Set and Map are data structures in JavaScript. They have been available for a while, but I feel like they are rarely used. (But maybe that’s just me.)

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Set is a collection of values. Each value (a primitive value, Object, DOM node) can only be unique inside the set.

const fruits = new Set();
fruits.add('Apple');
fruits.add('Banana');
fruits.add('Peach');
fruits.add('Apple');

(Common misconception: const does not mean that the content of the Set is immutable. You just cannot assign another thing to the variable fruits.)

The content of the set can be counted, iterated, filtered, matched.

fruits.size // 3
fruits.add('Lemon') // returns the Set
fruits.size // 4
fruits.add('Apple') // returns the Set. No indicator that the element is already contained.
fruits.has('Apple') // true

And you can compare or join sets.

const yellow = new Set();
yellow.add('Banana');
yellow.add('Bee');
yellow.add('Lemon');

fruits.difference(yellow) // Set [ "Apple", "Peach" ]
fruits.intersection(yellow) // Set [ "Banana", "Lemon" ]
everything = fruits.union(yellow) // [ "Apple", "Banana", "Peach", "Lemon", "Bee" ]

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Map is a collection of keyed values. A Map is a set of Key-Value pairs, similar to objects. But has some differences: Keys can be any value (compared to strings and integers for objects), the order is stable, and it is more performant for adding and removing values.

const fruits = new Map();
// the naming of this function is totally not confusing
fruits.set('Apple', {shape: 'round', color: 'many'});
fruits.set('Banana', {shape: 'long', color: 'yellow'});
fruits.set('Peach', {shape: 'round', color: 'red'});
fruits.set('Apple', {shape: 'round', color: 'all'});

The keys are be unique, of course. Adding another object with the same key overrides the existing one.

fruits.size // 3
fruits.has('Apple') // true
fruits.get('Apple') // Object { shape: "round", color: "all" }

There are multiple ways to access the content of a Map:

for (let [key, value] of fruits) {
  console.log(key, value);
}

fruits.forEach((value, key) => {
  console.log(key, value);
});

for (let key of fruits.keys()) {
  console.log(key);
}

for (let value of fruits.values()) {
  console.log(value);
}

for (let [key, value] of fruits.entries()) {
  console.log(key, value);
}

Source: https://javascript.info/map-set