Javascript applets don't exist. Java is an applet, JavaScript/JScript is not, just to straighten things out for you . Do you have an example of what you are trying to achieve?
XML is made to hold data and organize it and Java can read that data and use it in applets or applications. It is quite powerful to know, but quite old in the world of programming. It allowed portable data AND code which was very useful which is why almost every programmer around that time had to know XML.
Also they only did the newbie to Java/Javascript thing confusing them.