Originally Posted by Philip M
For the life of me I do not see the use of "two arrays with the same content. (Or, more correctly, two variables referencing the same array object.)".
How can it make sense to lose/replace the original array? That was the point I was trying to make.
if the array is a deck of cards in a game, and you want all players drawing from the same shuffled deck, it makes a lot of sense.
another mutative example from a real project is var filename=pathRay.pop(), path=pathRay.join("/");
another mutative example: gridRows.sort( sorter.textNoCase("lastName") ); to sort a grid before rendering it, uses should be intuitive.
i myself used to use a lot of custom array prototypes like sum(), filtered(), etc. but .map changed all that. I can now just use a tiny pure function method on an  method to do all that stuff: r.reduce(F.sum), r.filter(Boolean), etc. i still use .unique a lot, since it's slow to do with a filter method...
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