Maps (special kind of object)

// a key in the map may only occur once
// allows keys of any type

let recipeMap = new Map([
  ['cucumber', 500],
  ['tomatoes', 350],
  ['onion',    50]
]);

• map.keys() – returns an iterable for keys,
• map.values() – returns an iterable for values,
• map.entries() – returns an iterable for entries [key, value], it’s used by default in for..of.