"Option type" for Javascript

option.js

"use strict";

class Some {
    constructor(value) {
        this.value = value;
    }

    is_some(self) {
        return true;
    }

    is_none(self) {
        return false;
    }

    unwrap(self) {
        return self.value;
    }

    map(self, lambda) {
        return Some(lambda(self.value));
    }
}

class NoneType {
    constructor() {
    }

    is_some(self) {
        return false;
    }

    is_none(self) {
        return true;
    }

    unwrap(self) {
        throw new Error("unwrap of None");
    }

    map(self, lambda) {
        return self;
    }
}

var None = new NoneType();

function find_first_vowel(s) {
    for (var c in s) {
        if ("aeiou".indexOf(c) >= 0) {
            return new Some(c);
        }
    }
    return None;
}

var v = find_first_vowel("iou");
print(v === "a");
print(v === "i");
print(v.unwrap() === "i");