JavaScript で CSS プロパティ値を元の単位で取得するにはどうすればよいですか?

function getMatchedStyle(elem, property){
    // Element style has highest priority
    var val = elem.style.getPropertyValue(property);

    // Stop if important
    if(elem.style.getPropertyPriority(property))
        return val;

    // Get matched rules
    var rules = getMatchedCSSRules(elem);

    // Iterate rules backwards, prioritizing highest priority
    for(var i = rules.length; i --> 0;){
        var r = rules[i];

        var important = r.style.getPropertyPriority(property);

        // Reset if more important or not yet set
        if(val == null || important){
            val = r.style.getPropertyValue(property);

            // Stop if important
            if(important)
                break;
        }
    }

    return val;
}

<div class="b">div 1</div>
<div>

div      { width: 100px; }
.d3      { width: auto !important; }
div#b    { width: 80%;   }
div#c.c  { width: 444px; }
x, div.a { width: 50%;   }
.a       { width: 75%;   }

div 1:  100px
div 2:  50%
div 3:  auto
div 4:  44em