I found that the best tutorials are on the Ext JS site itself.
Ext JS Intro Tutorial
Ext JS is a large JS library that includes many features from standard DOM manipulation through one of the most comprehensive sets of JavaScript interface components. Since this is such a great API to learn, I found that looking at the samples and their associated source code is the best way to find out.
Ext JS Samples
This is convenient if the external JS API documents are easily accessible, and the Jack Slocum Ext JS Docs AIR application is nice.
Ext JS API Docs AIR App
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