scroll

The scroll event is fired when the document view or an element has been scrolled.

General info

Specification: DOM L3, CSSOM View
Interface: UIEvent
Bubbles: Not on elements, but bubbles to the default view when fired on the document
Cancelable: No
Target: defaultView, Document, Element
Default Action: None

Properties

Property	Type	Description
`target` Read only	`EventTarget`	The event target (the topmost target in the DOM tree).
`type` Read only	`DOMString`	The type of event.
`bubbles` Read only	`Boolean`	Whether the event normally bubbles or not
`cancelable` Read only	`Boolean`	Whether the event is cancellable or not?
`view` Read only	`WindowProxy`	`document.defaultView` (`window` of the document)
`detail` Read only	`long` (`float`)	0.

Example

Since scroll events can fire at a high rate, the event handler shouldn't execute computationally expensive operations such as DOM modifications. Instead, it is recommended to throttle the event using requestAnimationFrame, setTimeout or customEvent, as follows:

Scroll optimization with window.requestAnimationFrame

// Reference: https://www.html5rocks.com/en/tutorials/speed/animations/

var last_known_scroll_position = 0;
var ticking = false;

function doSomething(scroll_pos) {
  // do something with the scroll position
}

window.addEventListener('scroll', function(e) {
  last_known_scroll_position = window.scrollY;
  if (!ticking) {
    window.requestAnimationFrame(function() {
      doSomething(last_known_scroll_position);
      ticking = false;
    });
  }
  ticking = true;
});

More similar examples on resize event.

Browser compatibility

iOS UIWebView

In iOS UIWebViews, scroll events are not fired while scrolling is taking place; they are only fired after the scrolling has completed. See Bootstrap issue #16202. Safari and WKWebViews are not affected by this bug.