functors.js

function Maybe(x) {
  if (this instanceof Maybe === false) {
    return new Maybe(x);
  }
  this.value = x;
}

Maybe.prototype.map = fn => (this.value == null ? this : new Maybe(fn(this.value)));

function Either(left, right) {
  if (this instanceof Either === false) {
    return new Either(left, right);
  }
  this.left = left;
  this.right = right;
}

Either.prototype.map = f => (this.right == null ? this : new Either(this.left, f(this.right)));