Suppose I have this javascript function. How do I pass the element to it?

Question

function getLeftPos(elm){
  var left = 0;
  while(elm){
    left += elm.offsetLeft;
    elm= elm.offsetParent;
  }
  return left;
}

How come this doesn't work?

getLeftPos(document.getElementById("#frame"));

If I alert(elm) inside that function, it is NULL.

Answer 1

Don't put the # in the id.

getLeftPos(document.getElementById("frame"));

Answer 2

Another viable answer to your question "how come this doesn't work" is:

Javascript != JQuery

Not trying to be snarky but its a really important thing to remember.

For core Javascript documentation a good resource in general is developer.mozilla.org and for getElementById in particular