Redux Interview Questions and Answers

Flux is an application design paradigm used as a replacement for the more traditional mvc pattern. It is not a framework or a library but a new kind of architecture that complements React and the concept of Unidirectional Data Flow. Facebook used this pattern internally when working with React The workflow between dispatcher, stores and views components with distinct inputs and outputs as follows:

Redux is a predictable state container for JavaScript apps based on the Flux design pattern. Redux can be used together with ReactJS, or with any other view library. It is tiny (about 2kB) and has no dependencies.

You need to keep your application state as small as possible. You don't have to push everything in there. Only do it makes a lot of sense to keep something there Or if it makes your life easier when using Dev Tools. But we shouldn't overload its importance too much.

Redux DevTools is a live-editing time travel environment for redux with hot reloading, action replay, and customizable UI. If you don’t want to bother with installing Redux DevTools and integrating it into your project, consider using Redux DevTools Extension for Chrome and Firefox.

Instead of saying downsides we can say that there are few compromises of using Redux over Flux. Those are as follows:

1. You will need to learn avoiding mutations: Flux is un-opinionated about mutating data, but Redux doesn't like mutations and many packages complementary to Redux assume you never mutate the state. You can enforce this with dev-only packages like redux-immutable-state-invariant, Immutable.js, or your team to write non-mutative code.
2. You're going to have to carefully pick your packages: While Flux explicitly doesn't try to solve problems such as undo/redo, persistence, or forms, Redux has extension points such as middleware and store enhancers, and it has spawned a young but rich ecosystem. This means most packages are new ideas and haven't received the critical mass of usage yet
3. There is no nice Flow integration yet: Flux currently lets you do very impressive static type checks which Redux doesn't support yet.

Yes.You just need to export the store from the module where it created with createStore . Also, it shouldn't pollute the global window object

store = createStore(myReducer);
export default store;

You need to follow two steps to use your store in your container

1. Use mapStateToProps(): It maps the state variables from your store to the props that you specify
2. Connect the above props to your container: The object returned by the mapStateToProps component is connected to the container. You can import connect from react-redux like

import React from 'react';
import { connect } from 'react-redux';
class App extends React.Component {

  render() {
    return <div>{this.props.containerData}</div>;
  }
}
function mapStateToProps(state) {
  return { containerData: state.appData };
}
export default connect(mapStateToProps)(App);
function mapStateToProps(state) {
  return { containerData: state.data };
}
export default connect(mapStateToProps)(App);

Constants allow you to easily find all usages of that specific functionality across the project when you use an IDE. It also prevents you from introducing silly bugs caused by typos -- in which case, you will get a ReferenceError immediately. Normally we will save them in a single file (constants.js or actionTypes.js) For example:

export const ADD_TODO = 'ADD_TODO';
export const DELETE_TODO = 'DELETE_TODO';
export const EDIT_TODO = 'EDIT_TODO';
export const COMPLETE_TODO = 'COMPLETE_TODO';
export const COMPLETE_ALL = 'COMPLETE_ALL';
export const CLEAR_COMPLETED = 'CLEAR_COMPLETED';

In redux you use them in two places

1. During actions creation Let's take actions.js

import { ADD_TODO } from './actionTypes';
export function addTodo(text) {
  return { type: ADD_TODO, text };
}
2. Reducers Let's create reducer.js
import { ADD_TODO } from './actionTypes';
export default (state = [], action) => {
  switch (action.type) {
    case ADD_TODO:
    return [
    ...state,
    {
      text: action.text,
      completed: false
    }
];

default:

return state

}
};

Below are the major features of redux form:

1. Field values persistence via Redux store

2. Validation (sync/async) and submission

3. Formatting, parsing and normalization of field values

The store is a JavaScript object that holds application state. Along with this it also has the following responsibilities:

Allows access to state via getState() ;
Allows state to be updated via dispatch(action) ;
Registers listeners via subscribe(listener) ;
Handles unregistering of listeners via the function returned by subscribe(listener) .

Both Redux Thunk and Redux Saga take care of dealing with side effects. In most of the scenarios, Thunk allows Promises to deal with them, whereas Saga uses Generators. Thunk is simple to use and Promises are familiar to many developers, Saga/Generators are more powerful but you will need to learn them. But both the two middleware can coexists, so you can start with Thunks and introduce Sagas when/if you need them.

Redux Form works with React and Redux to enable a form in React to use Redux to store all of its state. Redux Form can be used with raw HTML5 inputs, but it also works with very well with common UI frameworks like Material UI, React Widgets and React Bootstrap.

These libraries are very different for very different purposes, but there are some vague similarities. Redux is a tool for managing state throughout the application. It is usually used as an architecture for UIs. Think of it as an alternative to (half of) Angular. RxJS is a reactive programming library. It is usually used as a tool to accomplish asynchronous tasks in JavaScript. Think of it as an alternative to Promises.

