JS: Set.prototype.size

New in ES2015.

set_obj.size → returns the total count of values in set_obj.

// size of set
const s = new Set([4, 5, 6]);
console.log(s.size); // 3

size is a getter property. [see JS: Getter/Setter Properties]

Reference

ECMAScript 2015 §Keyed Collection#sec-get-set.prototype.size