142 lines
3.7 KiB
Markdown
142 lines
3.7 KiB
Markdown
---
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title: Never Use .passive and .prevent Together
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impact: HIGH
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impactDescription: Conflicting modifiers cause .prevent to be ignored and trigger browser warnings
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type: gotcha
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tags: [vue3, events, modifiers, scroll, touch, performance]
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---
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# Never Use .passive and .prevent Together
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**Impact: HIGH** - The `.passive` modifier tells the browser you will NOT call `preventDefault()`, while `.prevent` does exactly that. Using them together causes `.prevent` to be ignored and triggers browser console warnings. This is a logical contradiction that leads to broken event handling.
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## Task Checklist
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- [ ] Never combine `.passive` and `.prevent` on the same event
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- [ ] Use `.passive` for scroll/touch events where you want better performance
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- [ ] Use `.prevent` when you need to stop the default browser action
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- [ ] If you need conditional prevention, handle it in JavaScript without `.passive`
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**Incorrect:**
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```html
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<!-- WRONG: Conflicting modifiers -->
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<template>
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<div @scroll.passive.prevent="handleScroll">
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<!-- .prevent will be IGNORED -->
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<!-- Browser shows warning -->
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</div>
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</template>
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```
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```html
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<!-- WRONG: On touch events -->
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<template>
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<div @touchstart.passive.prevent="handleTouch">
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<!-- Cannot prevent default - passive already promised not to -->
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</div>
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</template>
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```
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```html
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<!-- WRONG: On wheel events -->
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<template>
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<div @wheel.passive.prevent="handleWheel">
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<!-- Broken: will scroll anyway despite .prevent -->
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</div>
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</template>
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```
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**Correct:**
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```html
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<!-- CORRECT: Use .passive for performance (no prevention needed) -->
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<template>
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<div @scroll.passive="handleScroll">
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<!-- Good for scroll tracking without blocking -->
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</div>
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</template>
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```
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```html
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<!-- CORRECT: Use .prevent when you need to prevent default -->
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<template>
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<form @submit.prevent="handleSubmit">
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<!-- Correctly prevents form submission -->
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</form>
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</template>
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```
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```html
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<!-- CORRECT: For touch events where you need to prevent -->
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<template>
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<div @touchmove="handleTouchMove">
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<!-- Handle prevention conditionally in JS -->
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</div>
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</template>
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<script setup>
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function handleTouchMove(event) {
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if (shouldPreventScroll.value) {
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event.preventDefault()
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}
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// ... handle touch
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}
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</script>
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```
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## Understanding .passive
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```javascript
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// .passive tells the browser:
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// "I promise I won't call preventDefault()"
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// This allows the browser to:
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// 1. Start scrolling immediately without waiting for JS
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// 2. Improve scroll performance, especially on mobile
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// 3. Reduce jank and stuttering
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// Equivalent to:
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element.addEventListener('scroll', handler, { passive: true })
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```
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## When to Use .passive
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```html
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<!-- Good use cases for .passive -->
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<!-- Scroll tracking analytics -->
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<div @scroll.passive="trackScrollPosition">
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<!-- Touch gesture detection (no prevention needed) -->
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<div @touchmove.passive="detectGesture">
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<!-- Wheel event monitoring -->
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<div @wheel.passive="monitorWheel">
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```
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## When to Use .prevent (Without .passive)
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```html
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<!-- Good use cases for .prevent -->
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<!-- Form submission -->
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<form @submit.prevent="handleSubmit">
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<!-- Link clicks with custom navigation -->
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<a @click.prevent="navigate">
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<!-- Preventing context menu -->
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<div @contextmenu.prevent="showCustomMenu">
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```
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## Browser Warning
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When you combine `.passive` and `.prevent`, the browser console shows:
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```
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[Intervention] Unable to preventDefault inside passive event listener
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due to target being treated as passive.
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```
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## Reference
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- [Vue.js Event Handling - Event Modifiers](https://vuejs.org/guide/essentials/event-handling.html#event-modifiers)
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- [MDN - Improving scroll performance with passive listeners](https://developer.mozilla.org/en-US/docs/Web/API/EventTarget/addEventListener#improving_scrolling_performance_with_passive_listeners)
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