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
...
preventDefault()
console
...
')
}
3
...
(refer
“activateLasers” function in the first point for example)
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14
...
Binding in Constructor: In JavaScript classes, the methods are
not bound by default
...
Normally we bind them in constructor
...
handleClick = this
...
bind(this);
}
handleClick() {
console
...
handleClick}>Click Me;
}
}
2
...

jsx harmony
is:', this)

handleClick = () => {
}

console
...
handleClick}>
me'}

{'Click

3
...

jsx harmony handleClick() {
console
...
handleClick()}>Click Me; }
Note: If the callback is passed as prop to child components, those components might do an extra re-rendering
...
bind() or public class fields syntax approach considering performance
...

How to pass a parameter to an event handler or callback?
You can use an arrow function to wrap around an event handler and pass
parameters:

17

jsx harmony this
...
bind:
jsx harmony ...
bind(this,
id)} /> Apart from these two approaches, you can also pass arguments
to a function which is defined as arrow function jsx harmony onClick={this
...
log("Hello, your ticket number is", id) };
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16
...
Its API is same as the browser’s native event, including
stopPropagation() and preventDefault(), except the events work identically across all browsers
...

What are inline conditional expressions?
You can use either if statements or ternary expressions which are available
from JS to conditionally render expressions
...

jsx harmony

Hello!

{
!isLogin?

unread messages
...


messages
...
length}
:

You

}

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18
...
Key prop helps React identify which items have changed, are
added, or are removed
...
map((todo) =>
key={todo
...
text}
)
18


When you don’t have stable IDs for rendered items, you may use the item
index as a key as a last resort:
jsx harmony const todoItems = todos
...
text}
)
Note:
1
...
This can negatively impact performance and may cause
issues with component state
...
If you extract list item as separate component then apply keys on list
component instead of li tag
...
There will be a warning message in the console if the key prop is not
present on list items
...

What is the use of refs?
The ref is used to return a reference to the element
...

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
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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
...

� Back to Top
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
� Back to Top
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 differences are purely opinionated and it vary based on
the professional experience
...

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201
...
If the
website is not using React or if React fails to communicate with DevTools
then it won’t show up the tab
...

What are Styled Components?
styled-components is a JavaScript library for styling React applications
...

97

� Back to Top
203
...

import React from 'react'
import styled from 'styled-components'
// Create a
� Back to Top
204
...

� Back to Top
205
...
1
...
i
...
You
can just pass --typescript option as below

98

npx create-react-app my-app --typescript
# or
yarn create react-app my-app --typescript
But for lower versions of react scripts, just supply --scripts-version
option as react-scripts-ts while you create a new project
...

Now the project layout should look like the following:
my-app/
��
...
d
...

�� package
...
json
�� tsconfig
...
json
�� tsconfig
...
json
�� tslint
...

What are the main features of Reselect library?
Let’s see the main features of Reselect library,
1
...

2
...
A selector is not recomputed unless one of its
arguments changes
...
Selectors are composable
...

207
...
shop
...
shop
...
reduce((acc, item) => acc + item
...
20 },
{ name: 'orange', value: 0
...
log(subtotalSelector(exampleState)) // 2
...
log(taxSelector(exampleState))
// 0
...
log(totalSelector(exampleState))
// { total: 2
...

Does the statics object work with ES6 classes in React?
No, statics only works with React
...
createClass({
statics: {
someMethod: function() {

100

//
...
Component {
static propTypes = {
//
...

}
}
or writing them outside class as below,
class Component extends React
...

}
Component
...
}
Component
...
}
� Back to Top
210
...
The most common
usage is with React and React Native, but there are bindings available
for Angular, Angular 2, Vue, Mithril, and more
...

� Back to Top
211
...
You should be able to use it regardless of your
JavaScript build process
...

� Back to Top
212
...

const InitializeFromStateForm = reduxForm({
form: 'initializeFromState',
enableReinitialize : true
})(UserEdit)
If your initialValues prop gets updated, your form will update too
...

How React PropTypes allow different types for one prop?
You can use oneOfType() method of PropTypes
...
propTypes = {
size: PropTypes
...
string,
PropTypes
...

Can I import an SVG file as react component?
You can import SVG directly as component instead of loading it as a file
...
0
...

