push(a);
};

var clearUI = function(){
var white = "#fff";
nameField.value = "";
nameField.style.backgroundColor = white;
addressField.value = "";
addressField.style.backgroundColor = white;
emailField.value = "";
emailField.style.backgroundColor = white;
postcodeField.value = "";
postcodeField.style.backgroundColor = white;

};

var updateList = function(){
var tableDiv = document.getElementById("table"),
    table = "<table border='1'><thead><th>Name</th>
<th>Address</th><th>Email</th><th>Postcode</th><th>Completed</th></thead>";

for(var i=0, j=contacts.length; i<j; i++){
    var contacts1 = contacts[i];
     table += contacts1.tableRow();
}
table+="</table>";
tableDiv.innerHTML = table;
};

var add = function(){
addContact(nameField, addressField, emailField, postcodeField);
clearUI();
updateList();
};

var cancel = function(){
clearUI();
updateList();
};

window.onload = function(){
nameField = document.getElementById("name");
addressField = document.getElementById("address");
emailField = document.getElementById("email");
postcodeField = document.getElementById("postcode");
okButton = document.getElementById("ok");
okButton.onclick = add;
cancelButton = document.getElementById("cancel");
cancelButton.onclick = cancel;
clearUI();
}; 

var showTable = function(){
var tableDiv = document.getElementById("table"),
    table = "<table border='1'><thead><th>Name</th>
<th>Address</th><th>Email</th>  <th>Postcode</th></thead>";

for(var i=0, j=contacts.length; i<j; i++){
    var contacts1 = contacts[i];
    table += contacts1.tableRow();
}
table+="</table>";
tableDiv.innerHTML = table;
};