EDIT: see paavohtl's comment: I hadn't payed attention to the types, dumb me. You're right, because for a pair of functions f and g, you have an isomorphism if

EDIT: see paavohtl's comment: I hadn't payed attention to the types, dumb me. You're right, because for a pair of functions f and g, you have an isomorphism if: f(g(x)) == x g(f(x)) == x for every x. However, here of course (([a]) => {a})( (({ a }) => [a])({ key: 'data'}) ) is equal to { a: 'data' } The OP doesn't quite master what he's talking about…