Axios interceptors in Vue - Mini Project: Build & Apply

Using Axios Interceptors in Vue

📖 Scenario: You are building a Vue app that fetches user data from an API. You want to add a way to automatically add an authorization token to every request and handle errors globally.

🎯 Goal: Build a Vue component that uses Axios interceptors to add an authorization header to all requests and logs errors from responses.

📋 What You'll Learn

Create an Axios instance

Add a request interceptor to include an authorization token

Add a response interceptor to catch errors

Use the Axios instance in a Vue component to fetch data

💡 Why This Matters

🌍 Real World

Many web apps need to communicate with APIs securely and handle errors consistently. Axios interceptors help automate adding tokens and error handling.

💼 Career

Understanding Axios interceptors is essential for frontend developers working with Vue or other frameworks to build robust API integrations.

1

Create Axios instance

Create an Axios instance called apiClient using axios.create() with the base URL set to 'https://api.example.com'.

Vue

# Create an Axios instance called apiClient with baseURL 'https://api.example.com'
# Your code here

Hint

Use axios.create() and pass an object with baseURL.

2

Add request interceptor

Add a request interceptor to apiClient using apiClient.interceptors.request.use. The interceptor should add an Authorization header with the value 'Bearer mytoken123' to the config headers.

Vue

import axios from 'axios'

const apiClient = axios.create({
  baseURL: 'https://api.example.com'
})

# Add a request interceptor to apiClient to add Authorization header
# Your code here

Hint

The request interceptor receives config. Add the header and return config.

3

Add response interceptor

Add a response interceptor to apiClient using apiClient.interceptors.response.use. The interceptor should return the response if successful, and if there is an error, log 'API error:' followed by the error to the console, then rethrow the error.

Vue

import axios from 'axios'

const apiClient = axios.create({
  baseURL: 'https://api.example.com'
})

apiClient.interceptors.request.use(config => {
  config.headers.Authorization = 'Bearer mytoken123'
  return config
})

# Add a response interceptor to apiClient to log errors
# Your code here

Hint

The response interceptor takes two functions: one for success, one for error. Log the error and rethrow it.

4

Use Axios instance in Vue component

Create a Vue component named UserList using <script setup>. Import ref and onMounted from 'vue' and apiClient. Create a users ref initialized to an empty array. Use onMounted to fetch /users from apiClient and assign the response data to users.value. Add a template that lists user names in an unordered list.

Vue

// Import ref and onMounted from 'vue' and apiClient
// Create users ref and fetch users on mounted
// Your code here



  
    
  

Hint

Use ref for reactive data and onMounted to fetch data once the component loads. Use v-for to list users.

Practice

1. What is the main purpose of Axios interceptors in a Vue application?

easy

A. To modify requests or responses globally before they reach your components

B. To create new Axios instances for each API call

C. To replace Vue lifecycle hooks with Axios methods

D. To automatically refresh the Vue component state

Solution

1. Step 1: Understand Axios interceptors role

   Axios interceptors allow you to intercept and modify requests or responses globally before they reach your Vue components.

2. Step 2: Compare options

   Options B, C, and D describe unrelated or incorrect uses of Axios interceptors.

3. Final Answer:

   To modify requests or responses globally before they reach your components -> Option A

4. Quick Check:

   Interceptor purpose = Modify requests/responses globally [OK]

Hint: Interceptors act like global filters for requests and responses [OK]

Common Mistakes:

• Thinking interceptors create new Axios instances
• Confusing interceptors with Vue lifecycle hooks
• Assuming interceptors update component state automatically

2. Which of the following is the correct syntax to add a request interceptor using Axios in Vue?

easy

A. axios.use.interceptors.request(config => { return config })

B. axios.request.interceptors.use(config => { return config })

C. axios.interceptors.request.use(config => { return config })

D. axios.interceptors.use.request(config => { return config })

Solution

1. Step 1: Recall Axios interceptor syntax

   The correct syntax to add a request interceptor is axios.interceptors.request.use(callback).

2. Step 2: Identify correct option

   Only axios.interceptors.request.use(config => { return config }) matches the correct syntax; others have incorrect property order or method calls.

3. Final Answer:

   axios.interceptors.request.use(config => { return config }) -> Option C

4. Quick Check:

   Correct syntax = axios.interceptors.request.use [OK]

Hint: Remember: interceptors.request.use is the correct chain [OK]

Common Mistakes:

• Swapping 'request' and 'interceptors' order
• Using 'use' before 'interceptors'
• Missing the return of config in the callback

3. Given this Axios response interceptor code in Vue:

axios.interceptors.response.use(response => {
  response.data.modified = true;
  return response;
});

const result = await axios.get('/api/data');
console.log(result.data.modified);

What will be logged to the console?

medium

A. undefined

B. true

C. false

D. An error will occur

Solution

1. Step 1: Analyze the response interceptor

   The interceptor adds a property 'modified' with value true to response.data before returning it.

2. Step 2: Understand the logged value

   After the GET request, result.data.modified will be true because the interceptor modified it.

3. Final Answer:

   true -> Option B

4. Quick Check:

   Interceptor modifies data = logged true [OK]

Hint: Interceptors can add properties to response data before use [OK]

Common Mistakes:

• Assuming interceptor does not modify response
• Expecting undefined because property was added dynamically
• Thinking an error occurs due to property addition

4. Identify the error in this Vue Axios interceptor code:

axios.interceptors.request.use(config => {
  config.headers['Authorization'] = 'Bearer token';
});

medium

A. Missing return of config in the interceptor callback

B. Headers should be set on response, not config

C. Authorization header value must be a function

D. Interceptor should be added on axios.request, not axios.interceptors

Solution

1. Step 1: Check interceptor callback requirements

   Axios interceptors must return the config object after modifying it to continue the request chain.

2. Step 2: Identify missing return

   The code modifies headers but does not return config, causing the request to hang or fail.

3. Final Answer:

   Missing return of config in the interceptor callback -> Option A

4. Quick Check:

   Always return config in request interceptor [OK]

Hint: Always return config or response in interceptors [OK]

Common Mistakes:

• Forgetting to return config or response
• Setting headers on response instead of request config
• Misplacing interceptor on wrong Axios property

5. You want to globally handle 401 Unauthorized errors in your Vue app using Axios interceptors. Which approach correctly implements this behavior?

hard

A. Use a response interceptor to retry the request infinitely on 401 errors

B. Use a request interceptor to modify the request URL on 401 errors

C. Use a request interceptor to add a 401 status header before sending

D. Use a response interceptor to check error response status and redirect to login

Solution

1. Step 1: Understand 401 error handling

   401 Unauthorized errors come from server responses, so handling must be in a response interceptor.

2. Step 2: Choose correct handling method

   Use a response interceptor to check error response status and redirect to login correctly uses a response interceptor to detect 401 status and redirect user to login page.

3. Step 3: Eliminate incorrect options

   Retrying infinitely causes loops; using a request interceptor cannot handle response errors like 401; adding a 401 status header in the request is invalid.

4. Final Answer:

   Use a response interceptor to check error response status and redirect to login -> Option D

5. Quick Check:

   401 errors handled in response interceptor [OK]

Hint: Handle server errors in response interceptors, not request ones [OK]

Common Mistakes:

• Trying to handle response errors in request interceptors
• Retrying requests infinitely on 401 errors
• Adding fake status headers in requests