Redux Interview Questions and Answers

Redux is a javascript library for managing application state.

It is most commonly used with libraries such as React.js or Angular, etc.

Suppose, you need to pass data (state and props) in between such components that don't have any relationship, so while making such communication between two components if it difficult to pass data (state and props) and maintaining them is very difficult task. In such case Redux is very useful because Redux eliminates the need to continuously pass state from one component to another.

When using Redux with React, states will no longer need to be lifted up. Everything is handled by Redux. Redux simplifies the app and makes it easier to maintain.

1. Redux offers a solution of storing all your application state at one place, called store.
2. Components then dispatch state changes to the store, not directly to other components.
3. The components that need to be aware of state changes can subscribe to the store.
4. The store can be thought of as a "middleman" for all state changes in the application.
5. With Redux involved, components don't communicate directly with each other, but rather all state changes must go through the single source of truth, the store.

Redux provides a way to centralize the state of your application

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Following are the benefits of using Redux:

1. Single source of truth
2. Redux makes the state predictable.
3. Easy to Maintain
4. No need to uplift state in react
5. Easy to Debug

Redux is available as a package on NPM.

npm install redux --save

Redux can be described in three fundamental principles

1. Single source of truth
2. State is read-only
3. Changes are made with pure functions

The state of your whole application is stored in an object tree within a single store.

The only way to change the state is to dispatch an action, an object describing what happened.

To specify how the state tree is transformed by actions, you write pure reducers.

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Redux allows you to manage the state of the application using Store, A child component can directly access the state from the Store.

Following are deatils how redux works:

1. When UI Event triggers (OnClick, OnChange, etc) it can Dispatch Actions based on the Event.
2. Reducers process the Actions, and return new state as an Object.
3. The new state of the whole application goes into a single Store.
4. Components can easily Subscribe to Store.

Following are the main piller of Redux

1. Store
2. Action
3. Reducer
4. Dispatch
5. Subscribe
6. Provider
7. Connect

A Store is an Object which holds the whole state tree of your application.

import React from 'react'
import { render } from 'react-dom'
import { Provider } from 'react-redux'
import { createStore } from 'redux'
import rootReducer from './reducers'
import App from './components/App'

const store = createStore(rootReducer)

render(
  
    
  ,
  document.getElementById('root')
)

Actions are plain JavaScript objects and they must have a type property to indicate the type of action to be carried out. They must also have a payload that contains the information that should be worked on by the action.

Actions are sent using store.dispatch() method.

action: { 
  type: [TYPE], 
  payload: [DATA] 
}

Reducer are pure fucntions which take the previous state of the app and return a new state based on the action passed to it.

function(state, action) => newState

Always remember that we should never mutate state inside the reducer, always return a new copy of state.

You can achieve this using

1. Object.assign()
2. Spread (…) operator

Dispatch is a method which triggers action with type and payload to Reducer.

store.dispatch()

Subscribe is a method which is used to Subscribe data/state from Store.

store.subscribe()

The Provider is a component that has a reference to the Store and provides the data from the Store to the component it wraps.

Connect is a function communicates with the Provider.

A pure function is a function which

1. always return the same output for the same input
2. has no side effects, means it doesn't change any external state.

A function is only pure if, given the same input, it will always produce the same output.

Example

const add = (a, b) => a + b // A pure function

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Flux is a Design Pattern and Redux is a library

REDUX	FLUX
Single Store	Multiple Store
Redux is a container for JavaScript apps.	Flux is a container for application state and logic that are registered to callbacks.
It offers interesting features such as writing applications, testing in different environments such as a server, client, etc.	It is an observer pattern that is modified to fit React.
In redux, actions can be functions or promises.	It is a fancy name given to observer pattern and Facebook has developed tools to aid the implementation of these patterns.
Redux is the first choice for web developers because it offers live code editing.	Flux supports actions that are simple JavaScript objects.

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No, Redux can be used as a data store. it is most commonly used with React.js and React Native.

Redux can also be used with Angular, Vue.js etc