Optimizing Code
My experience is that JavaScript becomes unwieldy when the app gets large; I spend a long time optimizing the code at a low level. Also, because JavaScript is a scripting language and weakly typed, you often don't find bugs right away.
Migration
The move from lightweigt JavaScript to robust, heavy-weight Java isn't trivial, but it is possible to migrate slowly or to migrate just those problem areas of the application. To migrate existing JavaScript functionality to GWT, you must start with a pretty solid API. Usually, this means you're calling just one or two functions in your web page.
Start by building the components and functionality in GWT with the use of GWT components and features instead of raw HTML and JavaScript. It's important to stick to higher level components that provide XBrowser safety.
Next, expose specific parts of the GWT component to JavaScript with the use of "native" functions. These functions will probably look just like your existing API so that you keep backward compatibility. Finally, you just need to include the GWT-generated JavaScript and remove the old, native JavaScript from your web page.
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