An example of an iterable class

Fib.js

class Fib {
	a = 0;
	b = 0;
	next() {
		if (this.a == 0) {
			this.a = 1;
			return { value: 1, done: false };
		}
		if (this.b == 0) {
			this.b = 1;
			return { value: 1, done: false };
		}
		const value = this.a + this.b;
		this.a = this.b;
		this.b = value;
		return { value, done: false };
	}
	[Symbol.iterator]() {
		return this;
	}
}
for (let n of new Fib()) {
	console.log(n);
	if (n > 1000) break;
}
/* Output from the above loop:
1
1
2
3
..
1597
*/