Error handling in HTTP calls in Vue - Performance & Optimization

Performance: Error handling in HTTP calls

MEDIUM IMPACT

This affects the responsiveness and smoothness of the user interface during and after HTTP requests.

Handling errors during HTTP calls in a Vue component

Vue

import { ref } from 'vue';

const data = ref(null);
const error = ref(null);

async function fetchData() {
  try {
    const response = await fetch('https://api.example.com/data');
    if (!response.ok) throw new Error('Network response was not ok');
    data.value = await response.json();
    error.value = null;
  } catch (err) {
    error.value = err.message;
  }
}

Using reactive error state allows non-blocking UI updates and smooth user experience.

📈 Performance GainNo UI blocking, fewer re-renders, better INP score.

Handling errors during HTTP calls in a Vue component

Vue

fetch('https://api.example.com/data')
  .then(response => response.json())
  .then(data => {
    this.data = data;
  })
  .catch(error => {
    alert('Error occurred: ' + error.message);
  });

Using alert blocks the UI thread and interrupts user interaction, causing poor responsiveness.

📉 Performance CostBlocks rendering and user input until alert is dismissed, increasing INP.

Performance Comparison

Pattern	DOM Operations	Reflows	Paint Cost	Verdict
Blocking alert() in catch	Minimal DOM changes	Triggers reflow after alert dismissed	Blocks paint during alert	[X] Bad
Reactive error state update	Updates reactive DOM nodes	Single reflow for error message	Smooth paint without blocking	[OK] Good

Rendering Pipeline

Error handling in HTTP calls affects the interaction responsiveness stage by controlling UI updates and preventing blocking dialogs.

→JavaScript Execution

→Layout

→Paint

→Composite

⚠️ BottleneckJavaScript Execution when blocking dialogs like alert() are used

Core Web Vital Affected

INP

This affects the responsiveness and smoothness of the user interface during and after HTTP requests.

Optimization Tips

1Avoid blocking UI methods like alert() for error handling.

2Use reactive state to display errors asynchronously.

3Keep error UI updates minimal to reduce reflows and paints.

Performance Quiz - 3 Questions

Which error handling method improves interaction responsiveness during HTTP calls in Vue?

A Reloading the page on error

B Using reactive state to show error messages

C Using alert() to notify errors

D Ignoring errors silently

DevTools: Performance

How to check: Record a session while triggering an HTTP error and observe the Main thread activity and responsiveness.

What to look for: Look for long tasks caused by alert() blocking and check responsiveness metrics like INP.

Practice

1. What is the main purpose of using try...catch blocks in Vue HTTP calls?

easy

A. To handle errors when the HTTP request fails or returns an error

B. To speed up the HTTP request

C. To style the HTTP response data

D. To automatically retry the HTTP request

Solution

1. Step 1: Understand the role of try...catch

   The try...catch block is used to catch errors that happen during code execution, such as network failures or exceptions.

2. Step 2: Apply to HTTP calls in Vue

   When making HTTP calls, errors like failed requests or server errors can occur. Using try...catch helps catch these errors and handle them gracefully.

3. Final Answer:

   To handle errors when the HTTP request fails or returns an error -> Option A

4. Quick Check:

   Error handling = To handle errors when the HTTP request fails or returns an error [OK]

Hint: Use try...catch to catch HTTP errors in Vue calls [OK]

Common Mistakes:

• Thinking try...catch speeds up requests
• Using try...catch to style data
• Assuming try...catch retries requests automatically

2. Which of the following is the correct syntax to check if an HTTP response was successful in Vue?

easy

A. if (response.status === 200) { ... }

B. if (response.success) { ... }

C. if (response.error) { ... }

D. if (response.ok) { ... }

Solution

1. Step 1: Identify the standard property for success

   The response.ok property is a boolean that is true if the HTTP status code is in the 200-299 range, indicating success.

2. Step 2: Compare options

   if (response.status === 200) { ... } checks for exactly 200, which is valid but less flexible. if (response.ok) { ... } uses response.ok, which is the recommended and clearer way. Options C and D are not valid properties.

3. Final Answer:

   if (response.ok) { ... } -> Option D

4. Quick Check:

   Check response.ok for success = if (response.ok) { ... } [OK]

Hint: Use response.ok to check HTTP success easily [OK]

Common Mistakes:

• Checking response.error which does not exist
• Checking response.success which is invalid
• Only checking for status 200, missing other success codes

3. Consider this Vue code snippet:
<pre>async function fetchData() { try { const response = await fetch('https://api.example.com/data'); if (!response.ok) { throw new Error('Network response was not ok'); } const data = await response.json(); return data; } catch (error) { return 'Fetch failed'; } } What will fetchData() return if the server responds with status 404?

medium

A. The JSON data from the response

B. 'Fetch failed'

C. undefined

D. An unhandled error

Solution

1. Step 1: Analyze response status handling

   The code checks response.ok. For status 404, response.ok is false, so it throws an error.

2. Step 2: Understand catch block behavior

   The thrown error is caught by the catch block, which returns the string 'Fetch failed'.

3. Final Answer:

   'Fetch failed' -> Option B

4. Quick Check:

   404 triggers catch, returns 'Fetch failed' = 'Fetch failed' [OK]

Hint: Non-ok response triggers catch block return [OK]

Common Mistakes:

• Assuming JSON data is returned on 404
• Expecting undefined instead of catch return
• Thinking error is unhandled and crashes

4. Identify the error in this Vue HTTP call code:

async function getUser() {
  try {
    const response = await fetch('/user');
    if (response.ok) {
      const user = response.json();
      return user;
    }
  } catch (e) {
    console.error(e);
  }
}

medium

A. Missing await before response.json() causing a Promise to be returned

B. Incorrect use of try...catch block

C. Using response.ok instead of response.status

D. No error, code is correct

Solution

1. Step 1: Check response.json() usage

   The response.json() method returns a Promise, so it must be awaited to get the actual data.

2. Step 2: Identify missing await

   The code returns response.json() without await, so it returns a Promise instead of user data.

3. Final Answer:

   Missing await before response.json() causing a Promise to be returned -> Option A

4. Quick Check:

   Always await response.json() to get data [OK]

Hint: Always await response.json() to get actual data [OK]

Common Mistakes:

• Forgetting to await response.json()
• Thinking response.ok is incorrect usage
• Assuming try...catch is wrongly used

5. You want to show a user-friendly error message in your Vue app when an HTTP call fails. Which approach best combines error handling and accessibility?

hard

A. Log errors to console only without user notification

B. Ignore errors and reload the page automatically

C. Use try...catch to catch errors, set an error message in state, and display it with aria-live="polite" for screen readers

D. Show a popup alert without any ARIA attributes

Solution

1. Step 1: Combine error handling with user feedback

   Using try...catch allows catching errors. Setting an error message in Vue state lets the UI update to inform the user.

2. Step 2: Ensure accessibility with ARIA

   Using aria-live="polite" on the error message container ensures screen readers announce the message politely, improving accessibility.

3. Final Answer:

   Use try...catch to catch errors, set an error message in state, and display it with aria-live="polite" for screen readers -> Option C

4. Quick Check:

   Error handling + aria-live = Use try...catch to catch errors, set an error message in state, and display it with aria-live="polite" for screen readers [OK]

Hint: Use aria-live with error messages for accessibility [OK]

Common Mistakes:

• Ignoring errors or not showing messages
• Using alerts without ARIA support
• Logging errors only without user feedback