An attempt at implementing a maybe monad myself

maybe-monad.js

const Maybe = function(val) {
	this.__value = val instanceof Maybe ? val.__value : val
}

Maybe.from = function(val) {
	return new Maybe(val)
}

Maybe.none = function() {
	return new Maybe()
}

Maybe.prototype = {
	isPresent: function() {
		return this.__value !== null && this.__value !== undefined
	},

	map: function(cb) {
		if (this.isPresent()) {
			let val = cb(this.__value)

			if (val && val instanceof Maybe) {
				return Maybe.from(val.__value)
			} else {
				return Maybe.from(val)
			}
		} else {
			return Maybe.none()
		}
	},

	orElse: function(val) {
		return this.isPresent() ? this.__value : val
	}
}

const user = {
	id: 1,
	name: 'bob',
	email: '[email protected]',
	address: {
		number: '20',
		street: 'Nowhere road',
		town: 'Townsville',
		city: 'Gotham',
		country: 'The Vatican',
		planet: 'Mars',
		system: 'Sol',
		galaxy: 'The Milky Way'
	}
}

const myTown = Maybe.from(user)
	.map(user => Maybe.from(user.address)) // to prove that flattening happens
	.map(address => address.town)
	.orElse('none')
	
console.log('myTown', myTown)