Composable for API calls (useFetch pattern) in Vue - Performance & Optimization

Performance: Composable for API calls (useFetch pattern)

MEDIUM IMPACT

This pattern affects page load speed and interaction responsiveness by managing when and how API data is fetched and rendered.

Fetching data from an API in multiple components

Vue

import { ref } from 'vue';

export function useFetchUser() {
  const user = ref(null);
  const loading = ref(false);
  const error = ref(null);

  async function fetchUser() {
    loading.value = true;
    try {
      const res = await fetch('https://api.example.com/user');
      user.value = await res.json();
    } catch (e) {
      error.value = e;
    } finally {
      loading.value = false;
    }
  }

  return { user, loading, error, fetchUser };
}

// Components import and reuse this composable

Centralizes API call, caches data, and shares state, reducing network requests and re-renders.

📈 Performance GainSingle network request reused, fewer reflows, faster interaction response.

Fetching data from an API in multiple components

Vue

export default {
  data() {
    return { user: null };
  },
  async mounted() {
    const res = await fetch('https://api.example.com/user');
    this.user = await res.json();
  }
};

// Repeated in every component needing user data

Each component fetches the same data independently, causing multiple network requests and redundant state updates.

📉 Performance CostTriggers multiple network requests and reflows, increasing load time and interaction delay.

Performance Comparison

Pattern	DOM Operations	Reflows	Paint Cost	Verdict
Independent fetch in each component	Multiple redundant DOM updates	Multiple reflows per fetch	High paint cost due to repeated updates	[X] Bad
Composable useFetch pattern with shared state	Single DOM update per data change	Single reflow per data update	Lower paint cost with minimal updates	[OK] Good

Rendering Pipeline

The composable triggers the fetch during component setup or on demand, updating reactive state that causes minimal re-rendering only when data changes.

→Network Request

→JavaScript Execution

→Style Calculation

→Layout

→Paint

⚠️ BottleneckNetwork Request and JavaScript Execution when fetching and processing data

Core Web Vital Affected

INP

This pattern affects page load speed and interaction responsiveness by managing when and how API data is fetched and rendered.

Optimization Tips

1Reuse API call logic in a composable to avoid duplicate requests.

2Update reactive state minimally to reduce reflows and paints.

3Fetch data lazily or on demand to avoid blocking initial rendering.

Performance Quiz - 3 Questions

What is the main performance benefit of using a composable for API calls in Vue?

A It delays rendering until all components finish fetching.

B It increases the number of network requests for faster data.

C It reduces redundant network requests by sharing fetched data.

D It forces components to fetch data independently.

DevTools: Performance

How to check: Record a session while loading the page and interacting with components. Look for multiple network requests to the same API endpoint and repeated layout or paint events.

What to look for: Fewer network requests and reduced layout/paint events indicate better performance with composable pattern.

Practice

1. What is the main purpose of a composable like useFetch in Vue?

easy

A. To fetch data from an API and manage loading and error states reactively

B. To style components with CSS variables

C. To create Vue directives for DOM manipulation

D. To define routes in a Vue application

Solution

1. Step 1: Understand composable role

   A composable in Vue is a reusable function that encapsulates logic, often for state or side effects.

2. Step 2: Identify useFetch purpose

   The useFetch composable specifically handles API calls and manages reactive states like loading and error.

3. Final Answer:

   To fetch data from an API and manage loading and error states reactively -> Option A

4. Quick Check:

   Composable useFetch = fetch + reactive states [OK]

Hint: Composable useFetch manages API data and states reactively [OK]

Common Mistakes:

• Confusing composables with styling or routing
• Thinking composables only handle UI rendering
• Assuming composables are Vue components

2. Which of the following is the correct way to import and use a useFetch composable in a Vue component?

easy

A. const { useFetch } = Vue; const result = useFetch(url);

B. import useFetch from 'vue'; const data = useFetch(url);

C. import { useFetch } from '@/composables/useFetch'; const { data, error } = useFetch(url);

D. import { fetchData } from '@/composables'; fetchData(url);

Solution

1. Step 1: Check import syntax

   The composable is usually exported from a file and imported with curly braces if it's a named export.

