var videoThumbanils = document.querySelectorAll(".x-video-thumbnail");

for (let v of videoThumbanils) {
    let image = v.previousElementSibling;
    image.addEventListener("mouseover", function(event) { 
        image.style.display = "none";
        v.classList.remove("x-hidden");
        v.play();
    });
    v.addEventListener("mouseout", function(event) { 
        v.classList.add("x-hidden");
        image.style.display = "";
        v.pause();
    });
}