Axios & Error handling like a boss 😎
Tags: typescript, vue, programming, webdev
Published: 2022-08-07 00:31:57

## Introduction

I really love the problem/solution. approach. We see some problem, and then, a really nice solution. But for this talking, i think we need some introduction as well.

When you develop an web application, you generally want's to separate the _frontend_ and _backend_. Fo that, you need something that makes the communication between these guys.

To illustrate, you can build a _frontend_ (commonly named as GUI or user interface) using vanilla HTML, CSS and Javascript, or, frequently, using several frameworks like **Vue**, React and so many more avaliable online. I marked Vue because it's my personal preference.

Why? I really don't study the others so deeply that i can't assure to you that Vue is the best, but i liked the way he works, the syntax, and so on. It's like your crush, it's a personal choice.

But, beside that, any framework you use, you will face the same problem:_ How to communicate with you backend_ (that can be written in so many languages, that i will not dare mention some. My current crush? Python an [Flask](https://flask.palletsprojects.com/)).

One solution is to use [AJAX](https://developer.mozilla.org/en-US/docs/Web/Guide/AJAX) (What is AJAX? Asynchronous JavaScript And XML). You can use [XMLHttpRequest](https://developer.mozilla.org/en-US/docs/Web/API/XMLHttpRequest) directly, to make requests to _backend_ and get the data you need, but the downside is that the code is verbose. You can use [Fetch API](https://developer.mozilla.org/en-US/docs/Web/API/Fetch_API) that will make an abstraction on top of `XMLHttpRequest`, with a powerfull set of tools. Other great change is that `Fetch API` will use [Promises](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Promise), avoiding the callbacks from `XMLHttpRequest` (preventing the [callback hell](http://callbackhell.com/)).

Alternatively, we have a awesome library named [Axios](https://axios-http.com/), that have a nice API (for curiosity purposes, under the hood, uses `XMLHttpRequest`, giving a very wide browser support). The Axios API wraps the `XMLHttpRequest` into `Promises`, different from `Fetch API`. Beside that, nowadays `Fetch API` is well supported by the browsers engines available, and have polyfills for older browsers. I will not discuss which one is better because i really think is personal preference, like any other library or framework around. If you dont't have an opinion, i suggest that you seek some comparisons and dive deep articles. Has a nice article that i will mention to you written by [Faraz Kelhini](https://blog.logrocket.com/axios-vs-fetch-best-http-requests/).

My personal choice is `Axios` because have a nice API, has Response timeout, automatic JSON transformation, and Interceptors (we will use them in the proposal solution), and so much more. Nothing that cannot be accomplished by `Fetch API`, but has another approach.

## The Problem

Talking about `Axios`, a simple GET HTTP request can be made with these lines of code:

```typescript
import axios from 'axios'

//here we have an generic interface with basic structure of a api response:
interface HttpResponse<T> {
  data: T[]
}

// the user interface, that represents a user in the system
interface User {
  id: number
  email: string
  name: string
}

//the http call to Axios
axios.get<HttpResponse<User>>('/users').then((response) => {
  const userList = response.data
  console.log(userList)
})
```

We've used Typescript (interfaces, and generics), ES6 Modules, Promises, Axios and Arrow Functions. We will not touch them deeply, and will presume that you already know about them.

So, in the above code, if everything goes well, aka: the server is online, the network is working perfectly, so on, when you run this code you will see the list of users on console. The real life isn't always perfect.

We, developers, have a mission:

> Make the life of users simple!

So, when something is go bad, we need to use all the efforts in ours hands to resolve the problem ourselves, without the user even notice, and, when nothing more can be done, we have the obligation to show them a really nice message explaining what goes wrong, to _easy theirs souls_.

`Axios` like `Fetch API` uses `Promises` to handle asynchronous calls and avoid the callbacks that we mention before. `Promises` are a really nice _API_ and not to difficult to understand. We can chain actions (`then`) and error handlers (`catch`) one after another, and the API will call them in order. If an Error occurs in the _Promise_, the nearest `catch` is found and executed.

