Hydration behavior in Vue - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ component_behavior

intermediate

What happens during Vue hydration when server and client markup differ?

Consider a Vue 3 app using SSR. The server renders a list of items, but the client has a different list initially. What will Vue do during hydration?

Vue

<template>
  <ul>
    <li v-for="item in items" :key="item.id">{{ item.text }}</li>
  </ul>
</template>

<script setup>
import { ref } from 'vue'
const items = ref([{ id: 1, text: 'A' }, { id: 2, text: 'B' }])
// Assume server rendered items: [{ id: 1, text: 'A' }, { id: 2, text: 'C' }]
</script>

A Vue will keep the server markup as is and ignore the client data, causing the list to show 'C' instead of 'B'.

B Vue will discard the server-rendered DOM and fully re-render the list on the client from scratch.

C Vue will throw a hydration error and stop rendering because the server and client markup differ.

D Vue will patch the DOM to update the text of the second list item from 'C' to 'B' without re-rendering the entire list.

💡 Hint

Think about how Vue tries to reuse server markup and patch only differences.

Vue's hydration process reuses the server-rendered DOM and patches only the differences found in client data. It updates text content or attributes as needed without full re-rendering.

❓ lifecycle

intermediate

When does the mounted() hook run in a hydrated Vue component?

In a Vue 3 SSR app, the server renders the component and the client hydrates it. At what point does the mounted() lifecycle hook run?

A Mounted runs immediately after the server renders the HTML, before hydration starts.

B Mounted runs before hydration begins, during the initial client setup phase.

C Mounted runs after the client finishes hydration and the component is attached to the DOM.

D Mounted does not run at all during hydration to avoid duplicate side effects.

💡 Hint

Consider when the component is fully interactive on the client.

The mounted() hook runs after hydration completes and the component is attached to the DOM on the client. It does not run on the server.

🔧 Debug

advanced

Why does this Vue hydration warning appear in the console?

Given this Vue 3 SSR app, the console shows a hydration warning about mismatched content. What causes this warning?

Vue

<template>
  <div>{{ message }}</div>
</template>

<script setup>
import { ref } from 'vue'
const message = ref('Hello from client')
// Server rendered message was 'Hello from server'
</script>

A The client data differs from the server markup, causing Vue to detect mismatch during hydration.

B The component is missing a unique key, so Vue cannot match nodes properly.

C The server did not render the component, so hydration fails.

D Vue hydration warnings only appear if the component uses deprecated APIs.

💡 Hint

Check if the server and client render the same text content.

Hydration warnings occur when the client-side data produces different markup than the server-rendered HTML, causing Vue to detect mismatches.

📝 Syntax

advanced

Which Vue hydration code snippet correctly avoids hydration mismatch?

Choose the code snippet that prevents hydration mismatch by ensuring server and client render the same content.

A

<template>
  <div>{{ count }}</div>
</template>
<script setup>
import { ref, onMounted } from 'vue'
const count = ref(0)
onMounted(() => { count.value = 1 })
</script>

B

<template>
  <div>{{ count }}</div>
</template>
<script setup>
import { ref } from 'vue'
const count = ref(1)
</script>

C

<template>
  <div v-if="mounted">{{ count }}</div>
</template>
<script setup>
import { ref, onMounted } from 'vue'
const count = ref(1)
const mounted = ref(false)
onMounted(() => { mounted.value = true })
</script>

D

<template>
  <div>{{ count }}</div>
</template>
<script setup>
import { ref } from 'vue'
const count = ref(0)
</script>

💡 Hint

Think about what value the server rendered and what the client starts with.

To avoid hydration mismatch, the initial client state must match the server-rendered content. Setting count to 1 initially matches server output.

❓ state_output

expert

What is the final rendered output after hydration with this Vue 3 SSR component?

Given this Vue 3 SSR component, what will the user see after hydration completes?

Vue

<template>
  <button @click="increment">Clicked {{ count }} times</button>
</template>

<script setup>
import { ref } from 'vue'
const count = ref(5) // server rendered with count=5
function increment() {
  count.value++
}
</script>

A The button shows 'Clicked 5 times' initially, and increments count on each click.

B The button shows 'Clicked 0 times' initially because client state resets, then increments on click.

C The button shows 'Clicked 5 times' but clicking does not update the count due to hydration issues.

D The button shows 'Clicked 0 times' and clicking causes hydration errors.

💡 Hint

Consider how Vue preserves reactive state from server to client during hydration.

Vue hydrates with the server state (count=5). The button shows 'Clicked 5 times' initially and increments correctly on clicks.

Practice

1. What is the main purpose of hydration in Vue when using server-side rendering?

easy

A. To connect the static HTML from the server with Vue's client-side code for interactivity

B. To generate HTML on the client without any server involvement

C. To replace all server-rendered HTML with new client-rendered HTML

D. To disable JavaScript on the client for faster loading

Solution

1. Step 1: Understand hydration concept

   Hydration links the server-rendered HTML with Vue's client code to make the page interactive without re-rendering everything.

