let interval = 0;
let direction = 0;

setInterval(() => {
    if (direction === 0) {
        if (interval === 100) {
            direction = 1;
            return;
        }
        console.log(interval++);
    } else if (direction === 1) {
        if (interval === 0) {
            direction = 0;
            return;
        }
        console.log(interval--);
    }
}, 50);