Closure (JavaScript) with Fibonacci

function getFibonator() {
    let prevNum = 0;
    let currentNum = 1;

    return function() {
        let nextNum = prevNum + currentNum;
        prevNum = currentNum;
        currentNum = nextNum;
        return prevNum;
    }
}


let fib = getFibonator();
console.log(fib()); // 1
console.log(fib()); // 1
console.log(fib()); // 2
console.log(fib()); // 3
console.log(fib()); // 5
console.log(fib()); // 8
console.log(fib()); // 13