I didn't use JQuery before. I use Dojo in my daily work. I find the chance to extend the library is little. We usually do extending for widgets. From the table of content, it seems you do lots extending in JQuery. Is that because JQuery has a better capability to extend than other libraries?
Yes, jQuery is designed to be extended. The jQuery team recognised early on that the core jQuery couldn't and shouldn't do everything. They wanted to provide the basic functionality that would be useful in a wide range of situations, without bloating the library with seldom used functions. They then included various extension points so that third-parties could add the extra abilities and have them integrated with the basic abilities. The strength of this approach is seen in the large number of jQuery plugins available.