JavaScript hooks for transitions in Vue

Introduction

JavaScript hooks for transitions let you run code at specific moments during an animation. This helps you control what happens when elements enter or leave the page.

You want to start a sound or effect exactly when an element appears.

You need to fetch data right after an animation finishes.

You want to clean up or reset something when an element disappears.

You want to coordinate multiple animations with custom timing.

You want to add or remove classes dynamically during transitions.

Syntax

Vue

<transition
  @before-enter="hookBeforeEnter"
  @enter="hookEnter"
  @after-enter="hookAfterEnter"
  @before-leave="hookBeforeLeave"
  @leave="hookLeave"
  @after-leave="hookAfterLeave"
>
  <!-- element to animate -->
</transition>

Each hook is a JavaScript function that runs at a specific transition phase.

Use component to wrap the element you want to animate.

Examples

Runs the onEnter function when the element starts entering.

Vue

<transition @enter="onEnter">
  <div v-if="show">Hello</div>
</transition>

Runs onAfterLeave after the element finishes leaving.

Vue

<transition @after-leave="onAfterLeave">
  <div v-if="show">Bye</div>
</transition>

Runs three hooks at different stages of entering.

Vue

<transition
  @before-enter="beforeEnter"
  @enter="enter"
  @after-enter="afterEnter"
>
  <div v-if="show">Hi</div>
</transition>

Sample Program

This Vue component shows a box that fades in and out when you click the button. The JavaScript hooks control the fade effect by changing opacity step by step. Console logs show when each hook runs.

Vue

<template>
  <button @click="toggle">Toggle Box</button>
  <transition
    @before-enter="beforeEnter"
    @enter="enter"
    @after-enter="afterEnter"
    @before-leave="beforeLeave"
    @leave="leave"
    @after-leave="afterLeave"
  >
    <div v-if="show" class="box">I appear and disappear!</div>
  </transition>
</template>

<script setup>
import { ref } from 'vue'

const show = ref(false)

function toggle() {
  show.value = !show.value
}

function beforeEnter(el) {
  el.style.opacity = 0
  console.log('Before enter: opacity set to 0')
}

function enter(el, done) {
  let opacity = 0
  const step = () => {
    opacity += 0.1
    el.style.opacity = opacity
    if (opacity < 1) {
      requestAnimationFrame(step)
    } else {
      done()
    }
  }
  requestAnimationFrame(step)
  console.log('Enter: starting fade in')
}

function afterEnter(el) {
  console.log('After enter: fully visible')
}

function beforeLeave(el) {
  el.style.opacity = 1
  console.log('Before leave: opacity set to 1')
}

function leave(el, done) {
  let opacity = 1
  const step = () => {
    opacity -= 0.1
    el.style.opacity = opacity
    if (opacity > 0) {
      requestAnimationFrame(step)
    } else {
      done()
    }
  }
  requestAnimationFrame(step)
  console.log('Leave: starting fade out')
}

function afterLeave(el) {
  console.log('After leave: element hidden')
}
</script>

<style scoped>
.box {
  width: 200px;
  height: 100px;
  background-color: #42b983;
  color: white;
  display: flex;
  align-items: center;
  justify-content: center;
  margin-top: 10px;
  border-radius: 8px;
  user-select: none;
}
button {
  padding: 0.5rem 1rem;
  font-size: 1rem;
  cursor: pointer;
}
</style>

Important Notes

Hooks receive the element as the first argument and a done callback for async animations.

Use done() to tell Vue when your animation finishes, so it can proceed.

Console logs help you see the order of hook calls during transitions.

Summary

JavaScript hooks let you run code at key moments in transitions.

Use hooks to customize animations beyond CSS.

Hooks help coordinate complex effects and side actions during enter/leave.

Practice

1. What is the main purpose of JavaScript hooks in Vue transitions?

easy

A. To run custom code at specific moments during enter and leave transitions

B. To replace CSS animations completely

C. To define the HTML structure of a component

D. To manage Vue component state

Solution

1. Step 1: Understand what JavaScript hooks do in Vue transitions

   JavaScript hooks allow you to run code at key moments during the enter and leave phases of a transition.

2. Step 2: Compare options with this understanding

   Only To run custom code at specific moments during enter and leave transitions correctly describes this purpose. Options B, C, and D describe unrelated concepts.

