You absolutely should.

By editing a built-in prototype, you're assuming that by the time you need it, nobody else has done the same thing that you just did (but perhaps they figured "capitallize" would output 'nIfTy'). You're opening your code to error, or introducing error into someone else's code.

Hi, I don't know a lot of about javascript but why adding to String's prototype is bad practice? Are you talking about creating new constructor instead of using string prototype? Thank you)

I agree with the above comments but accidently clicked the 'Best practices' button...doh!