图像的视差效果

Question

我目前正在尝试在javaScript中在屏幕上移动鼠标时创建图像的视差移动。为此，我设法创建了以下代码：

var mainImage = document.getElementById("mainLogo");
var currentX = '';
var currentY = '';
var movementConst = .5;

function moveMainImage(event)
{
    if (currentX = '') currentX = event.screenX;
    var diffX = event.screenX - currentX;
    currentX = event.screenX;
    if (currentY = '') currentY = event.screenY;
    var diffY = event.screenY - currentY;
    currentY = event.screenY;

    var movementX = diffX * movementConst;
    var movementY = diffY * movementConst;
    var newX = mainImage.style.left + movementX;
    var newY = mainImage.style.top + movementY;

    mainImage.style.left = newX + "px";
    mainImage.style.top = newY + "px";
}

document.addEventListener("mousemove", moveMainImage); 

我遇到的问题是“ mousemove”事件侦听器仅发生一次，而每次鼠标移动时都不会调用。我对javaScript还是很陌生，所以如果这是一个简单的问题，我深表歉意。请帮忙吗？