The read-only scrollX property of the Window interface returns the number of pixels that the document is currently scrolled horizontally. This value is subpixel precise in modern browsers, meaning that it isn't necessarily a whole number. You can get the number of pixels the document is scrolled vertically from the scrollY property.

In practice, the returned value is a double-precision floating-point value with the range of E(min)=-1022 to E(max)=1023 indicating the number of pixels the document is currently scrolled horizontally from the origin, where a positive value means the content is scrolled to the left. If the document is rendered on a subpixel-precise device, then the returned value is also subpixel-precise and may contain a decimal component. If the document isn't scrolled at all left or right, then scrollX is 0.

In more technical terms, scrollX returns the X coordinate of the left edge of the current viewport. If there is no viewport, the returned value is 0.

This example checks the current horizontal scroll position of the document. If it's greater than 400 pixels, the window is scrolled back to the beginning.

if (window.scrollX > 400) {
  window.scroll(0, 0);
}

The pageXOffset property is an alias for the scrollX property:

window.pageXOffset === window.scrollX; // always true

• Window.scrollY