Get computed font size for DOM element in JS

You could try to use the non-standard IE element.currentStyle property, otherwise you can look for the DOM Level 2 standard getComputedStyle method if available :

function getStyle(el,styleProp) {
  var camelize = function (str) {
    return str.replace(/\-(\w)/g, function(str, letter){
      return letter.toUpperCase();
    });
  };

  if (el.currentStyle) {
    return el.currentStyle[camelize(styleProp)];
  } else if (document.defaultView && document.defaultView.getComputedStyle) {
    return document.defaultView.getComputedStyle(el,null)
                               .getPropertyValue(styleProp);
  } else {
    return el.style[camelize(styleProp)]; 
  }
}

Usage:

var element = document.getElementById('elementId');
getStyle(element, 'font-size');

More info:

• Get Styles (QuirksMode)

Edit: Thanks to @Crescent Fresh, @kangax and @Pekka for the comments.

Changes:

• Added camelize function, since properties containing hypens, like font-size, must be accessed as camelCase (eg.: fontSize) on the currentStyle IE object.
• Checking the existence of document.defaultView before accessing getComputedStyle.
• Added last case, when el.currentStyle and getComputedStyle are not available, get the inline CSS property via element.style.