Axios interceptors in Vue

Introduction

Axios interceptors let you catch and change requests or responses before they reach your app. This helps you add common tasks like adding tokens or handling errors in one place.

You want to add an authorization token to every request automatically.

You need to show a loading spinner while waiting for a response.

You want to catch errors globally and show friendly messages.

You want to log all requests and responses for debugging.

You want to modify request data or response data before using it.

Syntax

Vue

axios.interceptors.request.use(function (config) {
  // modify config before request is sent
  return config;
}, function (error) {
  // handle request error
  return Promise.reject(error);
});

axios.interceptors.response.use(function (response) {
  // modify response data
  return response;
}, function (error) {
  // handle response error
  return Promise.reject(error);
});

The use method takes two functions: one for success and one for error.

Always return the config or response in the success function to continue the flow.

Examples

Adds an authorization header to every request.

Vue

axios.interceptors.request.use(config => {
  config.headers.Authorization = 'Bearer mytoken';
  return config;
});

Returns only the data part of the response to simplify usage.

Vue

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

Shows an alert when any response error happens.

Vue

axios.interceptors.response.use(null, error => {
  alert('Request failed!');
  return Promise.reject(error);
});

Sample Program

This Vue component uses Axios interceptors to add an authorization header to every request and to catch errors globally. When you click the button, it fetches data and shows loading, data, or error messages.

Vue

<template>
  <div>
    <button @click="fetchData">Fetch Data</button>
    <p v-if="loading">Loading...</p>
    <pre v-if="data">{{ data }}</pre>
    <p v-if="error" style="color: red">{{ error }}</p>
  </div>
</template>

<script setup>
import { ref } from 'vue';
import axios from 'axios';

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

// Add request interceptor to add a fake token
axios.interceptors.request.use(config => {
  config.headers.Authorization = 'Bearer fake-token-123';
  return config;
});

// Add response interceptor to handle errors
axios.interceptors.response.use(
  response => response,
  err => {
    error.value = 'Failed to fetch data';
    loading.value = false;
    return Promise.reject(err);
  }
);

async function fetchData() {
  loading.value = true;
  error.value = null;
  data.value = null;
  try {
    const response = await axios.get('https://jsonplaceholder.typicode.com/posts/1');
    data.value = JSON.stringify(response.data, null, 2);
  } catch {
    // error handled by interceptor
  } finally {
    loading.value = false;
  }
}
</script>

Important Notes

Interceptors run for every request or response unless removed.

You can eject interceptors if you want to stop them later.

Be careful to handle errors in interceptors to avoid unhandled promise rejections.

Summary

Axios interceptors let you change requests or responses before your app sees them.

Use them to add headers, handle errors, or modify data globally.

They help keep your code clean by centralizing common tasks.

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