アラートボックスに変数値が表示されない

Question

私はJavaScriptを学んでいますが、今は関数を使って遊んでいます。ユーザーに何を入力したかを知らせようとしていますが、名前と姓と電子メールを入力し、 、警告ボックスは言う：あなたは要素のvalue、ない要素そのものを必要とし

<script> 
    function printMessage(firstNameID,lastNameID,emailID){ 
     var firstName = document.getElementById(firstNameID); 
     var lastName = document.getElementById(lastNameID); 
     var email = document.getElementById(emailID); 
     alert("Hello " + firstName + " " + lastName + "! Your email " + email + " was succesfully registered ..."); 
    } 
</script> 
<form name="subscribeform"> 
    First Name: <br> 
<input type="textbox" id="firstname" form="subscribeform"><br> 
    Last Name: <br> 
<input type="textbox" id="lastname" form="subscribeform"><br> 
    Email: <br> 
    <input type="textbox" id="email" form="subscribeform"><br><br> 
    <button id="subscribe" form="subscribeform" onclick="printMessage('firstname','lastname','email');">Subscribe, you know you want to ...</button> 
    </form> 

Answer 1

：

Hello [object HTMLInputElement] [object HTMLInputElement]! Your email [object HTMLInputElement] was succesfully registered ... 

これは私のコードです。

alert("Hello " + firstName.value + " " + lastName.value + "! Your email " + email.value + " was succesfully registered ..."); 

か...

var firstName = document.getElementById(firstNameID).value; 
var lastName = document.getElementById(lastNameID).value; 
var email = document.getElementById(emailID).value;