Fibonacci (Iterative)

function fib(n) {
    if(n === 0) return 0;
    if(n <= 2) return 1;

    let x = 1, y = 2;
    for(let i = 3; i < n; i++) {
        let oldX = x;
        x = y;
        y = y + oldX;  /*?.*/
    }

    return y; 
}