XPath

XPath (XML Path Language) is a language for addressing elements in a XML or HTML document. XPathExpressions describe paths in a tree represention of the document.

XPath is very fast compared to manual DOM traversal (see e.g. getElementsByClassName Speed Comparison).

This page is intended for tips and gotchas related to using XPath in Greasemonkey scripts, not complete documentation. See the links below for that.

Using XPath in Greasemonkey

The most convenient way to use XPath in Greasemonkey scripts is with a helper function. The insides of that function illustrate the less convenient way. The native method is through the document.evaluate function.

XPathResult resultTypes

• resultType Constants
• Further reading at Mozilla Developer Center

Relative paths

Note that relative XPathExpressions, that start from some specific node, do not begin with a /. Start them with a period, e.g. .//a, or an axis, e.g. descendant::a. The default axis is child, so just a works to find an immediate descendant.

Links

• Mozilla Developer Center: Introduction to using XPath in JavaScript
• Mozilla Developer Center: XPath axes
• Mozilla Developer Center: XPath functions
• XPath tutorial
• Wikipedia: XPath
• XPath Checker (Firefox addon)