JavaScript: Find DIV's line-height, not CSS property but actual line-height

The answer is actually using .clientHeight. As Gaby said, this is not really reliable/trustworthy. However, it is! Here:

function getLineHeight(el) {
    var temp = document.createElement(el.nodeName), ret;
    temp.setAttribute("style", "margin:0; padding:0; "
        + "font-family:" + (el.style.fontFamily || "inherit") + "; "
        + "font-size:" + (el.style.fontSize || "inherit"));
    temp.innerHTML = "A";

    el.parentNode.appendChild(temp);
    ret = temp.clientHeight;
    temp.parentNode.removeChild(temp);

    return ret;
}

"Clone" the properties of your element into a new one, get the new's clientHeight, delete the temporary element, and return it's height;

Explained at quirksmode : http://www.quirksmode.org/dom/getstyles.html

example: http://www.jsfiddle.net/gaby/UXNs2/

function getStyle(el,styleProp)
{
    var x = document.getElementById(el);
    if (x.currentStyle)
        var y = x.currentStyle[styleProp];
    else if (window.getComputedStyle)
        var y = document.defaultView.getComputedStyle(x,null).getPropertyValue(styleProp);
    return y;
}

and use it like

getStyle('test', 'line-height' )