How to check if a Textarea is empty in Javascript or jQuery?

How do I check if a textarea contains nothing?

I tried with this code:

if(document.getElementById("field").value ==null)
{
    alert("debug");
    document.getElementById("field").style.display ="none";
 }

But it doesn't do what I expect. I expect that it should appear a messagebox "debug" and that the textarea is not shown.

How can I fix that issue?