So, the code above with basic error handler will become:

```typescript
import axios from 'axios'

//..here go the types, equal above sample.

//here we call axios and passes generic get with HttpResponse<User>.
axios
  .get<HttpResponse<User>>('/users')
  .then((response) => {
    const userList = response.data
    console.log(userList)
  })
  .catch((error) => {
    //try to fix the error or
    //notify the users about somenthing went wrong
    console.log(error.message)
  })
```

Ok, and what is the problem then? Well, we have a hundred errors that, in every API call, the solution/message is the same. For curiosity, Axios show us a little list of them: `ERR_FR_TOO_MANY_REDIRECTS, ERR_BAD_OPTION_VALUE, ERR_BAD_OPTION, ERR_NETWORK, ERR_DEPRECATED, ERR_BAD_RESPONSE, ERR_BAD_REQUEST, ERR_CANCELED, ECONNABORTED, ETIMEDOUT`. We have the [HTTP Status Codes](https://developer.mozilla.org/en-US/docs/Web/HTTP/Status), where we found so many errors, like `404` (Page Not Found), and so on. You get the picture. We have too much common errors to elegantly handle in every API request.

## The very ugly solution

One **very ugly** solution that we can think of, is to write one big ass function that we increment every new error we found. Besides the ugliness of this approach, it will work, if you and your team remember to call the function in every API request.

```typescript
function httpErrorHandler(error) {
  if (error === null) throw new Error('Unrecoverable error!! Error is null!')
  if (axios.isAxiosError(error)) {
    //here we have a type guard check, error inside this if will be treated as AxiosError
    const response = error?.response
    const request = error?.request
    const config = error?.config //here we have access the config used to make the api call (we can make a retry using this conf)

    if (error.code === 'ERR_NETWORK') {
      console.log('connection problems..')
    } else if (error.code === 'ERR_CANCELED') {
      console.log('connection canceled..')
    }
    if (response) {
      //The request was made and the server responded with a status code that falls out of the range of 2xx the http status code mentioned above
      const statusCode = response?.status
      if (statusCode === 404) {
        console.log('The requested resource does not exist or has been deleted')
      } else if (statusCode === 401) {
        console.log('Please login to access this resource')
        //redirect user to login
      }
    } else if (request) {
      //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
    }
  }
  //Something happened in setting up the request and triggered an Error
  console.log(error.message)
}
```

With our magical badass function in place, we can use it like that:

```javascript
import axios from 'axios'

axios
  .get('/users')
  .then((response) => {
    const userList = response.data
    console.log(userList)
  })
  .catch(httpErrorHandler)
```

We have to remember to add this `catch` in every API call, and, for every new error that we can graciously handle, we need to increase our nasty `httpErrorHandler` with some more code and ugly `if's`.

Other problem we have with this approach, besides ugliness and lack of mantenability, is that, if in one, only single one API call, i desire to handle different from global approach, i cannot do.

The function will grow exponentially as the problems that came together. This solution will not scale right!

## The elegant and recommended solution

When we work as a team, to make them remember the _slickness_ of every piece of software is hard, very hard. Team members, come and go, and i do not know any documentation good enough to surpass this issue.

In other hand, if the code itself can handle these problems on a generic way, do-it! The developers cannot make mistakes if they need do nothing!

Before we jump into code (that is what we expect from this article), i have the need to speak some stuff to you understand what the codes do.

Axios allow we to use something called `Interceptors` that will be executed in every request you make. It's a awesome way of checking permission, add some header that need to be present, like a token, and preprocess responses, reducing the amount of boilerplate code.

We have two types of `Interceptors`. _Before (request)_ and _After (response)_ an AJAX Call.

It's use is simple as that:

```javascript
//Intercept before request is made, usually used to add some header, like an auth
const axiosDefaults = {}
const http = axios.create(axiosDefaults)
//register interceptor like this
http.interceptors.request.use(
  function (config) {
    // Do something before request is sent
    const token = window.localStorage.getItem('token') //do not store token on localstorage!!!
    config.headers.Authorization = token
    return config
  },
  function (error) {
    // Do something with request error
    return Promise.reject(error)
  }
)
```

But, in this article, we will use the response interceptor, because is where we want to deal with errors. Nothing stops you to extend the solution to handle request errors as well.

