What Dian said plus...
What exactly are you trying to acheive by clicking the enter button? Do you want to go to the next line in a text area, unfocus from a text area, submit a form???
You can't actually send a response the the browser that says the enter key was clicked (or any other key for that matter), but if you let us know what you are trying to do we may be able to help.
I.E.
If you want to mimic the letter A (caps) being pressed when a user is on a text field:
document.getElementById("tbxId").value = "A";
TIP: trying googling the answer before posting, you'll find that more times than not someone else somewhere has had the same request and posted an answer online.
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I have recently been semi-converted to ensuring all my code (well most of it) works in both javascript and non-javascript enabled browsers