symbol.js

var Fib = {
    [Symbol.iterator]() {
        var n1 = 1, n2 = 1;
        return {
            // make the iterator an iterable
            [Symbol.iterator]() { return this; },
            next() {
                var current = n2;
                n2 = n1;
                n1 = n1 + current;
                return { value: current, done: false };
            },
            return(v) {
                console.log(
                    "Fibonacci sequence abandoned."
                );
                return { value: v, done: true };
            }
        };
    }
};
for (var v of Fib) {
    console.log(v);
    if (v > 50) break;
}