Venezuela has accused the US of being behind a botched raid to oust President Maduro.

After writing my post a few months ago on building your own redux app, I have been asked a couple times to write a guide on creating redux middleware and how it works. This will be a quick post on how you can acheive anything with your own middleware!

##Basic middleware

const customMiddleware = store => next => action => {
  if(action.type !== 'custom') return next(action)
  //do stuff!
}

Applying it:

import { createStore, applyMiddleware, } from 'redux'
import reducer from './reducer'
import customMiddleware from './customMiddleware'

const store = createStore(
  reducer,
  applyMiddleware(customMiddleware)
)

Whaaa? store => next => action => I know that looks confusing. Essentially you are building a chain of functions, it will look like this when it gets called:

//next looks something like this:
let dispatched = null
let next = actionAttempt => dispatched = actionAttempt 

const dispatch = customMiddleware(store)(next)

dispatch({
  type: 'custom',
  value: 'test'
})

All you are doing is chaining function calls and passing in the neccesary data. When I first saw this I was confused a little due to the long chain, but it made perfect sense after reading the article on writing redux tests.

So now that we understand how those chained functions work, let’s explain the first line of our middleware.

if(action.type !== 'custom') return next(action)

There should be some way to tell what actions should go through your middleware. In this example, we are saying if the action’s type is not custom call next, which will pass it to any other middleware and then to the reducer.

##Doing Cool stuff

The official guide on redux middleware covers a few examples on this, I’m going to try to explain it in a more simple way.

Say we want an action like this:

dispatch({
  type: 'ajax',
  url: 'http://api.com',
  method: 'POST',
  body: state => ({
    title: state.title
    description: state.description
  }),
  cb: response => console.log('finished!', response)
})

We want this to do a post request, and then call the cb function. It would look something like this:

import fetch from 'isomorphic-fetch'

const ajaxMiddleware = store => next => action => {
  if(action.type !== 'ajax') return next(action)
  
  fetch(action.url, {
    method: action.method,
    body: JSON.stringify(action.body(store.getState()))
  })
  .then(response => response.json())
  .then(json => action.cb(json))
}

It’s pretty simple really. You have access to every method redux offers in middleware. What if we wanted the cb function to have access to dispatching more actions? We could change that last line of the fetch function to this:

.then(json => action.cb(json, store.dispatch))

Now in the callback, we can do:

  cb: (response, dispatch) => dispatch(newAction(response))

As you can see, middleware is very easy to write in redux. You can pass store state back to actions, and so much more. If you need any help or if I didn’t go into detail enough, feel free to leave a comment below!

One of the most common problems that I run into when using Redux is trying to figure out why an action is not being captured by a reducer. For someone just getting starting with Redux, debugging this issue can be especially overwhelming because of how Redux manages data flow. So before you start pouring over configuration code, or the logic contained in your action creators and reducers, please, make sure your action types are defined and spelled correctly.

One of the most common problems that I run into when using Redux is trying to figure out why an action is not being captured by a reducer. For someone just getting starting with Redux, debugging this issue can be especially overwhelming because of how Redux manages data flow. So before you start pouring over configuration code, or the logic contained in your action creators and reducers, please, make sure your action types are defined and spelled correctly.

In any application that I have built, most bugs that I have run into are simply due to typos. However, the solution to this particular problem is harder to spot because no errors are raised when the application is run. Take a look at the snippet below.

// actionTypes.js

export const FETCH_FILE_REQUEST = 'fetch_file_request';
export const FETCH_FILE_SUCCESS = 'fetch_file_success';
export const FETCH_FILE_FAIL = 'fetch_file_fail';


// filesReducer.js

import {
  FETCH_FILE_REQUEST,
  FETCH_FILE_SUCESS,
  FETCH_FILE_FAIL
} from '../actions/actionTypes';

const filesReducer = (state = {}, action) => {
  switch (action.type) {
    case FETCH_FILE_SUCESS:
      return { ...state, file: action.payload };
    default:
      return state;
  }
}

export default filesReducer;

Assuming we dispatched an action with type FETCH_FILE_SUCCESS, the filesReducer should catch the action before the default case is returned. But what if that is not happening? Where do we start the debugging process. There does not appear to be anything wrong with the code in the reducer; the action type was imported and matches the case in the switch statement. There are no errors in the browser. Where is the issue?

