JS: Set.prototype.has

New in ES2015.

set_obj.has(value)

Return true if value is in set. Else, false.

const s = new Set([3,4,5]);

// check existence

console.log ( s.has (5) ); // true
console.log ( s.has (99) ); // false

Reference

ECMAScript 2015 §Keyed Collection#sec-set.prototype.has