Window.screenTop

Window.screenTop 只读属性返回垂直距离，单位是 CSS 像素，从用户浏览器的上边界到屏幕最顶端。

备注： screenTop is an alias of the older Window.screenY property. screenTop was originally supported only in IE but was introduced everywhere due to popularity.

语法

topWindowPos = window.screenTop

返回值

A number equal to the number of CSS pixels from the top edge of the browser viewport to the top edge of the screen.

例子

In our screenleft-screentop example, you'll see a canvas onto which has been drawn a circle. In this example we are using screenLeft/screenTop plus Window.requestAnimationFrame() to constantly redraw the circle in the same physical position on the screen, even if the window position is moved.

js

initialLeft = window.screenLeft + canvasElem.offsetLeft;
initialTop = window.screenTop + canvasElem.offsetTop;

function positionElem() {
  let newLeft = window.screenLeft + canvasElem.offsetLeft;
  let newTop = window.screenTop + canvasElem.offsetTop;

  let leftUpdate = initialLeft - newLeft;
  let topUpdate = initialTop - newTop;

  ctx.fillStyle = "rgb(0, 0, 0)";
  ctx.fillRect(0, 0, width, height);
  ctx.fillStyle = "rgb(0, 0, 255)";
  ctx.beginPath();
  ctx.arc(
    leftUpdate + width / 2,
    topUpdate + height / 2 + 35,
    50,
    degToRad(0),
    degToRad(360),
    false,
  );
  ctx.fill();

  pElem.textContent =
    "Window.screenLeft: " +
    window.screenLeft +
    ", Window.screenTop: " +
    window.screenTop;

  window.requestAnimationFrame(positionElem);
}

window.requestAnimationFrame(positionElem);

Also in the code we include a snippet that detects whether screenLeft is supported, and if not, polyfills in screenLeft/screenTop using Window.screenX/Window.screenY.

js

if (!window.screenLeft) {
  window.screenLeft = window.screenX;
  window.screenTop = window.screenY;
}

规范

Specification
CSSOM View Module
# dom-window-screentop

浏览器兼容性

BCD tables only load in the browser

参考