2. Step 2: Using the composable

   Calling useFetch(url) returns reactive properties like data and error.

3. Final Answer:

   import { useFetch } from '@/composables/useFetch'; const { data, error } = useFetch(url); -> Option C

4. Quick Check:

   Correct import + destructure = import { useFetch } from '@/composables/useFetch'; const { data, error } = useFetch(url); [OK]

Hint: Import named composable and destructure returned refs [OK]

Common Mistakes:

• Importing from wrong path or package
• Using default import when composable is named export
• Not destructuring returned reactive properties

3. Given this useFetch composable usage:

const { data, loading, error } = useFetch('https://api.example.com/items');

What will loading.value be immediately after calling useFetch?

medium

A. null

B. true

C. false

D. undefined

Solution

1. Step 1: Understand loading state in useFetch

   Loading usually starts as true when the fetch begins to indicate data is being fetched.

2. Step 2: Check initial loading value

   Immediately after calling, loading is true until data or error updates it to false.

3. Final Answer:

   true -> Option B

4. Quick Check:

   Loading starts true during fetch [OK]

Hint: Loading is true while fetching data [OK]

Common Mistakes:

• Assuming loading starts false before fetch
• Confusing loading with data presence
• Expecting loading to be null or undefined

4. What is the main issue with this useFetch composable snippet?

export function useFetch(url) {
  const data = ref(null);
  const loading = ref(false);
  const error = ref(null);

  fetch(url)
    .then(res => res.json())
    .then(json => { data.value = json; loading.value = false; })
    .catch(err => { error.value = err; loading.value = false; });

  loading.value = true;
  return { data, loading, error };
}

medium

A. Setting loading.value = true after fetch call starts causes race condition

B. Not using async/await causes syntax error

C. data should be initialized as an empty array, not null

D. Error ref should be initialized as an empty string

Solution

1. Step 1: Analyze loading state timing

   Loading is set to false inside fetch callbacks but set to true after fetch starts, which can cause timing issues.

2. Step 2: Identify race condition

   Because loading is set to true after fetch call, it might overwrite loading false set in callbacks if fetch resolves quickly.

3. Final Answer:

   Setting loading.value = true after fetch call starts causes race condition -> Option A

4. Quick Check:

   Loading state timing error = Setting loading.value = true after fetch call starts causes race condition [OK]

Hint: Set loading true before fetch call to avoid race [OK]

Common Mistakes:

• Setting loading true after fetch call instead of before
• Confusing async/await with promises syntax errors
• Assuming initial data type must be array

5. You want to create a useFetch composable that automatically retries fetching up to 3 times if the API call fails. Which approach correctly adds this retry logic?

hard

A. Wrap fetch in a try/catch but do not retry on failure

B. Call fetch once and ignore errors, relying on component to retry

C. Use setTimeout outside the composable to call useFetch repeatedly

D. Use a recursive function inside the composable that calls fetch again on error until max retries reached

Solution

1. Step 1: Understand retry logic needs repeated fetch calls

   Retry means calling fetch again if it fails, up to a limit.

2. Step 2: Implement retry inside composable

   A recursive or loop function inside composable can retry fetch on error until max retries.

3. Step 3: Evaluate other options

   Ignoring errors or retrying outside composable breaks encapsulation; try/catch without retry misses requirement.

4. Final Answer:

   Use a recursive function inside the composable that calls fetch again on error until max retries reached -> Option D

5. Quick Check:

   Retry logic inside composable = Use a recursive function inside the composable that calls fetch again on error until max retries reached [OK]

Hint: Retry fetch inside composable with recursion or loop [OK]

Common Mistakes:

• Retrying outside composable losing reactive state control
• Ignoring errors instead of retrying
• Not limiting retry attempts causing infinite loops