functors.js

class Wrapper {
  constructor(value) {
  	this._value=value;
  }
  map(f) {
  	return f(this._value);
  }
  fmap(f) {
  	return new Wrapper(f(this._value));
  }
}
const wrap = v => new Wrapper(v);

const plus = a => b => a+b;
const plus3 = plus(3);

const two = wrap(2);

console.log(two.fmap(plus3).map(v => v));