Redux uses the Reactive paradigm little bit because the Store is reactive. The Store observes actions from a distance, and changes itself. RxJS also uses the Reactive paradigm, but instead of being an architecture, it gives you basic building blocks, Observables, to accomplish this "observing from a distance" pattern.

Redux Thunk, Redux Promise, Redux Saga

redux-saga is a library that aims to make side effects (i.e. asynchronous things like data fetching and impure things like accessing the browser cache) in React/Redux applications easier and better. It is available in NPM as npm install --save redux-saga

The reducer is a pure function that takes the previous state and an action, and returns the next state.

(previousState, action) => newState
It's called a reducer because it's the type of function you would pass to Array.prototype.reduce(reducer, ?
initialValue) . It's very important that the reducer stays

pure. Things you should never do inside a reducer:

Mutate its arguments;
Perform side effects like API calls and routing transitions;
Call non-pure functions, e.g. Date.now() or Math.random() .

You need to pass initial state as second argument to createStore

const rootReducer = combineReducers({
  todos: todos,
  visibilityFilter: visibilityFilter
});
const initialState = {
  todos: [{id:123, name:'sudheer', completed: false}]
};
const store = createStore(
rootReducer,
initialState
);

You can use applyMiddleware where you can pass each piece of middleware as a new argument. So you just need to pass each piece of middleware you'd like. For example, you can add Redux Thunk and logger middlewares as an argument as below,

import { createStore, applyMiddleware } from 'redux'
const createStoreWithMiddleware = applyMiddleware(ReduxThunk, logger)(createStore);

Selectors are functions that take Redux state as an argument and return some data to pass to the component. For example, to get user details from the state:

const getUserData = state => state.user.data;

Below are the major features of Redux devTools

1. Lets you inspect every state and action payload
2. Lets you go back in time by “cancelling” actions
3. If you change the reducer code, each “staged” action will be re-evaluated
4. If the reducers throw, you will see during which action this happened, and what the error was
5. With persistState() store enhancer, you can persist debug sessions across page reloads

Redux Thunk middleware allows you to write action creators that return a function instead of an action. The thunk can be used to delay the dispatch of an action, or to dispatch only if a certain condition is met. The inner function receives the store methods dispatch and getState() as parameters.

Most of the applications has several top-level directories as below

1. Components Used for “dumb” React components unaware of Redux
2. Containers Used for “smart” React components connected to Redux
3. Actions Used for all action creators, where file name corresponds to part of the app
4. Reducers Used for all reducers, where file name corresponds to state key
5. Store Used for store initialization This structure works well for small and mid-level size apps.

Component is part of the React API. A Component is a class or function that describes part of a React UI. Container is an informal term for a React component that is connected to a redux store. Containers receive Redux state updates and dispatch actions, and they usually don't render DOM elements; they delegate rendering to presentational child components.

You can use Context in your application directly and is going to be great for passing down data to deeply nested components which what it was designed for. Whereas Redux is much more powerful and provides a large number of features that the Context Api doesn't provide. Also, React Redux uses context internally but it doesn’t expose this fact in the public API. So you should feel much safer using context via React Redux than directly because if it changes, the burden of updating the code will be on React Redux instead developer responsibility.

Redux follows three fundamental principles:

1. Single source of truth: The state of your whole application is stored in an object tree within a single store. The single state tree makes it easier to keep track of changes over time and debug or inspect the application.
2. State is ready only: The only way to change the state is to emit an action, an object describing what happened. This ensures that neither the views nor the network callbacks will ever write directly to the state.
3. Changes are made with pure functions: To specify how the state tree is transformed by actions, you write pure reducers(Reducers are just pure functions that take the previous state and an action, and return the next state).

A reducer is simply a pure function that takes the previous state and an action, and returns the next state.

You need to write a root reducer in your application which delegate handling the action to the reducer generated by combineReducers() . For example, let us take rootReducer to return the initial state after USER_LOGOUT action. As we know, reducers are supposed to return the initial state when they are called with undefined as the first argument, no matter the action.

const appReducer = combineReducers({
  /* your app’s top-level reducers */
})
const rootReducer = (state, action) => {
  if (action.type === 'USER_LOGOUT') {
    state = undefined
  }
return appReducer(state, action)
}

In case of using redux-persist, you may also need to clean your storage. Redux-perist keeps a copy of your state in a storage engine. First, you need to import the appropriate storage engine and then, to parse the state before setting it to undefined and clean each storage state key,

const appReducer = combineReducers({
  /* your app’s top-level reducers */

})
const rootReducer = (state, action) => {
  if (action.type === 'USER_LOGOUT') {
    Object.keys(state).forEach(key => {
      storage.removeItem(`persist:${key}`);
    });
  state = undefined;
}
return appReducer(state, action)
}

