Axios interceptors in Vue - Step-by-Step Execution

Concept Flow - Axios interceptors

Request made by Vue component

↓

Request Interceptor runs

↓

Request sent to server

↓

Response received from server

↓

Response Interceptor runs

↓

Response delivered to Vue component

This flow shows how Axios interceptors catch requests before sending and responses after receiving, allowing modification or handling.

Execution Sample

Vue

import axios from 'axios';

axios.interceptors.request.use(config => {
  config.headers['X-Test'] = '123';
  return config;
});

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

This code adds headers before requests and modifies response data after receiving it.

Execution Table

Step	Action	Input/State	Output/State	Notes
1	Vue component calls axios.get('/api/data')	No headers	Request config with URL '/api/data'	Start request
2	Request interceptor runs	Request config	Request config with header 'X-Test: 123'	Header added
3	Request sent to server	Request config with header	Waiting for response	Request sent
4	Response received	Raw response {data: {value: 42}}	Raw response	Server replied
5	Response interceptor runs	Raw response	Response with data.modified = true	Response data modified
6	Vue component receives response	Modified response	Data with value 42 and modified true	Final data delivered
7	End	N/A	N/A	Request cycle complete

💡 Request and response cycle ends after interceptors modify and deliver data.

Variable Tracker

Variable	Start	After Step 2	After Step 5	Final
config.headers	{}	{"X-Test": "123"}	{"X-Test": "123"}	{"X-Test": "123"}
response.data	{value: 42}	{value: 42}	{value: 42, modified: true}	{value: 42, modified: true}

Key Moments - 2 Insights

Why does the request interceptor need to return the config object?

Because axios uses the returned config to send the request. If you don't return it (see Step 2 in execution_table), the request won't proceed.

What happens if the response interceptor does not return the response?

The Vue component will not receive the response data (see Step 5). Returning the response is necessary to continue the chain.

Visual Quiz - 3 Questions

Look at the execution table, what header is added by the request interceptor at Step 2?

A 'X-Test: 123'

B 'Authorization: Bearer token'

C 'Content-Type: application/json'

D No header added

💡 Hint

Check the Output/State column at Step 2 in the execution_table.

At which step does the response data get modified?

A Step 3

B Step 5

C Step 1

D Step 6

💡 Hint

Look for 'Response data modified' note in the execution_table.

If the request interceptor did not return the config, what would happen?

A Request would still send normally

B Response interceptor would run twice

C Request would be blocked and not sent

D Vue component would receive an error immediately

💡 Hint

Refer to the key_moments explanation about returning config in Step 2.

Concept Snapshot

Axios interceptors let you run code before requests and after responses.
Use axios.interceptors.request.use(fn) to modify requests.
Use axios.interceptors.response.use(fn) to modify responses.
Always return config or response to continue the chain.
Useful for adding headers, logging, or error handling.

Full Transcript

Axios interceptors in Vue let you catch requests before they go out and responses when they come back. The request interceptor can add headers or change the request settings. The response interceptor can change the data before your component sees it. Both interceptors must return their input to keep the process going. This way, you can add things like authentication tokens or modify data globally in one place.

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