Is there something wrong with extending native Javascript prototypes?

I have seen some people regard extending the natives prototypes in Javascript with disdain. I was thinking about doing it, because of syntactic convenience. I.e.

function(array, element)

can be more cumbersome to write and less readable than

array.function(element)

But the seco开发者_JAVA技巧nd can only be acheived (AFAIK) by extending the Array prototype. Is there something wrong with extending native prototypes, and will doing this somehow haunt be me later?

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It can conflict with other libraries trying to do the same.

It can conflict with future methods added to native objects.

If anyone is using for (var i in array) without proper hasOwnProperty() checks, their code may/probably will break because the new method may show up in the iteration in older browsers.