const zero = f => x => x;
const one  = f => x => f(x);
const two  = f => x => f(f(x));
const thr  = f => x => f(f(f(x)));
const add =
    m => n => f => x =>
        m(f)(n(f)(x));

const toNumber = n => n(x => x + 1)(0);

console.log(toNumber(add(thr)(two))); // 3