Map

The Map object holds key-value pairs and remembers the original insertion order of the keys. Any value (both objects and primitive values) may be used as either a key or a value.

Map objects are collections of key-value pairs. A key in the Map may only occur once; it is unique in the Map’s collection. A Map object is iterated by key-value pairs — a for…of loop returns a 2-member array of [key, value] for each iteration. Iteration happens in insertion order, which corresponds to the order in which each key-value pair was first inserted into the map by the set() method (that is, there wasn’t a key with the same value already in the map when set() was called).