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Akhter Rasool8,597 Points
why are we using  in front of the selector??
Also please tell me the difference between caching items to a jquery object($context) and a variable(context) ??
Steven Parker227,114 Points
The two slashes (//) makes this a comment, it does nothing. But without the slashes, $("canvas") returns a set of objects representing all canvas elements. The  after it gives you the first (and possibly only) single element from that set.
The only difference between "$canvas" and "canvas" is the "$" in the name, they are both variables. Either one can be assigned with a JQuery object. It's just a commonly used (and recommended) convention to use variable names that begin with $ to represent JQuery objects.