Fetch API in Vue components - Performance & Optimization

Performance: Fetch API in Vue components

MEDIUM IMPACT

This affects page load speed and interaction responsiveness by controlling when and how data is fetched and rendered in Vue components.

Fetching data inside the component's setup without caching or lazy loading

Vue

import { ref, onMounted } from 'vue';
const cache = {};
export default {
  setup() {
    const data = ref(null);
    onMounted(async () => {
      if (cache['data']) {
        data.value = cache['data'];
      } else {
        const response = await fetch('https://api.example.com/data');
        data.value = await response.json();
        cache['data'] = data.value;
      }
    });
    return { data };
  }
}

Caches data to avoid repeated fetches, reducing network requests and speeding up rendering on repeated visits.

📈 Performance GainReduces network calls and blocks rendering less often, improving LCP and INP.

Fetching data inside the component's setup without caching or lazy loading

Vue

import { ref, onMounted } from 'vue';
export default {
  setup() {
    const data = ref(null);
    onMounted(async () => {
      const response = await fetch('https://api.example.com/data');
      data.value = await response.json();
    });
    return { data };
  }
}

Fetch runs every time the component mounts, causing delays in rendering and blocking UI updates until data arrives.

📉 Performance CostBlocks rendering until fetch completes, increasing LCP and causing slower INP.

Performance Comparison

Pattern	DOM Operations	Reflows	Paint Cost	Verdict
Fetch in onMounted with caching	Minimal reactive updates	Single reflow after data arrives	Single paint update	[OK] Good
Fetch in onMounted without caching	Reactive updates each mount	Reflow per mount	Paint per mount	[!] OK
Fetch in template expressions	Multiple fetch triggers	Multiple reflows per render	Multiple paints	[X] Bad
Synchronous fetch blocking UI	Blocks initial render	Blocks reflow	Blocks paint	[X] Bad

Rendering Pipeline

Fetch API calls in Vue components trigger network requests that delay data availability. Until data arrives, Vue may render placeholders or empty states. Once data is set, Vue updates the DOM causing style recalculation, layout, paint, and composite steps.

→Network

→Style Calculation

→Layout

→Paint

→Composite

⚠️ BottleneckNetwork latency and Layout caused by reactive DOM updates after data fetch

Core Web Vital Affected

LCP, INP

This affects page load speed and interaction responsiveness by controlling when and how data is fetched and rendered in Vue components.

Optimization Tips

1Always fetch data asynchronously inside lifecycle hooks like onMounted.

2Cache fetch results to avoid repeated network requests and speed up rendering.

3Never fetch data directly in template expressions to prevent multiple fetch calls and reflows.

Performance Quiz - 3 Questions

What is a performance benefit of caching fetch results in Vue components?

A Increases bundle size significantly

B Triggers more reflows on each render

C Reduces repeated network requests and speeds up rendering

D Blocks rendering until cache is cleared

DevTools: Performance

How to check: Open DevTools, go to Performance tab, record while loading the Vue page, and look for long tasks or network delays during component mount.

What to look for: Look for long blocking times in Main thread and delayed DOM updates indicating slow fetch or blocking code.

Practice

1. What is the main purpose of using the Fetch API inside a Vue component?

easy

A. To handle user input events like clicks

B. To style the component with CSS

C. To get data from the internet and use it in the component

D. To create new Vue components dynamically

Solution

1. Step 1: Understand Fetch API role

   The Fetch API is used to request data from a server or internet resource.

2. Step 2: Connect Fetch API to Vue component

   Inside Vue components, fetch is used to load data that the component will display or use.

3. Final Answer:

   To get data from the internet and use it in the component -> Option C

4. Quick Check:

   Fetch API = get data [OK]

Hint: Fetch API fetches data from internet for Vue components [OK]

Common Mistakes:

• Thinking Fetch API styles components
• Confusing fetch with event handling
• Using fetch to create components

2. Which is the correct way to use fetch inside a Vue 3 <script setup> component to load data on mount?

easy

A. Call fetch inside the onMounted() lifecycle hook

B. Call fetch inside a computed property

C. Use fetch directly in the template section

D. Use fetch inside a watchEffect without dependencies

Solution

1. Step 1: Identify lifecycle hook for data loading

   onMounted() runs code after the component appears, perfect for fetching data.

2. Step 2: Match fetch usage to lifecycle hook

   Calling fetch inside onMounted() ensures data loads once component is ready.

