How to start a marquee with JavaScript based on user input?

I am trying to use JavaScript to start a marquee when a user puts their name into a textbox and then clicks a button. I've got an idea as to how to do it, but my script never fully works. Any help is appreciated!

Here's what I have so far:

<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">

<html xmlns="http://www.w3.org/1999/xhtml">
<head>
<title></title>
<script type="text/javascript">
    function StartMarquee() {
        var text = document.getElementById(namebox);
        if (text != null) {
            document.write("<marquee behavior='scroll' direction='right'>Hello " + text + "!</marquee>");
        }
        else {
            alert("Enter your name first!!!");
        }
    } 
</script>
</head>
<body>
<table style="margin:0px auto 0px auto;">
<tr><td>Enter your name!</td>
<td><input type="text" id="namebox"/></td>
<td><input type="button" value="Enter" onclick="StartMarquee()"/></td></tr>
</table>
</body>
</html>

Solution

Your JavaScript has a few problems.

You are passing an undefined variable namebox to getElementById. You need to put this in quotes ('namebox').
You need to check the value of text against the empty string, not null.
You need to use the value of the input (text.value as opposed to just text) in the element you're creating.

Here is what your code would look like with these fixes:

function StartMarquee() {
  var text = document.getElementById('namebox');
  if (text.value !== '') {
    document.write("<marquee behavior='scroll' direction='right'>Hello " + text.value + "!</marquee>");
  }
  else {
    alert("Enter your name first!!!");
  }
}

Some other general suggestions:

Don't use document.write. Instead, use DOM methods to create a new element and insert it into the DOM.
Use unobtrusive JavaScript. Attach your behavior after the document loads instead of using inline event handlers.
Use === and !== for conditions to avoid type coercion and to ensure you're getting the result you think you are.
Never, ever use marquee.