An simple use of response interceptor, is to call ours big ugly function to handle all sort of errors.

As every form of automatic handler, we need a way to bypass this (disable), when we want. We are gonna extend the `AxiosRequestConfig` interface and add two optional options `raw` and `silent`. If `raw` is set to `true`, we are gonna do nothing. `silent` is there to mute notifications that we show when dealing with global errors.

```typescript
declare module 'axios' {
  export interface AxiosRequestConfig {
    raw?: boolean
    silent?: boolean
  }
}
```

Next step is to create a `Error` class that we will throw every time we want to inform the error handler to assume the problem.

```typescript
export class HttpError extends Error {
  constructor(message?: string) {
    super(message) // 'Error' breaks prototype chain here
    this.name = 'HttpError'
    Object.setPrototypeOf(this, new.target.prototype) // restore prototype chain
  }
}
```

Now, let's write the interceptors:

```typescript
// this interceptor is used to handle all success ajax request
// we use this to check if status code is 200 (success), if not, we throw an HttpError
// to our error handler take place.
function responseHandler(response: AxiosResponse<any>) {
  const config = response?.config
  if (config.raw) {
    return response
  }
  if (response.status == 200) {
    const data = response?.data
    if (!data) {
      throw new HttpError('API Error. No data!')
    }
    return data
  }
  throw new HttpError('API Error! Invalid status code!')
}

function responseErrorHandler(response) {
  const config = response?.config
  if (config.raw) {
    return response
  }
  // the code of this function was written in above section.
  return httpErrorHandler(response)
}

//Intercept after response, usually to deal with result data or handle ajax call errors
const axiosDefaults = {}
const http = axios.create(axiosDefaults)
//register interceptor like this
http.interceptors.response.use(responseHandler, responseErrorHandler)
```

Well, we do not need to remember our magical badass function in every ajax call we made. And, we can disable when we want, just passing `raw` to request config.

```javascript
import axios from 'axios'

// automagically handle error
axios
  .get('/users')
  .then((response) => {
    const userList = response.data
    console.log(userList)
  })
  //.catch(httpErrorHandler) this is not needed anymore

// to disable this automatic error handler, pass raw
axios
  .get('/users', {raw: true})
  .then((response) => {
    const userList = response.data
    console.log(userList)
  }).catch(() {
    console.log("Manually handle error")
  })
```

Ok, this is a nice solution, but, this bad-ass ugly function will grow so much, that we cannot see the end. The function will become so big, that anyone will want to maintain.

Can we improve more? Oh yeahhh.

## The IMPROVED and elegant solution

We are gonna develop an `Registry` class, using [Registry Design Pattern](https://java-design-patterns.com/patterns/registry/). The class will allow you to register error handling by an key (we will deep dive in this in a moment) and a action, that can be an string (message), an object (that can do some nasty things) or an function, that will be executed when the error matches the key. The registry will have parent that can be placed to allow you override keys to custom handle scenarios.

Here are some types that we will use througth the code:

```typescript
// this interface is the default response data from ours api
interface HttpData {
  code: string
  description?: string
  status: number
}

// this is all errrors allowed to receive
type THttpError = Error | AxiosError | null

// object that can be passed to our registy
interface ErrorHandlerObject {
  after?(error?: THttpError, options?: ErrorHandlerObject): void
  before?(error?: THttpError, options?: ErrorHandlerObject): void
  message?: string
  notify?: QNotifyOptions
}

//signature of error function that can be passed to ours registry
type ErrorHandlerFunction = (error?: THttpError) => ErrorHandlerObject | boolean | undefined

//type that our registry accepts
type ErrorHandler = ErrorHandlerFunction | ErrorHandlerObject | string

//interface for register many handlers once (object where key will be presented as search key for error handling
interface ErrorHandlerMany {
  [key: string]: ErrorHandler
}

// type guard to identify that is an ErrorHandlerObject
function isErrorHandlerObject(value: any): value is ErrorHandlerObject {
  if (typeof value === 'object') {
    return ['message', 'after', 'before', 'notify'].some((k) => k in value)
  }
  return false
}
```