2. Step 2: Compare options

   To connect the static HTML from the server with Vue's client-side code for interactivity correctly describes hydration. Options B, C, and D misunderstand hydration's role.

3. Final Answer:

   To connect the static HTML from the server with Vue's client-side code for interactivity -> Option A

4. Quick Check:

   Hydration = connect server HTML + client code [OK]

Hint: Hydration links server HTML to client Vue code [OK]

Common Mistakes:

• Thinking hydration generates HTML only on client
• Believing hydration replaces server HTML entirely
• Assuming hydration disables JavaScript

2. Which of the following Vue code snippets correctly enables hydration for a server-rendered app?

easy

A. createApp(App).mount('#app')

B. new Vue({ el: '#app' })

C. createSSRApp(App).mount('#app')

D. Vue.hydrate(App, '#app')

Solution

1. Step 1: Identify hydration method in Vue 3 SSR

   Vue 3 uses createSSRApp() to create an app that supports hydration of server-rendered HTML.

2. Step 2: Evaluate options

   createSSRApp(App).mount('#app') uses createSSRApp(). createApp(App).mount('#app') is for client-only apps. new Vue({ el: '#app' }) is Vue 2 syntax. Vue.hydrate(App, '#app') is invalid.

3. Final Answer:

   createSSRApp(App).mount('#app') -> Option C

4. Quick Check:

   SSR hydration uses createSSRApp() [OK]

Hint: Use createSSRApp() for hydration in Vue 3 SSR [OK]

Common Mistakes:

• Using createApp() instead of createSSRApp() for hydration
• Using Vue 2 syntax in Vue 3 projects
• Assuming Vue has a hydrate() method

3. Given this Vue 3 SSR hydration code snippet, what will be the console output?

const app = createSSRApp({
  data() { return { count: 0 } },
  mounted() { console.log('Mounted with count:', this.count) }
})
app.mount('#app')

medium

A. No output because mounted is not called during hydration

B. Mounted with count: 0

C. Error: createSSRApp is not a function

D. Mounted with count: undefined

Solution

1. Step 1: Understand mounted hook during hydration

   During hydration, the mounted hook runs after the client connects to server HTML, so it logs the current data state.

2. Step 2: Check data initialization

   Data count is initialized to 0, so the console logs 'Mounted with count: 0'.

3. Final Answer:

   Mounted with count: 0 -> Option B

4. Quick Check:

   Mounted logs count 0 during hydration [OK]

Hint: Mounted runs after hydration with initialized data [OK]

Common Mistakes:

• Thinking mounted does not run during hydration
• Assuming data is undefined at mount
• Confusing createSSRApp with createApp

4. You notice that after hydration, your Vue app's client-side interactivity is broken and console shows a mismatch warning. What is the most likely cause?

medium

A. The server-rendered HTML does not match the client-side template output

B. You forgot to import Vue in your client entry file

C. You used createApp() instead of createSSRApp() on the server

D. The mounted hook is missing in your component

Solution

1. Step 1: Understand hydration mismatch warning

   Vue warns when server HTML and client render output differ, causing hydration failure.

2. Step 2: Analyze options

   The server-rendered HTML does not match the client-side template output correctly identifies mismatch as cause. You forgot to import Vue in your client entry file causes import errors, not mismatch. You used createApp() instead of createSSRApp() on the server is about server setup, not client mismatch. The mounted hook is missing in your component does not cause hydration mismatch.

3. Final Answer:

   The server-rendered HTML does not match the client-side template output -> Option A

4. Quick Check:

   Mismatch = server HTML ≠ client template [OK]

Hint: Mismatch warning means server and client HTML differ [OK]

Common Mistakes:

• Ignoring mismatch warnings
• Confusing import errors with hydration issues
• Assuming missing mounted hook breaks hydration

5. You have a Vue 3 SSR app that renders a list of items on the server. Some items have a dynamic property that changes on the client only. How should you handle hydration to avoid mismatch errors?

hard

A. Ignore mismatch warnings since they don't affect user experience

B. Render all dynamic properties on the server to keep HTML identical

C. Disable hydration and fully re-render on client with createApp()

D. Use v-if or v-show to conditionally render client-only parts after hydration

Solution

1. Step 1: Identify client-only dynamic content issue

   Dynamic properties that differ between server and client cause hydration mismatches.

2. Step 2: Use conditional rendering for client-only parts

   Using v-if or v-show to render client-only content after hydration avoids mismatch.

3. Step 3: Evaluate other options

   Render all dynamic properties on the server to keep HTML identical is impractical if data is truly client-only. Disable hydration and fully re-render on client with createApp() disables hydration benefits. Ignore mismatch warnings since they don't affect user experience ignores errors.

4. Final Answer:

   Use v-if or v-show to conditionally render client-only parts after hydration -> Option D

5. Quick Check:

   Client-only content = conditional render after hydration [OK]

Hint: Render client-only parts conditionally after hydration [OK]

Common Mistakes:

• Trying to render client-only data on server
• Disabling hydration unnecessarily
• Ignoring mismatch warnings