What is the shape of error object inside axios request interceptor error handler?

Question

Technical note: As axios uses different libraries/mechanisms for Node and browser, this question touches only Node.js usage of [email protected].

I can set up the following interceptor for the axios library ( https://github.com/axios/axios#interceptors ):

// Add a request interceptor
axios.interceptors.request.use(function (config) {
    // Do something before request is sent
    return config;
  }, function (error) {
    // Do something with request error
    //
    // I am asking about this error handler and this error object
    //
    return Promise.reject(error);
  });

// Add a response interceptor
axios.interceptors.response.use(function (response) {
    // Do something with response data
    return response;
  }, function (error) {
    // Do something with response error
    return Promise.reject(error);
  });

When the callback described in error handler for request interceptor is triggered and and what is the shape of that error object?

P.S. I see that there is this code describing work with errors in axios:

axios.get('/user/12345')
  .catch(function (error) {
    if (error.response) {
      // The request was made and the server responded with a status code
      // that falls out of the range of 2xx
      console.log(error.response.data);
      console.log(error.response.status);
      console.log(error.response.headers);
    } else if (error.request) {
      //
      //
      //  !!!! This is a request error handler !!!
      //
      //
      // The request was made but no response was received
      // `error.request` is an instance of XMLHttpRequest in the browser and an instance of
      // http.ClientRequest in node.js
      console.log(error.request);
    } else {
      // Something happened in setting up the request that triggered an Error
      console.log('Error', error.message);
    }
    console.log(error.config);
  });

What will the error inside the request error handler represent in the latter code?

Answer 1

I think this source code might help you:

createError.js

It looks like an instance of Error, so it has error.message, and axios adds error.config, error.code, error.request and error.response, per enhanceError.js.

Answer 2

When the callback described in error handler for request interceptor is triggered and and what is the shape of that error object?

the error handler (.catch clause) will be triggered by the interceptor, when it "rejects" a promise like in this part of your code:

axios.interceptors.response.use(function (response) {
    // Do something with response data
    return response;
  }, function (error) {
    // Do something with response error
    return Promise.reject(error); // <---- HERE
  });

The shape of the axios error object is a JSON object as stated on the handling error section of axios docs on github

• message: the error message text.
• response: the response object (if received) as described in the previous section. Inside response you will have data, status, and headers objects
• request: the actual XMLHttpRequest object when running on browser or an instance of http.ClientRequest in node.js.
• config: the original request configuration.

What will the error inside the request error handler represent in the latter code?

It will be the error response from the request bypassed by your axios interceptor