“‘jsx harmony import { ReactComponent as Logo } from ‘
...
svg’
const App = () => (
{/* Logo is an actual react component */}

) “‘
Note: Don’t forget about the curly braces in the import
...


102

Why are inline ref callbacks or functions not recommended?
If the ref callback is defined as an inline function, it will get called twice
during updates, first with null and then again with the DOM element
...

class UserForm extends Component {
handleSubmit = () => {
console
...
input
...
handleSubmit}>
type='text'
ref={(input) => this
...
One quick fix is to use the ES7 class property syntax
to define the function
class UserForm extends Component {
handleSubmit = () => {
console
...
input
...
input = input
}
render () {
return (

...
setSearchInput} /> // Access DOM input in handle submit

Submit

)

103

}
}
**Note:** In React v16
...

What is render hijacking in react?
The concept of render hijacking is the ability to control what a component
will output from another component
...
By wrapping,
you can inject additional props or make other changes, which can cause
changing logic of rendering
...

� Back to Top
217
...

1
...
Inheritance Inversion (II)
...

Props Proxy
In this approach, the render method of the HOC returns a React Element
of the type of the WrappedComponent
...

function ppHOC(WrappedComponent) {
return class PP extends React
...
this
...
It is called Inheritance Inversion because instead of
the WrappedComponent extending some Enhancer class, it is passively

104

extended by the Enhancer
...

function iiHOC(WrappedComponent) {
return class Enhancer extends WrappedComponent {
render() {
return super
...

How to pass numbers to React component?
You should be passing the numbers via curly braces({}) where as strings
in quotes

React
...
getElementById('container
� Back to Top
219
...
e
...
Some users prefer to keep every single piece of data
in Redux, to maintain a fully serializable and controlled version of their
application at all times
...

Below are the thumb rules to determine what kind of data should be put
into Redux
1
...
Do you need to be able to create further derived data based on this
original data?
3
...
Is there value to you in being able to restore this state to a given
point in time (ie, time travel debugging)?
5
...
e, use what’s in state if it’s already
there instead of re-requesting it)?
� Back to Top

105

220
...

The service worker is a web API that helps you cache your assets and
other files so that when the user is offline or on a slow network, he/she
can still see results on the screen, as such, it helps you build a better user
experience, that’s what you should know about service worker for now
...

import
import
import
import

React from 'react';
ReactDOM from 'react-dom';
App from '
...
/registerServiceWorker';

ReactDOM
...
getElementById('root'));
registerServiceWorker();
� Back to Top
221
...
Now you can do the same with function components by wrapping
them in React
...

const MyComponent = React
...

What is React lazy function?
The React
...
It will automatically load the bundle containing the
OtherComponent when the component gets rendered
...

const OtherComponent = React
...
/OtherComponent'));
function MyComponent() {
return (
106



);
}
Note: React
...
If you want to do code-splitting in a server rendered app, we still
recommend React Loadable
...

How to prevent unnecessary updates using setState?
You can compare the current value of the state with an existing state value
and decide whether to rerender the page or not
...

For example, the user profile information is conditionally rendered as follows,
getUserProfile = user
const latestAddress
this
...
address
return null;
} else {
return { title:
}
});
};

=> {
= user
...

How do you render Array, Strings and Numbers in React 16
Version?
Arrays: Unlike older releases, you don’t need to make sure render
method return a single element in React16
...

For example, let us take the below list of developers,
const ReactJSDevs = () => {
return [

John

,
107



Jackie

,

Jordan

];
}
You can also merge this array of items in another array component
...

render() {
return 'Welcome to ReactJS questions';
}
// Number
render() {
return 2018;
}
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225
...
You can initialize the local state without using the
constructor and declare class methods by using arrow functions without
the extra need to bind them
...
setState(prevState => ({
value: prevState
...
setState(prevState => ({
value: prevState
...
state
...
handleIncrement}>+
...

What are hooks?
Hooks is a new feature(React 16
...

Let’s see an example of useState hook:
import { useState } from 'react';
function Example() {
// Declare a new state variable, which we'll call "count"
const [count, setCount] = useState(0);
return (

You clicked {count} times

setCount(count + 1)}>
Click me