How to fill in form field, and submit, using javascript?

It would be something like:

document.getElementById("username").value="Username";
document.forms[0].submit()

Or similar edit: you guys are too fast ;)

document.getElementById('username').value="moo"
document.forms[0].submit()

document.getElementById('username').value = 'foo';
document.getElementById('login_form').submit();

You can try something like this:

    <script type="text/javascript">
        function simulateLogin(userName)
        {
            var userNameField = document.getElementById("username");
            userNameField.value = userName;
            var goButton = document.getElementById("go");
            goButton.click();
        }

        simulateLogin("testUser");
</script>