---
title: Window.screenX
slug: Web/API/Window/screenX
tags:
  - API
  - CSSOM View
  - Property
  - Read-only
  - Reference
  - Window
  - screenX
browser-compat: api.Window.screenX
---
{{APIRef}}

The **`Window.screenX`** read-only property returns the
horizontal distance, in CSS pixels, of the left border of the user's browser viewport to
the left side of the screen.

> **Note:** An alias of `screenX` was implemented across modern
> browsers in more recent times — {{domxref("Window.screenLeft")}}. This was originally
> supported only in IE but was introduced everywhere due to popularity.

## Syntax

```js
leftWindowPos = window.screenX
```

### Returns

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

## Examples

In our [screenleft-screentop](https://mdn.github.io/dom-examples/screenleft-screentop/) ([source
code](https://github.com/mdn/dom-examples/blob/master/screenleft-screentop/index.html)) example, you'll see a canvas onto which has been drawn a circle. In this
example we are using {{domxref("Window.screenLeft")}}/{{domxref("Window.screenTop")}}
plus {{domxref("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);
```

These work in exactly the same way as `screenX`/`screenY`.

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

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

## Specifications

{{Specifications}}

## Browser compatibility

{{Compat}}

## See also

- {{domxref("window.screenLeft")}}
- {{domxref("Window.screenY")}}