So, with types done, let's see the class implementation. We are gonna use an Map to store object/keys and a parent, that we will seek if the key is not found in the current class. If parent is null, the search will end. On construction, we can pass an parent,and optionally, an instance of `ErrorHandlerMany`, to register some handlers.

```typescript
class ErrorHandlerRegistry {
  private handlers = new Map<string, ErrorHandler>()

  private parent: ErrorHandlerRegistry | null = null

  constructor(parent: ErrorHandlerRegistry = undefined, input?: ErrorHandlerMany) {
    if (typeof parent !== 'undefined') this.parent = parent
    if (typeof input !== 'undefined') this.registerMany(input)
  }

  // allow to register an handler
  register(key: string, handler: ErrorHandler) {
    this.handlers.set(key, handler)
    return this
  }

  // unregister a handler
  unregister(key: string) {
    this.handlers.delete(key)
    return this
  }

  // search a valid handler by key
  find(seek: string): ErrorHandler | undefined {
    const handler = this.handlers.get(seek)
    if (handler) return handler
    return this.parent?.find(seek)
  }

  // pass an object and register all keys/value pairs as handler.
  registerMany(input: ErrorHandlerMany) {
    for (const [key, value] of Object.entries(input)) {
      this.register(key, value)
    }
    return this
  }

  // handle error seeking for key
  handleError(
    this: ErrorHandlerRegistry,
    seek: (string | undefined)[] | string,
    error: THttpError
  ): boolean {
    if (Array.isArray(seek)) {
      return seek.some((key) => {
        if (key !== undefined) return this.handleError(String(key), error)
      })
    }
    const handler = this.find(String(seek))
    if (!handler) {
      return false
    } else if (typeof handler === 'string') {
      return this.handleErrorObject(error, { message: handler })
    } else if (typeof handler === 'function') {
      const result = handler(error)
      if (isErrorHandlerObject(result)) return this.handleErrorObject(error, result)
      return !!result
    } else if (isErrorHandlerObject(handler)) {
      return this.handleErrorObject(error, handler)
    }
    return false
  }

  // if the error is an ErrorHandlerObject, handle here
  handleErrorObject(error: THttpError, options: ErrorHandlerObject = {}) {
    options?.before?.(error, options)
    showToastError(options.message ?? 'Unknown Error!!', options, 'error')
    return true
  }

  // this is the function that will be registered in interceptor.
  resposeErrorHandler(this: ErrorHandlerRegistry, error: THttpError, direct?: boolean) {
    if (error === null) throw new Error('Unrecoverrable error!! Error is null!')
    if (axios.isAxiosError(error)) {
      const response = error?.response
      const config = error?.config
      const data = response?.data as HttpData
      if (!direct && config?.raw) throw error
      const seekers = [
        data?.code,
        error.code,
        error?.name,
        String(data?.status),
        String(response?.status),
      ]
      const result = this.handleError(seekers, error)
      if (!result) {
        if (data?.code && data?.description) {
          return this.handleErrorObject(error, {
            message: data?.description,
          })
        }
      }
    } else if (error instanceof Error) {
      return this.handleError(error.name, error)
    }
    //if nothings works, throw away
    throw error
  }
}
// create ours globalHandlers object
const globalHandlers = new ErrorHandlerRegistry()
```

Let's deep dive the `resposeErrorHandler` code. We choose to use `key` as an identifier to select the best handler for error. When you look at the code, you see that has an order that `key` will be searched in the registry. The rule is, search for the most specific to the most generic.

```typescript
const seekers = [
  data?.code, //Our api can send an error code to you personalize the error messsage.
  error.code, //The AxiosError has an error code too (ERR_BAD_REQUEST is one).
  error?.name, //Error has a name (class name). Example: HttpError, etc..
  String(data?.status), //Our api can send an status code as well.
  String(response?.status), //respose status code. Both based on Http Status codes.
]
```

