Axios Custom Instance Configuration Options

Creating a custom instance in Axios using axios.create() allows developers to define reusable, centralized HTTP configurations for specific APIs or microservices. This guide provides a detailed overview of the core configuration options available when instantiating a custom Axios client, covering base settings, request behavior, security, performance limits, and data transformation properties.

Core Connection Settings

baseURL: A string representing the base URL prepended to any relative URL passed to request methods.
```
baseURL: 'https://api.example.com/v1/'
```
timeout: The number of milliseconds before the request times out and aborts. If set to 0 (the default), no timeout is applied.
```
timeout: 5000
```
headers: An object containing default custom headers sent with every request made by the instance.
```
headers: { 'Authorization': 'Bearer token', 'Content-Type': 'application/json' }
```

Request and Query Parameters

params: Default URL query parameters to be appended to every request.
```
params: { api_key: '12345' }
```
paramsSerializer: A custom function or configuration object used to serialize params (e.g., handling array formats).
```
paramsSerializer: { indexes: false }
```

Authentication and Credentials

auth: Automatically sets an HTTP Basic Authorization header using provided credentials.
```
auth: { username: 'admin', password: 'secretpassword' }
```
withCredentials: A boolean indicating whether cross-site Access-Control requests should include credentials such as cookies, authorization headers, or TLS client certificates.
```
withCredentials: true
```

Data Transformations and Encodings

transformRequest: An array of functions allowing modifications to the request data before it is sent to the server.
transformResponse: An array of functions allowing modifications to the response data before it is passed to then or catch.
responseType: Defines the format of the data returned by the server. Options include 'json', 'text', 'blob', 'arraybuffer', 'document', and 'stream'.
responseEncoding: Sets the encoding used for decoding response data (e.g., 'utf8').

Request Control and Validation

validateStatus: A function determining whether to resolve or reject the returned promise based on the HTTP status code.
```
validateStatus: (status) => status >= 200 && status < 300
```
signal: An AbortSignal instance generated by AbortController to cancel active requests.
proxy: Defines the host, port, and optional credentials for an HTTP proxy server.
```
proxy: { host: '127.0.0.1', port: 9000 }
```
maxContentLength: The maximum allowed size (in bytes) of the HTTP response content.
maxBodyLength: The maximum allowed size (in bytes) of the HTTP request body.
maxRedirects: Defines the maximum number of redirects to follow in Node.js (defaults to 5).

Example Implementation

import axios from 'axios';

const apiClient = axios.create({
  baseURL: 'https://api.example.com/v1',
  timeout: 10000,
  headers: {
    'Content-Type': 'application/json',
    'Accept': 'application/json'
  },
  withCredentials: true,
  validateStatus: (status) => status >= 200 && status < 400
});