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

{'Welcome to React
world!'}

)
} }
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...
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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...

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
...
If the React
element was previously rendered into container, it will perform an update
on it and only mutate the DOM as necessary to reflect the latest changes
...
render(element, container, [callback])
If the optional callback is provided, it will be executed after the component
is rendered or updated
...

What is ReactDOMServer?
The ReactDOMServer object enables you to render components to static
markup (typically used on node server)
...
The following methods can be used in both
the server and browser environments:
1
...
renderToStaticMarkup()
For example, you generally run a Node-based web server like Express,
Hapi, or Koa, and you call renderToString to render your root component
to a string, which you then send as response
...
/MyPage'
app
...
write('')
res
...
write(renderToString())
res
...
end()
})
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...
Just like innerHTML, it is risky to use
this attribute considering cross-site scripting (XSS) attacks
...

In this example MyComponent uses dangerouslySetInnerHTML attribute
for setting HTML markup:
“‘jsx harmony function createMarkup() { return { __html: ‘First · Second’ } }
function MyComponent() { return
} “‘
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...
This is consistent with the DOM style
JavaScript property, is more efficient, and prevents XSS security holes
...
g
...
style
...

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...
React event handlers are named using camelCase, rather than lowercase
...
With JSX you pass a function as the event handler, rather than a
string
...

What will happen if you use setState() in constructor?
When you use setState(), then apart from assigning to the object state
React also re-renders the component and all its children
...
So
we need to use this
...

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...

In the below code snippet each element’s key will be based on ordering,
rather than tied to the data that is being represented
...

jsx harmony {todos
...
todo}

If you use element data for unique key, assuming todo
...

jsx harmony {todos
...
todo}

key={todo
...

Is it good to use setState() in componentWillMount() method?
Yes, it is safe to use setState() inside componentWillMount() method
...
componentWillMount() is
invoked immediately before mounting occurs
...
Avoid
introducing any side-effects or subscriptions in this method
...

jsx harmony componentDidMount() {
axios
...
then((result) => {
this
...
result
...

What will happen if you use props in initial state?
If the props on the component are changed without the component being
refreshed, the new prop value will never be displayed because the constructor function will never update the current state of the component
...

The below component won’t display the updated input value:
“‘jsx harmony class MyComponent extends React
...
state = {
records: [],
inputValue: this
...
inputValue
};
}
render() { return

