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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
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