Mozilla's Rhino scripting engine is used by screen-scraper to allow scripts to be written in JavaScript. Documentation for Rhino is sparse, but the interpreter does adhere strictly to the established ECMAScript standard, so just about any reference on JavaScript could be referred to. If you try writing scripts using JavaScript, and run into difficulties (because of lack of documentation), you may want to consider using Interpreted Java instead, which has very similar syntax and provides significantly better documentation. If you've worked with client-side JavaScript in web programming, you'll probably be comfortable using JavaScript in screen-scraper.
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