Passive Events Support

- Introduction
- The Issue
- What's a passive option?
- Why is it necessary?
- The solution
- How It Works
- Installation
- Usage
- Configuration
- Configurable Options
- Option: debug
- Option: events
- Option: listeners (Recommended)
- Supported Events
- Known Issue
- Browser Support
- Debugging the browser support
- Manually assigning the passive option

Introduction

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How many times have you yourself forgotten to make an event listener as passive, or installed a library such as Bootstrap, jQuery or Materialize and suddenly in the Google Chrome you see:

> [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.

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According to Official Documentation a passive option is:

> A boolean value that, if true, indicates that the function specified by listener will never call preventDefault(). If a passive listener does call preventDefault(), the user agent will do nothing other than generate a console warning.

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It is well docummented that:

> According to the specification, the default value for the passive option is always false. However, this introduces the potential for event listeners handling certain touch events (among others) to block the browser's main thread while it is attempting to handle scrolling, resulting in possibly enormous reduction in performance during scroll handling.

See Improving scrolling performance with passive listeners to learn more.

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Making event listeners as passive manually could be a repetetive and time consuming experience. Also if caused by a 3rd party, modifying it's source code should never be an option!

Here comes the Passive Events Support package! This is the package that will help you debug the source, solve the issue, but also improve the performance. It is flexible and highly configurable package!

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 being called in the handler or not.

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

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

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

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

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

`bash
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
// With JS
import { passiveSupport } from 'passive-events-support/src/utils'
passiveSupport({/.../})
`

`html



`

By default, importing this package will not update non-passive event listeners. For this package to act, you must specify which event listeners should be made as passive. See the Configuration section below.

Configuration

It is highly recommended to configure and only pass the custom list of event listeners, that trigger the console or Lighthouse warning.

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| Option | Type | Default |
| --- | --- | --- |
| debug | boolean | false |
| events | array | [] |
| listeners | array | [] |

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When enabled, all the non-passive event listeners will be console logged.

`js
{
debug: true
}
`

Console output

`js
[Passive Events Support] Non-passive Event Listener
element: div.some-element
event: 'touchstart'
handler:
fn: ƒ (e)
fnArgument: 'e'
fnContent: 'console.log(e)'
fnPrevented: false
arguments: false
updatedArguments: { passive: true }
`

The updatedArguments parameter will be shown only if the event listener was updated by this package.

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The list of events whose event listeners will have a passive option assigned with the value of true or false decided by the package as it is documented in the How It Works section.

#### Supported Events:

| Type | Events |
| --- | --- |
| Touch | touchstart, touchmove, touchenter, touchend, touchleave |
| Wheel | wheel, mousewheel |

`js
{
events: ['touchstart', 'touchmove']
}
`

Events that are not supported will be ignored.

#### Known events option issue:

While this option enables the package to assign the correct passive option value to all the event listeners for the listed events it also might break certain event listeners. The issue appear when preventDefault() is not being called from the handler itself, but rather from the another method called by the handler. In this case this package loses the track of preventDefault() and it marks the event listener as passive. This causes the event listener to break prompting an error message:

> Unable to preventDefault inside passive event listener invocation.

Luckilly, this can easilly be debugged in debug mode and fixed by the listeners configuration option!

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With this option, instead of certain events, you target certain event listeners.

When working altogether with events option, this option could be used to fix the event listeners broken by events option.

`js
{
listeners: [
{
element: '.select-choice',
event: 'touchstart',
prevented: true // (optional) will force { passive: false }
},
{
element: '.select-choice',
event: 'touchmove'
}
]
}
`

When prevented option is not presented, the package will calculate the passive value automatically as it is documented in the How It Works section.

Events that are not supported will be ignored. See supported events list.

Browser Support

Even tho the passive option is not supported by all the browsers (see Browser compatibility), this package, by default, checks the browser support for passive option before assigning it.

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You can yourself access the variable indicating the support:

`js
// With JS
import { passiveSupported } from 'passive-events-support/src/utils'
console.log(passiveSupported())
`

`html



`

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You, just like this package, yourself can manually add the passive option:

`js
// use window.passiveSupported if imported with