Replacing an empty cell's contents

hacksaw Posts 1 Registration date Sunday October 18, 2015 Status Member Last seen October 18, 2015 - Oct 18, 2015 at 10:45 PM
I have some empty cells in "J" column which I want to replace with data from another cell which is in "X" column. The formula has to be in "AA" column - I have tried all sorts of formulas:-
=if(J1 = "",X1,J1)

but all of these just put the data from "X" column into the "AA" column when I want it in the "J" column

Can someone help with this