41

{this
...
inputValue}
}}
Using props inside render method will update the value:
```jsx harmony
class MyComponent extends React
...
state = {
record: []
}
}
render() {
return

{this
...
inputValue}

}
}
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67
...
JSX does not render false or undefined, so you can use conditional
short-circuiting to render a given part of your component only if a certain
condition is true
...

jsx harmony const MyComponent = ({ name, address }) => (

{name}

{address
?

{address}

:

{'Address is not available'}

}

)
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...
Instead we can use prop destructuring
42

with
...

For example,
“‘jsx harmony const ComponentA = () =>
const ComponentB = ({ isDisplay, …domProps }) =>

{‘ComponentB’}
“‘
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...
Decorators are flexible and readable way of
modifying component functionality
...
Component
{ //…
...
Component { componentDidMount() { document
...
this
...

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...

For example moize library can memoize the component in another component
...
/components/Component’ // this module exports a non-memoized component

43

const MemoizedFoo = moize
...
6
...
memo`
...
memo(function MemoComponent(props) {
/* render using props */
});
OR
export default React
...

How you implement Server Side Rendering or SSR?
React is already equipped to handle rendering on Node servers
...

“‘jsx harmony import ReactDOMServer from ‘react-dom/server’ import
App from ‘
...
renderToString() “‘
This method will output the regular HTML as a string, which can be then
placed inside a page body as part of the server response
...

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...
Apart from this, if you minify the code, for example,
Uglify’s dead-code elimination to strip out development only code and
comments, it will drastically reduce the size of your bundle
...

44

What is CRA and its benefits?
The create-react-app CLI tool allows you to quickly create & run React
applications with no configuration step
...

2
...

4
...

Language extras beyond ES6 like the object spread operator
...

A fast interactive unit test runner with built-in support for coverage
reporting
...
A live development server that warns about common mistakes
...
A build script to bundle JS, CSS, and images for production, with
hashes and sourcemaps
...

What is the lifecycle methods order in mounting?
The lifecycle methods are called in the following order when an instance
of a component is being created and inserted into the DOM
...

2
...

4
...


45

What are the lifecycle methods going to be deprecated in React
v16?
The following lifecycle methods going to be unsafe coding practices and
will be more problematic with async rendering
...
componentWillMount()
2
...
componentWillUpdate()
Starting with React v16
...

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76
...
It can
return an object to update state, or null to indicate that the new props
do not require any state updates
...
Component {
static getDerivedStateFromProps(props, state) {
//
...

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77
...
The return value from this method will be passed
as the third parameter to componentDidUpdate()
...
Component {
getSnapshotBeforeUpdate(prevProps, prevState) {
//
...

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78
...

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79
...

Using displayName for naming component:
export default React
...

})
The recommended approach:
export default class TodoApp extends React
...

}
also
const TodoApp = () => {
//
...

What is the recommended ordering of methods in component
class?
Recommended ordering of methods from mounting to render stage:
1
...
constructor()
47

3
...

5
...

7
...

9
...

11
...
getter methods for render like getSelectReason() or getFooterContent()
13
...
render()
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81
...
We need to use object to map prop values to components
...
/HomePage’ import AboutPage
from ‘
...
/ServicesPage’ import ContactPage from ‘
...
page] || ContactPage
return }
// The keys of the PAGES object can be used in the prop types to catch
dev-time errors
...
propTypes = { page: PropTypes
...
keys(PAGES))
...

Why we need to pass a function to setState()?
The reason behind for this is that setState() is an asynchronous operation
...
That means you

48

should not rely on the current state when calling setState() since you
can’t be sure what that state will be
...
By doing this you
can avoid issues with the user getting the old state value on access due to
the asynchronous nature of setState()
...
After three consecutive increment
operations, the value is going to be incremented only by one
...
state
...
setState({ count: this
...
count + 1 })
this
...
state
...
setState({ count: this
...
count + 1 })
// this
...
count === 1, not 3
If we pass a function to setState(), the count gets incremented correctly
...
setState((prevState, props) => ({
count: prevState
...
increment
}))
// this
...
count === 3 as expected
(OR)
Why function is preferred over object for setState()?
React may batch multiple setState() calls into a single update for performance
...
props and this
...

This counter example will fail to update as expected:
// Wrong
this
...
state
...
props
...
That function will receive the previous state as the first argument,
and the props at the time the update is applied as the second argument
...
setState((prevState, props) => ({
counter: prevState
...
increment
}))
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83
...
StrictMode is a useful component for highlighting potential problems in an application
...
It activates additional checks and warnings for its descendants
...

“‘jsx harmony import React from ‘react’
function ExampleApplication() { return (

...
StrictMode>

) } “‘
In the example above, the strict mode checks apply to
and components only
...

What are React Mixins?
Mixins are a way to totally separate components to have a common functionality
...

One of the most commonly used mixins is PureRenderMixin
...
createClass({
mixins: [PureRenderMixin],
//
...


50

Why is isMounted() an anti-pattern and what is the proper solution?
The primary use case for isMounted() is to avoid calling setState() after
a component has been unmounted, because it will emit a warning
...
isMounted()) {
this
...
})
}
Checking isMounted() before calling setState() does eliminate the warning, but it also defeats the purpose of the warning
...

An optimal solution would be to find places where setState() might be
called after a component has unmounted, and fix them
...
Ideally, any callbacks should be canceled in componentWillUnmount(), prior to unmounting
...

What are the Pointer Events supported in React?
Pointer Events provide a unified way of handling all input events
...
We need to remember that
these events will only work in browsers that support the Pointer Events
specification
...

2
...

4
...

6
...

8
...

10
...

51

Why should component names start with capital letter?
If you are rendering your component using JSX, the name of that
component has to begin with a capital letter otherwise React will throw
an error as an unrecognized tag
...
jsx harmony
class SomeComponent extends Component { // Code goes here }
You can define component class which name starts with lowercase letter,
but when it’s imported it should have capital letter
...
/MyComponent'
What are the exceptions on React component naming? The component names should start with an uppercase letter but there are few exceptions to this convention
...
For example, the
below tag can be compiled to a valid component,
jsx harmony
render() {
return (
// `React
...
component)`

...

Are custom DOM attributes supported in React v16?
Yes
...
If you
wrote JSX with an attribute that React doesn’t recognize, React would
just skip it
...

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89
...
createClass()
...
This way it can conditionally render component
...
loggedIn) {
return null;
}
return (

welcome, {props
...
Component {
constructor(props) {
super(props);
this
...
state
...
state
...

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267
...

1
...
The items in the list have no ids
3
...

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268
...
i
...

For example, the below Book component uses two arrays with different
arrays,
function Book(props) {
const index = (

{props
...
map((page) =>

• ...
  title}
  

)}

);
const content = props
...
map((page) =>

...
title}

{page
...
pageNumber}