fib.js

function fib() {
	let a = 0;
	let b = 1;
	let c = 0;
	return function() {
		a = b;
		b = c;
		c = a + b;
		return b;
	}
}

const f = fib();

for (let i = 0; i < 20; i++) {
	console.log(f());
}