3. Final Answer:

   To run custom code at specific moments during enter and leave transitions -> Option A

4. Quick Check:

   JavaScript hooks = run code during transitions [OK]

Hint: Hooks run code during enter/leave, not replace CSS [OK]

Common Mistakes:

• Thinking hooks replace CSS animations
• Confusing hooks with component state management
• Believing hooks define HTML structure

2. Which of the following is the correct syntax to define a JavaScript hook for the beforeEnter phase in a Vue transition component?

easy

A. <transition v-on:before-enter="myHook">

B. <transition v-on:beforeEnter="myHook">

C. <transition @before-enter="myHook">

D. <transition @beforeEnter="myHook">

Solution

1. Step 1: Recall Vue event naming conventions for transitions

   Vue uses kebab-case event names for transition hooks, like before-enter, and the shorthand @ for v-on:.

2. Step 2: Match the correct syntax

   <transition @before-enter="myHook"> uses @before-enter, which is correct. Options A, C, and D use camelCase or incorrect casing.

3. Final Answer:

   <transition @before-enter="myHook"> -> Option C

4. Quick Check:

   Vue transition hooks use kebab-case with @ [OK]

Hint: Use kebab-case and @ for Vue transition hooks [OK]

Common Mistakes:

• Using camelCase event names
• Mixing v-on: and @ incorrectly
• Capitalizing event names

3. Given this Vue transition component snippet, what will be logged when the element starts entering?

<transition @enter="onEnter">
  <div v-if="show">Hello</div>
</transition>

medium

A. Nothing is logged because show is initially false

B. Throws an error because done() is called synchronously

C. Logs 'Enter started' immediately on page load

D. Logs 'Enter started' after 1 second when element appears

Solution

1. Step 1: Understand the timing of the show variable

   Initially, show is false, so the element is not rendered. After 1 second, show becomes true, triggering the enter transition.

2. Step 2: Analyze the enter hook behavior

   The onEnter hook logs 'Enter started' and calls done() immediately. This happens after 1 second when the element appears.

3. Final Answer:

   Logs 'Enter started' after 1 second when element appears -> Option D

4. Quick Check:

   Enter hook logs on element show [OK]

Hint: Enter hook runs when element appears, not on load [OK]

Common Mistakes:

• Thinking enter hook runs immediately on load
• Assuming no log because initial show is false
• Believing done() call causes error

4. Identify the error in this Vue transition JavaScript hook code:

<transition @leave="onLeave">
  <div v-if="visible">Bye</div>
</transition>

medium

A. Calling done() immediately before animation finishes causes transition to end too soon

B. Missing a return statement in onLeave function

C. Using setTimeout inside onLeave is not allowed

D. v-if should be v-show for transitions

Solution

1. Step 1: Understand the leave hook timing

   The onLeave hook should call done() only after the animation finishes to keep the element during transition.

2. Step 2: Analyze the code's done() call

   Here, done() is called immediately, but the opacity change happens after 500ms, so the transition ends too soon.

3. Final Answer:

   Calling done() immediately before animation finishes causes transition to end too soon -> Option A

4. Quick Check:

   done() must wait for animation end [OK]

Hint: Call done() only after animation completes [OK]

Common Mistakes:

• Calling done() too early
• Confusing v-if and v-show for transitions
• Thinking setTimeout is disallowed in hooks

5. You want to create a fade transition that delays the leave animation until a custom API call finishes. Which approach correctly uses JavaScript hooks to achieve this?

<transition @leave="handleLeave">
  <div v-if="show">Content</div>
</transition>

<script setup>
const show = ref(true)
async function apiCall() {
  return new Promise(resolve => setTimeout(resolve, 1000))
}

function handleLeave(el, done) {
  // What should go here?
}
</script>

hard

A. Set opacity to 0, call done(), then call apiCall()

B. Call apiCall(), then in .then() set opacity to 0 and call done()

C. Call done() immediately, then call apiCall()

D. Do nothing in handleLeave; rely on CSS only

Solution

1. Step 1: Understand the goal

   The leave transition should wait for the API call to finish before starting the fade out and completing the transition.

2. Step 2: Use async logic in the leave hook

   Call apiCall(), then after it resolves, set opacity to 0 and call done() to signal transition end.

3. Final Answer:

   Call apiCall(), then in .then() set opacity to 0 and call done() -> Option B

4. Quick Check:

   Wait async, then animate and done() [OK]

Hint: Wait async call before done() in leave hook [OK]

Common Mistakes:

• Calling done() before async finishes
• Ignoring async and relying on CSS only
• Calling done() too early, skipping animation