Build map in function closure

map.js

// At first, we need to build empty map.
// Empty map is just a function, it will return undefined when invoked.
// When you set new key-value into map, it will wrap a new function around previous map,
// and when you get something from map with key, it will pass key to every nested function until it match or hit empty map.
const map = () => undefined;
const set = map => (key, value) => q => q === key ? value: map;
const get = map => key => typeof map === 'function' ? get(map(key))(key) : map;


// Demo time!
const map1 = set(map)('key1', 1);

get(map1)('key1') // 1
get(map1)('key2') // undefiend

const map2 = set(map1)('key2', 2)

get(map2)('key1') // 1
get(map2)('key2') // 2

const map3 = set(map2)('key1', 3)

get(map3)('key1') // 3
get(map3)('key2') // 2