Sets are a new object type with ES6 (ES2015) that allow to create collections of unique values. The values in a set can be either simple primitives like strings or integers, but more complex object types like object literals or arrays can also be part of a set.
JavaScript:
let animals = new Set();
animals.add('1');
animals.add('2');
animals.add('3');
animals.add('4');
console.log(animals.size); // 4
animals.add('2');
console.log(animals.size); // 4