function assert(b) {
    if (!b)
        throw new Error("Bad!")
}

let set = new Set;
let items = [];
for (let i = 0; i < 3000; i++) {
    items.push(i);
    set.add(i);
}

let counter = 1000000;
while (items.length) {
    if (Math.random() < 0.85) {
        let item = items.pop();
        let removed = set.delete(item);
        assert(removed);
        assert(items.length === set.size); 
    } else {
        let newItem = ++counter;
        items.push(newItem);
        set.add(newItem);
        assert(items.length === set.size); 
    }
}