This is an example of an error sent by API:

```json
{
  "code": "email_required",
  "description": "An e-mail is required",
  "error": true,
  "errors": [],
  "status": 400
}
```

Other example, as well:

```json
{
  "code": "no_input_data",
  "description": "You doesnt fill input fields!",
  "error": true,
  "errors": [],
  "status": 400
}
```

So, as an example, we can now register ours generic error handling:

```typescript
globalHandlers.registerMany({
  //this key is sent by api when login is required
  login_required: {
    message: 'Login required!',
    //the after function will be called when the message hides.
    after: () => console.log('redirect user to /login'),
  },
  no_input_data: 'You must fill form values here!',
  //this key is sent by api on login error.
  invalid_login: {
    message: 'Invalid credentials!',
  },
  '404': { message: 'API Page Not Found!' },
  ERR_FR_TOO_MANY_REDIRECTS: 'Too many redirects.',
})

// you can registre only one:
globalHandlers.register('HttpError', (error) => {
  //send email to developer that api return an 500 server internal console.error
  return { message: 'Internal server errror! We already notify developers!' }
  //when we return an valid ErrorHandlerObject, will be processed as whell.
  //this allow we to perform custom behavior like sending email and default one,
  //like showing an message to user.
})
```

We can register error handler in any place we like, group the most generic in one typescript file, and specific ones inline. You choose. But, to this work, we need to attach to ours `http` axios instance. This is done like this:

```typescript
function createHttpInstance() {
  const instance = axios.create({})
  const responseError = (error: any) => globalHandlers.resposeErrorHandler(error)
  instance.interceptors.response.use(responseHandler, responseError)
  return instance
}

export const http: AxiosInstance = createHttpInstance()
```

Now, we can make ajax requests, and the error handler will work as expected:

```javascript
import http from '/src/modules/http'

// automagically handle error
http.get('/path/that/dont/exist').then((response) => {
  const userList = response.data
  console.log(userList)
})
```

The code above will show a Notify ballon on the user screen, because will fire the `404` error status code, that we registered before.

### Customize for one http call

The solution doesn't end here. Let's assume that, in one, only one http request, you want to handle `404` differently, but just `404`. For that, we create the `dealsWith` function below:

```typescript
export function dealWith(solutions: ErrorHandlerMany, ignoreGlobal?: boolean) {
  let global
  if (ignoreGlobal === false) global = globalHandlers
  const localHandlers = new ErrorHandlerRegistry(global, solutions)
  return (error: any) => localHandlers.resposeErrorHandler(error, true)
}
```

This function uses the `ErrorHandlerRegistry` parent to personalize one key, but for all others, use the global handlers (if you wanted that, `ignoreGlobal` is there to force not).

So, we can write code like this:

```javascript
import http from '/src/modules/http'

// this call will show the message 'API Page Not Found!'
http.get('/path/that/dont/exist')

// this will show custom message: 'Custom 404 handler for this call only'
// the raw is necessary because we need to turn off the global handler.
http.get('/path/that/dont/exist', { raw: true }).catch(
  dealsWith({
    404: { message: 'Custom 404 handler for this call only' },
  })
)

// we can turn off global, and handle ourselves
// if is not the error we want, let the global error take place.
http
  .get('/path/that/dont/exist', { raw: true })
  .catch((e) => {
    //custom code handling
    if (e.name == 'CustomErrorClass') {
      console.log('go to somewhere')
    } else {
      throw e
    }
  })
  .catch(
    dealsWith({
      404: { message: 'Custom 404 handler for this call only' },
    })
  )
```

## The Final Thoughts

All this explanation is nice, but code, ah, the code, is so much better. So, i've created an github repository with all code from this article organized to you try out, improve and customize.

- [Click here to access the repo in github](https://github.com/mperon/error-handler-vite).



*FOOTNOTES*:
- This post became so much bigger than a first realize, but i love to share my thoughts.
- If you have some improvement to the code, please let me know in the comments.
- If you see something wrong, please, fix-me!