Im getting a error in the Web Inspector as shown below:

TypeError: 'null' is not an object (evaluating 'myButton.onclick = function() {

var userName = myTextfield.value;

greetUser(userName);



return false;



}')

Here is my Code (HTML):

<h2>Hello World!</h2>

<p id=myParagraph>This is an example website</p>



<script src=js/script.js type=text/javascript></script>



<form>

  <input type=text id=myTextfield placeholder=Type your name />

  <input type=submit id=myButton value=Go />

</form>

Here is the JS:

var myButton = document.getElementById(myButton);

var myTextfield = document.getElementById(myTextfield);



function greetUser(userName) {

var greeting = Hello  + userName + !;

document.getElementsByTagName (h2)[0].innerHTML = greeting;

}



myButton.onclick = function() {

var userName = myTextfield.value;

greetUser(userName);



return false;



}

Any Idea why I am getting the error?