This document is an introduction to the JavaScript Programming Language for professional programmers. It is a small language, so if you are familiar with other languages, then this won't be too demanding.
JavaScript is not Java. They are two very different languages. JavaScript is not a subset of Java. It is not interpreted Java. (Java is interpreted Java!) JavaScript shares C-family syntax with Java, but at a deeper level it shows greater similarity to the languages Scheme and Self. It is a small language, but it is also a suprisingly powerful and expressive language.You should take a look at it. You will find that it is not a toy language, but a full programming language with many distinctive properties.
XPath is a language for finding information in an XML document. XPath is used to navigate through elements and attributes in an XML document.
XPath is a major element in the W3C's XSLT standard - and XQuery and XPointer are both built on XPath expressions.
So an understanding of XPath is fundamental to a lot of advanced XML usage.