Set

https://developer.mozilla.org/en/JavaScript/Reference/Global_Objects/Set

JavaScript

var Set = function Set() {
    var values = [];
    return {
        add: function(value) {
            var index = values.indexOf(value);
            if (!~index) {
                values.push(value);
            }
        },
        'delete': function(value) {
            var index = values.indexOf(value);
            if (~index) {
                values.splice(index, 1);
            }
        },
        has: function(value) {
            var index = values.indexOf(value);
            return !!~index;

        }
    }
}
var set = new Set;
var object = {
    a: 3
}
set.add(object)
console.log(set.has(object));
set.add(object)
console.log(set.has(object));
set.delete(object);
console.log(set.has(object));