require-passive-events.md

Require Passive Events

This rule enforces adding passive: true to high frequency event listeners (touchstart, touchmove, wheel, mousewheel).

Rule Details

Adding these events listeners can block the main thread as it waits to find out if the callbacks call preventDefault. This can cause large amounts UI lag, which will be noticeable for users.

Adding passive: true informs the browser that this event will not be calling preventDefault and as such is safe to asynchronously dispatch, freeing up the main thread for lag-free operation.

See also: https://developer.mozilla.org/en-US/docs/Web/API/EventTarget/addEventListener#Improving_scrolling_performance_with_passive_listeners

👎 Examples of incorrect code for this rule:

// bad
window.addEventListener('touchstart', () => {
  /* ... */
})

👍 Examples of correct code for this rule:

// good
window.addEventListener(
  'touchstart',
  () => {
    /* ... */
  },
  {passive: true}
)

Version

4.3.2