either - left - right

Either.js

const Left = function(x) {
  this.__value = x;
};

Left.of = function(x) {
  return new Left(x);
};

Left.prototype.map = function(f) {
  return this;
};

const Right = function(x) {
  this.__value = x;
};

Right.of = function(x) {
  return new Right(x);
};

Right.prototype.map = function(f) {
  return Right.of(f(this.__value));
};

const Either = function(x) {
  if (x instanceof Right) {
    this.right = x
  } else {
    this.left = x
  }
};

Either.prototype.map = function(f) {
  return this.right 
    ? this.right.map(f) 
    : this.left.map(f);
};

Either.of = function(x) {
  return new Either(x);
};