tag is returned as JavaScript function to the render function
...
Component {
render()
{
return(

{'Welcome to React
world!'}

)
} }
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4
...
Elements can
11

contain other Elements in their props
...

Once an element is created, it is never mutated
...
createElement(
'div',
{id: 'login-btn'},
'Login'
)
The above React
...
render():

Login

Whereas a component can be declared in several different ways
...

In either case, it takes props as an input, and returns a JSX tree as the
output:
const Button = ({ onLogin }) =>

Login

Then JSX gets transpiled to a React
...
createElement(
'div',
{ id: 'login-btn', onClick: onLogin },
'Login'
)
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5
...

1
...
Those are pure JavaScript functions that accept props object
as the first parameter and return React elements:

12

“jsx harmony function Greeting({ message }) {

{Hello, ${message}‘}

return

} “‘
2
...
The above function component can be written as:
jsx harmony
render() {
} }

class Greeting extends React
...
props
...

When to use a Class Component over a Function Component?
If the component needs state or lifecycle methods then use class component otherwise use function component
...
8 with
the addition of Hooks, you could use state , lifecycle methods and other
features that were only available in class component right in your function
component
...

What are Pure Components?
React
...
Component except
that it handles the shouldComponentUpdate() method for you
...
Component on the other hand won’t compare current props and state to next out of the box
...

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8
...
We should always try to make
our state as simple as possible and minimize the number of stateful components
...
Component { constructor(props)
{ super(props)
this
...
state
...
e
...

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9
...
They are single values or objects containing a set of values that are passed to components on creation using a
naming convention similar to HTML-tag attributes
...


14

The primary purpose of props in React is to provide following component
functionality:
1
...

2
...

3
...
props
...

For example, let us create an element with reactProp property:
jsx harmony



This reactProp (or whatever you came up with) name then becomes a
property attached to React’s native props object which originally already
exists on all components created using React library
...
reactProp
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10
...
While both of them
hold information that influences the output of render, they are different
in their functionality with respect to component
...

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11
...

//Wrong
this
...
message = 'Hello world'
Instead use setState() method
...
When state changes, the component responds by rerendering
...
setState({ message: 'Hello World' })
Note: You can directly assign to the state object either in constructor or
using latest javascript’s class field declaration syntax
...

What is the purpose of callback function as an argument of
setState()?
The callback function is invoked when setState finished and the component
gets rendered
...

Note: It is recommended to use lifecycle method rather than this callback
function
...
log('The name has updated and component re-ren
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13
...
In HTML, the event name usually represents in lowercase as a convention:

Whereas in React it follows camelCase convention:
jsx harmony



2
...
log("The link was clicked
...
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
...
type === 'USER_LOGOUT') {
state = undefined
}
return appReducer(state, action)
}
In case of using redux-persist, you may also need to clean your storage
...
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
...
type === 'USER_LOGOUT') {
Object
...
forEach(key => {
storage
...

Whats the purpose of at symbol in the Redux connect decorator?
The **@** symbol is in fact a JavaScript expression used to signify decorators
...

Let’s take an example setting up Redux without and with a decorator
...
/actionCreators'
{ bindActionCreators } from 'redux'
{ connect } from 'react-redux'

function mapStateToProps(state) {
return { todos: state
...
Component {
//
...
/actionCreators'
{ bindActionCreators } from 'redux'
{ connect } from 'react-redux'

function mapStateToProps(state) {
return { todos: state
...
Component {
//
...
The
decorator syntax isn’t built into any JavaScript runtimes yet, and is still
experimental and subject to change
...

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165
...

Whereas Redux is much more powerful and provides a large number of
features that the Context API doesn’t provide
...

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166
...
Therefore, they act as a reducer of state
...
This state is then reduced (or accumulated) based on the action, and
then the next state is returned
...

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167
...


84

Let’s take an example of fetching specific account as an 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
...
json)) // Use a normal function to set the receive
} else {
dispatch(someError)
}
})
}
}
function setAccount(data) {
return { type: 'SET_Account', data: data }
}
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168
...

� Back to Top
169
...
This pattern is called Higher-Order Components, and is generally
the preferred way of extending a component’s functionality in React
...

Let’s take an example of component using connect:
import { connect } from 'react-redux'
import { setVisibilityFilter } from '
...
/components/Link'
const mapStateToProps = (state, ownProps) => ({
85

active: ownProps
...
visibilityFilter
})
const mapDispatchToProps = (dispatch, ownProps) => ({
onClick: () => dispatch(setVisibilityFilter(ownProps
...

class MyComponent {
someMethod() {
doSomethingWith(this
...
store)
}
}
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170
...

Container is an informal term for a component that is connected to a
Redux store
...

