javascript submit() is not a function?

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submit is not a function in javascript

Why is the following basic javascript function giving me an error document.getElementById("form").submit is not a function.

The only thing i have on a page is a form and this javascript function. I want he form to auto submit when page is accessed.

<script>
 window.onload = function(){
 document.getElementById('form').submit();
}
</script>