You have to know what information can be accessed with JavaScript. You have to understand that all such information can be easily uploaded to web-server. All such information is harmless. However you can think differently. I will show you what javascript can take from you. And you will decide how is it terrible for you.

Let me show you live example (it doesn't work without JavaScript enabled):
window.screen - information about your screen-resolution and color-depth

window.navigator - your browser name, cpu type, operation system

window.navigator.plugins - some information about your plugins installed

window.document - your current URL, URL where you come here from

window.document.cookie - cookies for current domain

Source of JavaScript program here:

<script>
document.write("<br><b>window.screen</b><br>");
document.write("window.screen.height = " +  window.screen.height + "<br>");
document.write("window.screen.width = " +  window.screen.width + "<br>");
document.write("window.screen.pixelDepth  = " +  window.screen.pixelDepth  + "<br>");
document.write("window.screen.colorDepth  = " +  window.screen.colorDepth  + "<br>");
 
document.write("<br><b>window.navigator</b><br>");
for (var key in window.navigator) {
	if (typeof(window.navigator[key]) == "function") continue;
	if (typeof(window.navigator[key]) == "object") continue;
	document.write("window.navigator."+key+" = "+ window.navigator[key] +"<br>");
}
document.write("<br><b>window.navigator.plugins</b><br>");
for (var key in window.navigator.plugins) {
	if (typeof(window.navigator.plugins[key]) == "function") continue;
	document.write("window.navigator.plugins."+key+" = "+ window.navigator.plugins[key].name +" : " +window.navigator.plugins[key].filename +"<br>");
}

document.write("<br><b>window.document</b><br>");
document.write("document.referrer: "+document.referrer +"<br>");
document.write("document.URL: "+document.URL +"<br>");

document.write("<br><b>window.document.cookie</b><br>");
if (document.cookie == "") document.write("No cookies<br>");
document.write(document.cookie.toString().replace(/;/g, "<br>"));

</script>