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171
...
It also prevents you from
introducing silly bugs caused by typos – in which case, you will get a
ReferenceError immediately
...
js or
actionTypes
...

export
export
export
export
export
export

const
const
const
const
const
const

ADD_TODO = 'ADD_TODO'
DELETE_TODO = 'DELETE_TODO'
EDIT_TODO = 'EDIT_TODO'
COMPLETE_TODO = 'COMPLETE_TODO'
COMPLETE_ALL = 'COMPLETE_ALL'
CLEAR_COMPLETED = 'CLEAR_COMPLETED'

In Redux, you use them in two places:
1
...
js:
import { ADD_TODO } from '
...
In reducers:
Let’s create reducer
...
/actionTypes'
export default (state = [], action) => {
switch (action
...
state,
{
text: action
...

What are the different ways to write mapDispatchToProps()?
There are a few ways of binding action creators to dispatch() in
mapDispatchToProps()
...

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173
...
So, if you
use a connected component:
“‘jsx harmony import ConnectedComponent from ‘
...

� Back to Top
174
...
Components: Used for dumb components unaware of Redux
...
Containers: Used for smart components connected to Redux
...
Actions: Used for all action creators, where file names correspond
to part of the app
...
Reducers: Used for all reducers, where files name correspond to
state key
...
Store: Used for store initialization
...

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88

175
...

It is available in NPM:
$ npm install --save redux-saga
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176
...
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
...

What are the differences between call() and put() in reduxsaga?
Both call() and put() are effect creator functions
...
put() function creates an effect, which instructs middleware to
dispatch an action to the store
...


function* fetchUserSaga(action) {
// `call` function accepts rest arguments, which will be passed to `api
...
fetchUser, action
...

yield put({
type: 'FETCH_USER_SUCCESS',
userData
})
}

89

� Back to Top
178
...
The thunk can be used to delay the
dispatch of an action, or to dispatch only if a certain condition is met
...

� Back to Top
179
...

In most of the scenarios, Thunk uses Promises to deal with them, whereas
Saga uses Generators
...
But both middleware can coexist, so you can start with
Thunks and introduce Sagas when/if you need them
...

What is Redux DevTools?
Redux DevTools is a live-editing time travel environment for Redux with
hot reloading, action replay, and customizable UI
...

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181
...
Lets you inspect every state and action payload
...
Lets you go back in time by cancelling actions
...
If you change the reducer code, each staged action will be reevaluated
...
If the reducers throw, you will see during which action this happened,
and what the error was
...
With persistState() store enhancer, you can persist debug sessions
across page reloads
...

What are Redux selectors and why to use them?
Selectors are functions that take Redux state as an argument and return
some data to pass to the component
...
user
...
The selector can compute derived data, allowing Redux to store the
minimal possible state
2
...

What is Redux Form?
Redux Form works with React and Redux to enable a form in React to
use Redux to store all of its state
...

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184
...
Field values persistence via Redux store
...
Validation (sync/async) and submission
...
Formatting, parsing and normalization of field values
...

How to add multiple middlewares to Redux?
You can use applyMiddleware()
...

How to set initial state in Redux?
You need to pass initial state as second argument to createStore:
const rootReducer = combineReducers({
todos: todos,
visibilityFilter: visibilityFilter
})
const initialState = {
todos: [{ id: 123, name: 'example', completed: false }]
}
const store = createStore(
rootReducer,
initialState
)
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187
...
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)
...

188
...
They are the only source of
information for the store
...

For example, let’s take an action which represents adding a new todo item:
92

{
type: ADD_TODO,
text: 'Add todo item'
}
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React Native
� Back to Top
188
...

React Native is a mobile framework that compiles to native app components, allowing you to build native mobile applications (iOS, Android,
and Windows) in JavaScript that allows you to use React to build your
components, and implements React under the hood
...

How to test React Native apps?
React Native can be tested only in mobile simulators like iOS and Android
...
io)
Where it syncs using QR code, your mobile and computer should be in
same wireless network
...

How to do logging in React Native?
You can use console
...
warn, etc
...
29
you can simply run the following to see logs in the console:
$ react-native log-ios
$ react-native log-android
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191
...
Run your application in the iOS simulator
...
Press Command + D and a webpage should open up at http://localhost:8081/debugger-ui
...
Enable Pause On Caught Exceptions for a better debugging experience
...
Press Command + Option + I to open the Chrome Developer tools,
or open it via View -> Developer -> Developer Tools
...
You should now be able to debug as you normally would
...

What is reselect and how it works?
Reselect is a selector library (for Redux) which uses memoization concept
...

Reselect keeps a copy of the last inputs/outputs of the last call, and recomputes the result only if one of the inputs changes
...

It’s memoization and cache are fully customizable
...

What is Flow?
Flow is a static type checker designed to find type errors in JavaScript
...
For example, Flow helps you catch errors involving
null, unlike most type systems
...

