Learn D3.js

Maps and sets

Besides arrays, ES2015 also introduced two new data structures: Map, an associative array with key-value pairs easier to use than simple objects, and Set, which doesn't allow repeated values. Both can be transformed to and from arrays.

You can create a new Set object using:

const set = new Set();

And add elements to it using:

set.add(5);set.add(7);

If you try to add elements that already exist, they won't be included in the set:

set.add(1);set.add(5);   // not addedset.add(7);   // not addedconsole.log(set.size); // prints 3

You can check if a Set contains an element:

console.log(set.has(3)); // false

And convert a Set object into an array:

const array1 = [... set]; // spread operator

A Map can be more efficient than using an object ...