Maybe-Monad.js

/**
 * Custom Maybe Monad used in FP in JS book written in ES6
 * Author: Luis Atencio
 * https://github.com/luijar/functional-programming-js/blob/master/src/model/monad/Maybe.js
*/ 
class Maybe {	
	static just(a) { return new Just(a); }
	static nothing() { return new Nothing(); }
	static fromNullable(a) { return (a !== null && a !== undefined) ? Maybe.just(a) : Maybe.nothing(); }
	static of(a) { return Maybe.just(a); }
	get isNothing() { return false; }
	get isJust() { return false; }  };

class Just extends Maybe {	
	constructor(value) { super(); this._value = value; }
	get value() { return this._value; }
	map(f) { return Maybe.fromNullable(f(this._value)); }
	chain(f) { return f(this._value); }
	getOrElse() { return this._value; }
	filter(f) { Maybe.fromNullable(f(this._value) ? this._value : null); }
	get isJust() { return true; }
	toString () { return `Maybe.Just(${this._value})`; }  };

class Nothing extends Maybe {	
	map(f) { return this; }
	chain(f) { return this; }
	get value() { throw new TypeError("Can't extract the value of a Nothing."); }
	getOrElse(other) { return other; }
	filter() { return this._value; }
	get isNothing() { return true; }
	toString() { return 'Maybe.Nothing'; }  };
								
const getProp = key_name => an_object => an_object[key_name];

const maybeMiddleName = (a_person) => {	
	const middle_name = Maybe.fromNullable(a_person)
				.map(getProp('person'))
				.map(getProp('address'))
				.map(getProp('name'))
				.map(getProp('middle'))
				.getOrElse('No-Middle-Name');
	return middle_name;  }

///////////////////////////////////////////////////////////////////////////
var no_middle    = {person : { address: { name: {first: 'Abraham', last:'Lincoln'}}}};
var no_name      = {person : { address: { moniker: {first: 'Abraham', last:'Lincoln'}}}};
var no_address   = {person : { info: { name: {first: 'Abraham', last:'Lincoln'}}}};
var no_person    = {human : { address: { name: {first: 'Abraham', last:'Lincoln'}}}};
var empty_object = {};
var no_object    = [];
var null_val     = null;
var has_middle   = {person : { address: { name: {first: 'Abraham', middle:'The Best', last:'Lincoln'}}}};

var the_people = [no_middle, no_name, no_address, no_person, empty_object, no_object, null_val, has_middle];

the_middles = the_people.map(a_person=> maybeMiddleName(a_person));
console.dir(the_middles);