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The Map() object

The Map object has key-value pairs and remembers the original insertion order of the keys.

It differs from the normal Object because:

Any values can be used as a key or value. (In Object, keys have to be Strings or Symbols).
The keys in Map are ordered, so iterating over the object will return the keys in order of insertion.
Using the size property on a Map will return the size of a Map.
Map objects are iterable.

Example:

const cities = new Map([
  [0, 'Paris'],
  [1, 'London'],
  [2, 'Sydney']
])

for (var value of cities.value()){
  console.log(value)
}

or

var cities = new Map()

cities.set('0', "Paris");
cities.set({}, "London");
cities.set(() => {}, "Sydney");

for (var [key, value] of cities.entries()) {
  console.log(key + ' = ' + value);
}

// 0 = Paris
// [object Object] = London
// () => {} = Sydney