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I'm not sure whether or when it is useful (to improve performance) to dereference variables. var x = a.b.c.d[some_key].f; while (loop) { do_something_with(x); } seems to be better than while (loop) { do_somthing_with(a.b.c.d[some_key].f); } Is that needed or is this done automagically by smart JavaScript engines? But my actual question is whether I should do this, for example, in a library. (function() { var slice = Array.prototype.slice; Function.prototype.x = function x() { var args = slice.call(arguments, 0); ... }; })(); or just Function.prototype.x = function x() { var args = Array.prototype.slice.call(arguments, 0); ... }; The engine can't improve this automatically because it doesn't know whether Array.prototype.slice might change during the run time. So: does creating a closure for creating the local reference to the slice function make the script faster? Or does the additional closure scope make it slower than accessing the property "slice" of the property "prototype" of Array?

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