What is the difference between Flow and PropTypes?
Flow is a static analysis tool (static checker) which uses a superset of the
language, allowing you to add type annotations to all of your code and
catch an entire class of bugs at compile time
...
It can’t check anything other than the types of
the props being passed to a given component
...

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195
...
Install font-awesome:
$ npm install --save font-awesome
2
...
js file:
import 'font-awesome/css/font-awesome
...
css'
3
...

What is React Dev Tools?
React Developer Tools let you inspect the component hierarchy, including
component props and state
...

The official extensions available for different browsers or environments
...
Chrome extension
2
...
Standalone app (Safari, React Native, etc)
� Back to Top
197
...
) then you
must first open Chrome Extensions and check Allow access to file
URLs
...

How to use Polymer in React?
You need to follow below steps to use Polymer in React,
1
...
/bower_components/polymer/polymer
...
textContent = 'I am a calender'
} })
2
...
g
...
html of your React
application:

3
...
Component {
render() {
return (

)
}
}
export default MyComponent
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199
...
js?
React has the following advantages over Vue
...

2
...

4
...

Easier to test
...

More information and solutions available
...
But they are helpful as base parameters
...

What is the difference between React and Angular?
Let’s see the difference between React and Angular in a table format
...
e it has
two-way data binding between children
and parent and hence debugging is often
difficult

Note: The above list of diare the main benefits of React Router V4 module,
1
...

A router can be visualized as a single component() which wraps specific child router components()
...
You don’t need to manually set history
...

3
...

Can you describe about componentDidCatch lifecycle method
signature?
The componentDidCatch lifecycle method is invoked after an error has
been thrown by a descendant component
...
error: - The error object which was thrown
2
...

The method structure would be as follows
componentDidCatch(error, info)
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232
...
Inside Event handlers
2
...
During Server side rendering
4
...

111

Why do you not need error boundaries for event handlers?
Error boundaries do not catch errors inside event handlers
...
Unlike the render method and lifecycle methods, the event handlers
don’t happen during rendering
...

If you need to catch an error inside an event handler, use the regular
JavaScript try / catch statement:
class MyComponent extends React
...
state = { error: null };
this
...
handleClick
...
setState({ error });
}
}
render() {
if (this
...
error) {
return

Caught an error
...
handleClick}>Click Me

}
}
Note that the above example is demonstrating regular JavaScript behavior
and doesn’t use error boundaries
...

What is the difference between try catch block and error boundaries?
Try catch block works with imperative code whereas error boundaries are
meant for declarative code to render on the screen
...

}
Whereas error boundaries wrap declarative code as below,



So if an error occurs in a componentDidUpdate method caused by a
setState somewhere deep in the tree, it will still correctly propagate to
the closest error boundary
...

What is the behavior of uncaught errors in react 16?
In React 16, errors that were not caught by any error boundary will result
in unmounting of the whole React component tree
...
For example, it is worse for a payments app to
display a wrong amount than to render nothing
...

What is the proper placement for error boundaries?
The granularity of error boundaries usage is up to the developer based on
project needs
...
You can wrap top-level route components to display a generic error
message for the entire application
...
You can also wrap individual components in an error boundary to
protect them from crashing the rest of the application
...

What is the benefit of component stack trace from error boundary?
Apart from error messages and javascript stack, React16 will display the
component stack trace with file names and line numbers using error boundary concept
...

What is the required method to be defined for a class component?
The render() method is the only required method in a class component
...
e, All methods other than render method are optional for a class component
...

What are the possible return types of render method?
Below are the list of following types used and return from render method,
1
...
It includes html elements such as

and user defined
elements
...
Arrays and fragments: Return multiple elements to render as
Arrays and Fragments to wrap multiple elements
3
...

4
...
Booleans or null: Doesn’t render anything but these types are used
to conditionally render content
...

What is the main purpose of constructor?
The constructor is mainly used for two purposes,
1
...
state

114

2
...
setState() here!
this
...
handleClick = this
...
bind(this);
}
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241
...
i
...

� Back to Top
242
...
This is used for undefined props, but not
for null props
...
Component {
//
...
defaultProps = {
color: 'red'
};
If props
...

i
...
color will be set to red
}
Note: If you provide null value then it remains null value
...


115

Why should not call setState in componentWillUnmount?
You should not call setState() in componentWillUnmount() because
once a component instance is unmounted, it will never be mounted again
...

What is the purpose of getDerivedStateFromError?
This lifecycle method is invoked after an error has been thrown by a descendant component
...

The signature of the lifecycle method is as follows,
static getDerivedStateFromError(error)
Let us take error boundary use case with the above lifecycle method for
demonstration purpose,
class ErrorBoundary extends React
...
state = { hasError: false };
}
static getDerivedStateFromError(error) {
// Update state so the next render will show the fallback UI
...
state
...