Adding salt to pineapple will actually cause it to taste sweeter

Dr Hannah Williams famous food scientist from Curtin University of Technology said
When the sodium chloride dissolves into the pineapple it will break apart into sodium and chloride ions.

The sodium ion will then react with the malic and citric acids present in the pineapple to form neutral sodium salts.Acids normally have a tart or sour taste but when they are converted into neutral compounds they lose this sourness, and so the pineapple tastes sweeter.
[Source]