How can I do it? .click();

I have a very unusual problem. I want to make a script like this:

<script>
x = document.getElementById("a");
x.click();
</script>

So it's click on an element witch one Id's is "a", but I want that it make mouseup in this element. How can I do it, because if I write:

<script>
x = document.getElementById("a");
x.mouseup();
</script>

it's invaild.

Please help!

Máté