This function will help you to get the computed width(actual width) of an HTML Element/Object. For example you may be using a div to show some contents inside that. But want to calculate the actual width of the div after loading the contents inside it. There comes this function to help you.
function getComputedWidth(theElt){
var browserName=navigator.appName;
if (browserName=="Microsoft Internet Explorer"){
var is_ie=true;
} else {
var is_ie=false;
tmphght = document.getElementById(theElt).offsetWidth;
docObj = document.getElementById(theElt);
var tmphght1 = document.defaultView.getComputedStyle(docObj, "").getPropertyValue("width");
tmphght = tmphght1.split('px');
tmphght = tmphght[0];
return tmphght;
< div id="demo" onclick="alert(getComputedWidth('demo'));">hello there< /div>

To know the computed height of an element see here

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