Check descendant by id
<html> <head> <script type="text/javascript"> var djConfig = { baseScriptUri : "js/dojo/" }; </script> <script type="text/javascript" src="js/dojo/dojo/dojo.js"></script> <script> function testIt() { alert(dojo.isDescendant("div3", "div2")); } </script> </head> <body onLoad="testIt();"> <span id="div1"></span> <span id="div2"><span id="div3"></span></span> </body> </html>