You may have noticed that I misspelled SUCCESS in filesReducer.js, but the reason this can be hard to catch is because importing undefined types does not cause an error, so when we import FETCH_FILE_SUCESS, its value is actually undefined, so our reducer always hits the default case.

It would be nice if the existing import/export system could help us catch this. Unfortunately, since action types are just strings, validating their existence is challenging. Luckily, we have another option.

Enter Proxies

Proxies are a feature of ES2015 that allow us to customize operations on a object. They can be used in many different ways, and you can find some useful examples here and here. For our problem, this example from Mozilla looks promising:

let validator = {
  set: function(obj, prop, value) {
    if (prop === 'age') {
      if (!Number.isInteger(value)) {
        throw new TypeError('The age is not an integer');
      }
      if (value > 200) {
        throw new RangeError('The age seems invalid');
      }
    }

    // The default behavior to store the value
    obj[prop] = value;

    // Indicate success
    return true;
  }
};

let person = new Proxy({}, validator);

person.age = 100;
console.log(person.age); // 100
person.age = 'young'; // Throws an exception
person.age = 300; // Throws an exception

So if proxies can be used to validate that properties assigned to an object are of a certain type and value, we should definitely be able to ensure that our action types are never undefined, or else throw an error that will be easy for us to fix. Let’s refactor our actionTypes.js file.

// actionTypes.js

const types = {
  FETCH_FILE_REQUEST: 'fetch_file_request',
  FETCH_FILE_SUCCESS: 'fetch_file_success',
  FETCH_FILE_FAIL: 'fetch_file_fail'
}

const typeValidator = {
  get(obj, prop) {
    if (obj[prop]) {
      return prop;
    } else {
      throw new TypeError(`${prop} is not a valid action type`);
    }
  }
}

module.exports = new Proxy(types, typeValidator);

First, we define a object containing all our action types. Then we define our validator handler typeValidator. The get method inside our handler is called a trap, and provides access to the properties of a object. If the property we are looking for, an action type, in this case, exists in the types object, return that prop, unmodified. Otherwise, throw an error because the prop does not exist.

Finally, export a new proxy, passing the types object as the target and the typeValidator as the handler. However, it is important to note that the ES2015 module system does not work well with proxies, so module.exports and require() must be used for exporting and importing the types.

Barely any code needs to change in the reducer and action creator files, but in order for the action types to be imported successfully, we just need couple lines of code in a new file:

// actionTypesProxy.js

export const {
  FETCH_FILE_REQUEST,
  FETCH_FILE_SUCCESS,
  FETCH_FILE_FAIL,
} = require('./actionTypes');

// in the reducer and action creator files
// change '../actions/actionTypes' to
// '../actions/actionTypesProxy'

By creating a proxy to verify the existence of an action type, we no longer have to worry about correctly naming a property upon import because an error will be thrown in the browser console as soon as the application starts. So, reduce the number headaches you get when developing an application using Redux and start using proxies.

Interested in learning how to build applications using Redux with ReactJS. Check out this online course! Modern React with Redux

Bruno Coelho speaks to ​FIFA.com ​after his side fell at the final hurdle, defeated by Argentina in the semi-finals of the FIFA Futsal World Cup Colombia 2016.

Ahead of the match for third place, Al Hilal skipper Carlos Eduardo spoke to FIFA.com about the reasons for his side’s defeat to Flamengo and the importance of their upcoming fixture against Monterrey.