Count visible characters in overflow'd div

HTML

<div id="thediv">This is my text inside a div and I want the overflow of the text to be cut</div>

CSS

#thediv{
    width: 302px;
    height: 20px;
    overflow: hidden;
    /* white-space: nowrap; */
}

JavaScript

var div = document.getElementById('thediv');
var numc = count_visible(div);

// You can do something like this to hide partially visible characters
// div.firstChild.nodeValue = div.firstChild.data.substr(0,numc);

alert(numc + ' completely visible characters!');

function count_visible(el){
    var padding, em, numc;
    var text = el.firstChild.data;
    var max = el.clientWidth;

    var tmp = document.createElement('span');
    var node = document.createTextNode();
    tmp.appendChild(node);
    document.body.appendChild(tmp);

    if(getComputedStyle)
        tmp.style.cssText = getComputedStyle(el, null).cssText;
    else if(el.currentStyle)
        tmp.style.cssText = el.currentStyle.cssText;

    tmp.style.position = 'absolute';
    tmp.style.overflow = 'visible';
    tmp.style.width = 'auto';

    // Estimate number of characters that can fit.
    padding = tmp.style.padding;
    tmp.style.padding = '0';
    tmp.innerHTML = 'M';
    el.parentNode.appendChild(tmp);
    em = tmp.clientWidth;
    tmp.style.padding = padding;      
    numc = Math.floor(max/em);

    var width = tmp.clientWidth;
    
    // Only one of the following loops will iterate more than one time
    // Depending on if we overestimated or underestimated.

    // Add characters until we reach overflow width
    while(width < max && numc <= text.length){
        node.nodeValue = text.substring(0, ++numc);
        width = tmp.clientWidth;
    }
    
    // Remove characters until we no longer have overflow
    while(width > max && numc){
        node.nodeValue = text.substring(0, --numc);
        width = tmp.clientWidth;
    }

    // Remove temporary div
    document.body.removeChild(tmp);

    return numc;
}