A mapping type is a collection that stores objects by key, rather than by position. Mapping types maintain no reliable item order.

Because JavaScript objects derive from Object.prototype, you should not use a JavaScript object as a mapping type (dictionary hash map, etc.). Instead, use Map(), which included set(), get(), and has():

> let m = new Map()
undefined
> m.set('foo', 1)
Map { 'foo' => 1 }
> m.set('bar', 2)
Map { 'foo' => 1, 'bar' => 2 }
> m.get('foo')
1
> m.get('bar')
2
> m.has('foo')
true
> m.has('qux')
false

Sources

Haverbeke, Marijn. Eloquent Javascript: A Modern Introduction to Programming. http://eloquentjavascript.net.