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Map

1. Map is a collection of keyed data items, just like an Object. But the main difference is that Map allows keys of any type.

2. Maps Main Methods

   1. new Map() - creates the map
   2. map.set(key, value) - stores value by key
   3. map.get(key) – returns the value by the key, undefined if key doesn’t exist in map.
   4. map.has(key) – returns true if the key exists, false otherwise.
   5. map.delete(key) – removes the value by the key.
   6. map.clear() – clears the map
   7. map.size – returns the current element count.

let map = new Map();

map.set('1', 'str1');   // a string key
map.set(1, 'num1');     // a numeric key
map.set(true, 'bool1'); // a boolean key

// remember the regular Object? it would convert keys to string
// Map keeps the type, so these two are different:
alert( map.get(1)   ); // 'num1'
alert( map.get('1') ); // 'str1'

alert( map.size ); // 3