The @ symbol is in fact a JavaScript expression used to signify decorators. Decorators make it possible to annotate and modify classes and properties at design time. Let's take an example setting up Redux without and with a decorator Without decorator

import React from 'react';
import * as actionCreators from './actionCreators';
import { bindActionCreators } from 'redux';
import { connect } from 'react-redux';
function mapStateToProps(state) {
  return { todos: state.todos };
}
function mapDispatchToProps(dispatch) {
  return { actions: bindActionCreators(actionCreators, dispatch) };
}
class MyApp extends React.Component {
  // ...define your main app here
}
export default connect(mapStateToProps, mapDispatchToProps)(MyApp);

With decorator

import React from 'react';
import * as actionCreators from './actionCreators';
import { bindActionCreators } from 'redux';
import { connect } from 'react-redux';
function mapStateToProps(state) {
  return { todos: state.todos };
}
function mapDispatchToProps(dispatch) {
  return { actions: bindActionCreators(actionCreators, dispatch) };
}
@connect(mapStateToProps, mapDispatchToProps)
export default class MyApp extends React.Component {

  // ...define your main app here
}

The above examples are almost similar except the usage of decorator. The decorator syntax isn't built into any Javascript runtimes yet, and is still experimental and subject to change. You can use babel for the decorators support.

Reducers always return the accumulation of the state (based on all previous and current actions) not only default values. Therefore, they act as a reducer of state. Each time a redux reducer is called, the state is passed in with the action (state, action). This state is then reduced (or accumulated) based on the action, and then the next state is returned. i.e, You could "reduce" a collection of actions and an initial state (of the store) on which to perform these actions to get the resulting final state.

You can use redux-thunk middleware which allows you to define async actions. Let's take an example of fetching specific account as a ajax call using fetch API,

export function fetchAccount(id) {
  return dispatch => {
    dispatch(setLoadingAccountState());
    // Show a loading spinner
    fetch(`/account/${id}`, (response) => {
      dispatch(doneFetchingAccount());
      // Hide loading spinner
      if (response.status == 200){
        dispatch(setAccount(response.json));
        // Use a normal function to set the received state
      } else {
        dispatch(someError)
      }
  })
}
}
function setAccount(data) {
  return { type: 'SET_Account', data: data };
}

The best way to access your store through a component is using the connect() function. Actually creates a new component that wraps around your existing one! This pattern is called Higher-Order Components, and is generally the preferred way of extending a component's functionality in React. This allows you to map state and

action creators to your component, and have them passed in automatically as your store updates. Let's take an example of FilterLink component using connect,

import { connect } from 'react-redux'
import { setVisibilityFilter } from '../actions'
import Link from '../components/Link'
const mapStateToProps = (state, ownProps) => {
  return {
    active: ownProps.filter === state.visibilityFilter
  }
}
const mapDispatchToProps = (dispatch, ownProps) => {
  return {
    onClick: () => {
      dispatch(setVisibilityFilter(ownProps.filter))
    }
}
}
const FilterLink = connect(
mapStateToProps,
mapDispatchToProps
)(Link)
export default FilterLink

Due to it having quite a few performance optimizations and generally being less likely to cause bugs, the Redux devs almost always recommend using connect over accessing the store directly (using context API).

class MyComponent {
  someMethod() {
    doSomethingWith(this.context.store);
  }
}

Both call and put are effects creators functions. call function is used to create effect description, which instructs middleware to call the promise. put function creates effect, in which instructs middleware to dispatch an action to the store. Let's take example of how these effects work for fetching particular user data

function* fetchUserSaga(action) {
  // `call` function accepts rest arguments, which will be passed to `api.fetchUser` function.
  // Instructing middleware to call promise, it resolved value will be assigned to `userData` variable
  const userData = yield call(api.fetchUser, action.userId);
  // Instructing middleware to dispatch corresponding action.
  yield put({
    type: 'FETCH_USER_SUCCESS',
    userData
  });
}

Redux thunk is middleware that allows you to write action creators that return a function instead of an action. The thunk can then be used to delay the dispatch of an action if a certain condition is met. This allows you to handle the asyncronous dispatching of actions.

The mental model is that a saga is like a separate thread in your application that's solely responsible for side effects. redux-saga is a redux middleware, which means this thread can be started, paused and cancelled from the main application with normal redux actions, it has access to the full redux application state and it can dispatch redux actions as well.

Relay is similar to redux in that they both use a single store. The main difference is that relay only manages state originated from the server, and all access to the state is used via GraphQL queries (for reading data) and mutations (for changing data). Relay caches the data for you and optimizes data fetching for you, by fetching only changed data and nothing more.