passive-events-support

Passive Events Support

Introduction

How many times have you installed a library such as Bootstrap, jQuery, Materialize CSS or other and suddenly in the Google Chrome console you're prompted with a warning:

[Violation] Added non-passive event listener to a scroll-blocking 'touchstart' event. Consider marking event handler as 'passive' to make the page more responsive.

Or when running Lighthouse you get a lower score with a message:

Does not use passive listeners to improve scrolling performance

Consider marking your touch and wheel event listeners as passive to improve your page's scroll performance.

With many bad practice advices online, it is frustrating to find a proper solution that does not only hide the warnings, but also improves scrolling performance.

Common suggestion is to modify 3rd party source code and assign passive option manually. Despite the fact that you should never modify 3rd party source code, searching event listeners and assigning certain passive value depending on preventDefault() usage is a time consuming experience.

Another suggestion is to apply passive: false to all the event listeners that cause the warning. The warning will be gone, but the performance will not be improved whatsoever. What's the point then? On the other side, blindly assigning passive: true to all the event listeners manually or with a package such as default-passive-events will lead you to nothing else but errors in case of preventDefault() usage.

Here comes Passive Events Support. This is the package that will not only solve the issue, but also improve the performance.

How it works

When event listener does not have a passive option, it will be added and its value will depend on whether preventDefault() is called or not.

When event listener is not calling preventDefault() and passive option is not passed, it will add a { passive: true }

element.addEventListener('touchstart', (e) => {}) // { passive: true }

When event listener is calling preventDefault() and passive option are not passed, it will add a { passive: false }

element.addEventListener('touchstart', (e) => { e.preventDefault() }) // { passive: false }

When passive or other option is passed, their values will not be overwritten

element.addEventListener('touchstart', handler) // { passive: true }
element.addEventListener('touchstart', handler, { passive: false }) // { passive: false }
element.addEventListener('touchstart', handler, { capture: true) // { capture: true, passive: true }
element.addEventListener('touchstart', handler, { capture: false, passive: false }) // { capture: false, passive: false }

Installation

yarn add passive-events-support

Usage

This package must be imported before any package or code that is causing an issue or Lighthouse warning.

JS

require 'passive-events-support'
// or
import 'passive-events-support'

HTML

<script type="text/javascript" src="node_modules/passive-events-support/dist/main.js"></script>

With this code the default behaviour will be applied, passive option will be added automatically to all the listeners defined after the import.

Default behaviour

By default, importing this package will automatically resolve the issue. The passive option will be assigned to these events:

Type	Events
Scroll	scroll, wheel
Touch	touchstart, touchmove, touchenter, touchend, touchleave
Mouse	mouseout, mouseleave, mouseup, mousedown, mousemove, mouseenter, mousewheel, mouseover

Customization

It is recommended to customize and only pass the events that seems to trigger the warning. Passing just the necessary events will decrease the possibility of incompatibility. Some real life scenarios:

• For Materialize CSS just touchstart, touchmove and touchend were needed. It worked fine with all the default events too.
• For jQuery just mousewheel was needed. It did break when using all the default events instead of just mousewheel.

To customize the event list, you will need to pass an array of events manually:

import { passiveSupport } from 'passive-events-support/src/utils'

passiveSupport(['touchstart', 'touchmove'])

<script>
  window.passiveEvents = ['touchstart', 'touchmove']
</script>
<script type="text/javascript" src="node_modules/passive-events-support/dist/main.js"></script>

If you want to add passive option manually to a certain event listener, use passiveSupported() helper to find out if passive option is even supported by your browser:

import { passiveSupported } from 'passive-events-support/src/utils'

element.addEventListener('touchstart', handler, passiveSupported() ? { passive: true } : false)