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I have been looking at the source code of raphael.js and I see a lot of stuff like !variable && function() (e.g.: !svg.bottom && (svg.bottom = this); )
What does that exactly do? Does it check first and execute only if not true?
Correct. This is (ab)using short-circuit evaluation. A boolean expression is only executed as far as is needed to determine the result. In your example, if
svg.bottomis non-null, then!svg.bottomisfalse, and the result of the&&isfalse, so execution of the right hand side does not happen. It’s basically equivalent toif(!svg.bottom)svg.bottom = this;