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The comma is not needed at all. But you will still find some interpreters/compilers still allowing to use it. You can think of it this way: the interpreters is allowing you to make little mistakes and wisely fixing it internally. This is just a user-friendly way. Nothing else at all.
And you should not use that extra comma because a lot of strict ones will not let you pass through! Like Internet Explorer while interpreting Javascript. But Firefox will allow it.