3. Final Answer:

   Call fetch inside the onMounted() lifecycle hook -> Option A

4. Quick Check:

   Fetch in onMounted = correct usage [OK]

Hint: Use onMounted() to fetch data after component loads [OK]

Common Mistakes:

• Trying to fetch data in template (not allowed)
• Using computed for async fetch (wrong usage)
• Using watchEffect without dependencies causing repeated fetch

3. What will be the output in the console when this Vue 3 <script setup> code runs?

import { ref, onMounted } from 'vue';
const data = ref(null);
const error = ref(null);
onMounted(async () => {
  try {
    const res = await fetch('https://jsonplaceholder.typicode.com/posts/1');
    data.value = await res.json();
    console.log(data.value.title);
  } catch (e) {
    error.value = 'Failed to load';
    console.log(error.value);
  }
});

medium

A. Logs undefined because data.value is never set

B. Logs 'Failed to load' always

C. Throws a syntax error because async is not allowed

D. Logs the title of the post with id 1 from the API

Solution

1. Step 1: Analyze fetch and await usage

   The code fetches a post from the API and waits for the JSON response.

2. Step 2: Check console.log output

   It logs the title property of the fetched post, so it prints the post's title.

3. Final Answer:

   Logs the title of the post with id 1 from the API -> Option D

4. Quick Check:

   Fetch success logs title [OK]

Hint: Await fetch then log data property for output [OK]

Common Mistakes:

• Assuming fetch always fails
• Thinking async is not allowed in onMounted
• Expecting undefined because of missing await

4. Identify the error in this Vue 3 <script setup> snippet that fetches data:

import { ref, onMounted } from 'vue';
const data = ref(null);
const error = ref(null);
onMounted(() => {
  fetch('https://api.example.com/data')
    .then(response => response.json())
    .then(json => data = json)
    .catch(() => error.value = 'Error fetching');
});

medium

A. Assigning json directly to data instead of data.value

B. Missing await keyword before fetch

C. Using onMounted instead of created lifecycle hook

D. Not returning the fetch promise

Solution

1. Step 1: Check how ref is updated

   Refs in Vue must be updated by setting their .value property, not the ref variable itself.

2. Step 2: Identify incorrect assignment

   The code assigns json directly to data, which breaks reactivity; it should be data.value = json.

3. Final Answer:

   Assigning json directly to data instead of data.value -> Option A

4. Quick Check:

   Update ref with .value [OK]

Hint: Update ref variables with .value, not direct assignment [OK]

Common Mistakes:

• Forgetting .value when setting ref
• Confusing async/await with then chaining
• Thinking onMounted is wrong lifecycle hook here

5. You want to fetch user data and show a loading message while waiting. Which Vue 3 <script setup> code correctly implements fetch with loading and error states?

hard

A. const loading = false;
const error = null;
const user = null;
onMounted(() => {
fetch('/user')
.then(res => res.json())
.then(data => user = data)
.catch(() => error = 'Failed');
loading = false;
});

B. const loading = ref(false);
const error = ref(null);
const user = ref(null);
onMounted(async () => {
loading.value = true;
try {
const res = await fetch('/user');
user.value = await res.json();
} catch {
error.value = 'Failed to load user';
} finally {
loading.value = false;
}
});

C. const loading = ref(true);
const error = ref('');
const user = ref({});
fetch('/user').then(res => res.json()).then(data => {
user.value = data;
loading.value = false;
}).catch(() => {
error.value = 'Error';
loading.value = false;
});

D. const loading = ref(false);
const error = ref(null);
const user = ref(null);
async function loadUser() {
loading.value = true;
const res = await fetch('/user');
user.value = await res.json();
loading.value = false;
}
onMounted(loadUser);

Solution

1. Step 1: Check reactive state setup

   const loading = ref(false); const error = ref(null); const user = ref(null); onMounted(async () => { loading.value = true; try { const res = await fetch('/user'); user.value = await res.json(); } catch { error.value = 'Failed to load user'; } finally { loading.value = false; } }); uses ref() for loading, error, and user, enabling reactivity and UI updates.

2. Step 2: Verify fetch with loading and error handling

   It sets loading true before fetch, uses try/catch for errors, and sets loading false in finally block.

3. Final Answer:

   Uses refs with onMounted, try/catch/finally for loading and error states -> Option B

4. Quick Check:

   Loading true before fetch, false after [OK]

Hint: Use ref and try/catch with finally to manage loading state [OK]

Common Mistakes:

• Not using ref for reactive state
• Setting loading false too early
• Missing error handling in async fetch