Position a dialog box in center of viewable region of a browser window using javascript/DOM

I need to position a dialog box(read div) in the center of v开发者_如何转开发iewable region of a browser window. I need to use javascript DOM for doing this - making use of scrollHeight, scrollTop, clientHeight, etc. is permissible. The dialog box needs to appear upon clicking a link, it remains invisible otherwise.

CAN'T USE JQUERY TO CREATE A MODAL DIALOG.

Can somebody help me with the centering part of this problem

Regards

(function () {
  var getVieportWidth,
      getViewportHeight;

  if (window.innerWidth) {
    // All browsers except IE
    getViewportWidth = function() { return window.innerWidth; };
    getViewportHeight = function() { return window.innerHeight; };
  }
  else if (document.documentElement && document.documentElement.clientWidth) {
    // IE6 with DOCTYPE
    getViewportWidth = function() { return document.documentElement.clientWidth; };
    getViewportHeight = function() { return document.documentElement.clientHeight; };
  }
  else if (document.body.clientWidth) {
    // IE4, IE5, IE6 without DOCTYPE
    getViewportWidth = function() { return document.body.clientWidth; };
    getViewportHeight = function() { return document.body.clientHeight; };
  }

  var dialogDIVNode = document.getElementById('someID'),
      dialogDIVNodeWidth = dialogDIVNode.offsetWidth,
      dialogDIVNodeHeight = dialogDIVNode.offsetHeight;

  document.getElementById('someLinkID').addEventListener('click', function (e) {
    e.preventDefault();

    dialogDIVNode.style.left = ( getViewportWidth() / 2 - dialogDIVNodeWidth / 2 ) + 'px';
    dialogDIVNode.style.top = ( getViewportHeight() / 2 - dialogDIVNodeHeight / 2) + 'px';
    dialogDIVNode.style.display = 'block';
  }, false);
}());

Here is how to successfully center a dialog box, regardless of the position in the document:

dialogue.style.left = window.innerWidth - (window.innerWidth - dialogue.offsetWidth) / 2 
                      - dialogue.offsetWidth + pageXOffset;

dialogue.style.top = window.innerHeight - (window.innerHeight - dialogue.offsetHeight) / 2 
                     - dialogue.offsetHeight / 2 + pageYOffset;