Question

I have a function called makehash()
I have to fill the value of hash field with the return value of makehash()

<form action="http://example.com/test.php" enctype="multipart/form-data" method="post">
<input type="file" name="datafile" size="40">
<input type="hidden" name="upload_mode" value="0">
<input type="hidden" name="hash" value="">
<input type="submit" value="Upload">
</form>

Answer 1

document.forms[0].elements["hash"].value = makehash();

This assumes you have only one form, if you have more specify the form name instead of 0 or give ID to the hidden input and use document.getElementById instead.