for (variablename in object){
 statement or block to execute
}

<script type="text/javascript">
<!--
var aProperty;
document.write("Navigator Object Properties<br /> ");
for (aProperty in navigator)
{
 document.write(aProperty);
 document.write("<br />");
}
document.write("Exiting from the loop!");
//-->
</script>

Navigator Object Properties
appCodeName
appName
appMinorVersion
cpuClass
platform
plugins
opsProfile
userProfile
systemLanguage
userLanguage
appVersion
userAgent
onLine
cookieEnabled
mimeTypes
Exiting from the loop!