javascript - Trying to Understand How jQuery calculates computed properties for IE8 using currentStyle -

i'm trying understand how jquery arrives @ pixel values ie8 when dealing non-pixel based property values, such margin-top: 2em, or height: auto. ie9+, getcomputedstyle() can provide easily, in case of ie8, currentstyle not. trying arrive @ solution can calculate total height of element, including css height, padding, border, , margin browsers ie8+. have come across following answer, can't understand going on in accepted answer.

cross-browser (ie8-) getcomputedstyle javascript?

i wondering if explain going on in code?

here shim computed style webplatform.

if (!window.wpo) { window.wpo = {}; }  if (!wpo.utils) { wpo.utils = {}; }    wpo.utils.getcomputedstyle = function(_elem, _style)  {// wpo getcomputedstyle shim.     var computedstyle;     if (typeof _elem.currentstyle != 'undefined')       { computedstyle = _elem.currentstyle; }     else       { try{computedstyle = document.defaultview.getcomputedstyle(_elem, null);}catch(e){return '';} }      return computedstyle[_style];  }