JavaScript Maybe monad

Maybe.js

module.exports = Maybe;

function Maybe(x) {
	this._value = x;
}

Maybe.of = of;
Maybe.empty = empty;

var nothingValue = void 0;
var nothing = Maybe.nothing = new Maybe(nothingValue);

function of(x) {
	return new Maybe(x);
}

function empty() {
	// For Semigroup/Monoid
	return nothing;
}

Maybe.catMaybes = function(list) {
	return list.reduce(function(justs, maybe) {
		if(maybe.isJust()) {
			justs.push(maybe);
		}
		return justs;
	}, []);
};

Maybe.mapMaybes = function(f, list) {
	return list.reduce(function(justs, maybe) {
		var mappedMaybe = maybe.map(f);
		if(mappedMaybe.isJust()) {
			justs.push(mappedMaybe);
		}
		return justs;
	}, []);
};

Maybe.prototype.isNothing = function() {
	return this._value === nothingValue;
};

Maybe.prototype.isJust = function() {
	return !this.isNothing();
};

Maybe.prototype.map = function(f) {
	return this.isNothing() ? this : of(f(this._value));
};

Maybe.prototype.ap = function(maybe) {
	return this.flatMap(function(f) {
		return maybe.map(f);
	});
};

Maybe.prototype.flatMap = function(f) {
	return this.isNothing() ? this : f(this._value);
};

Maybe.prototype.concat = function(maybe) {
	// Semigroup/Monoid
	// See http://en.wikibooks.org/wiki/Haskell/MonadPlus#Definition
	return this.isNothing() ? maybe : this;
};

Maybe.prototype.filter = function(predicate) {
	return this.isJust() && predicate(this._value) ? this : nothing;
};

Maybe.prototype.maybe = function(f, defaultValue) {
	return this.isNothing() ? defaultValue : f(this._value);
};

Maybe.prototype.fromMaybe = function(defaultValue) {
	return this.isNothing() ? defaultValue : this._value;
};

Maybe.prototype.fromJust = function() {
	if(this.isNothing()) {
		throw new TypeError('fromJust: Maybe is nothing');
	}

	return this._value;
};

Maybe.prototype.foldl = Maybe.prototype.foldr = function(f, initial) {
	return this.isNothing() ? initial : f(initial, this._value);
};

Maybe.prototype.foldl1 = Maybe.prototype.foldr1 = function(f) {
	return this.fromJust();
}

Maybe.prototype.reduce = function(f) {
	return arguments.length < 2 ? this.foldl1(f) : this.foldl(f, arguments[1]);
};

Maybe.prototype.reduceRight = function(f) {
	return arguments.length < 2 ? this.foldr1(f) : this.foldr(f, arguments[1]);
};

Maybe.prototype.listToMaybe = function(list) {
	return list.length === 0 ? nothing : of(list[0]);
};

Maybe.prototype.maybeToList = function() {
	return this.isNothing() ? [] : [this._value];
};