```
Instead you need to move curly braces outside (don't forget to include spaces between class names):
```jsx harmony
```
_Template strings_ will also work:
```jsx harmony
```
**[⬆ Back to Top](#table-of-contents)**
96. ### What is the difference between React and ReactDOM?
The `react` package contains `React.createElement()`, `React.Component`, `React.Children`, and other helpers related to elements and component classes. You can think of these as the isomorphic or universal helpers that you need to build components. The `react-dom` package contains `ReactDOM.render()`, and in `react-dom/server` we have _server-side rendering_ support with `ReactDOMServer.renderToString()` and `ReactDOMServer.renderToStaticMarkup()`.
**[⬆ Back to Top](#table-of-contents)**
97. ### Why ReactDOM is separated from React?
The React team worked on extracting all DOM-related features into a separate library called _ReactDOM_. React v0.14 is the first release in which the libraries are split. By looking at some of the packages, `react-native`, `react-art`, `react-canvas`, and `react-three`, it has become clear that the beauty and essence of React has nothing to do with browsers or the DOM.
To build more environments that React can render to, React team planned to split the main React package into two: `react` and `react-dom`. This paves the way to writing components that can be shared between the web version of React and React Native.
**[⬆ Back to Top](#table-of-contents)**
98. ### How to use React label element?
If you try to render a `
` element bound to a text input using the standard `for` attribute, then it produces HTML missing that attribute and prints a warning to the console.
```jsx harmony
{'User'}
{'User'}
{"Submit"}
```
If you're using React Native then you can use the array notation:
```jsx harmony
{"Submit"}
```
**[⬆ Back to Top](#table-of-contents)**
100. ### How to re-render the view when the browser is resized?
You can listen to the `resize` event in `componentDidMount()` and then update the dimensions (`width` and `height`). You should remove the listener in `componentWillUnmount()` method.
```javascript
class WindowDimensions extends React.Component {
constructor(props) {
super(props);
this.updateDimensions = this.updateDimensions.bind(this);
}
componentWillMount() {
this.updateDimensions();
}
componentDidMount() {
window.addEventListener("resize", this.updateDimensions);
}
componentWillUnmount() {
window.removeEventListener("resize", this.updateDimensions);
}
updateDimensions() {
this.setState({
width: window.innerWidth,
height: window.innerHeight,
});
}
render() {
return (
{this.state.width} x {this.state.height}
);
}
}
```
**[⬆ Back to Top](#table-of-contents)**
101. ### What is the difference between `setState()` and `replaceState()` methods?
When you use `setState()` the current and previous states are merged. `replaceState()` throws out the current state, and replaces it with only what you provide. Usually `setState()` is used unless you really need to remove all previous keys for some reason. You can also set state to `false`/`null` in `setState()` instead of using `replaceState()`.
**[⬆ Back to Top](#table-of-contents)**
102. ### How to listen to state changes?
The `componentDidUpdate` lifecycle method will be called when state changes. You can compare provided state and props values with current state and props to determine if something meaningful changed.
```
componentDidUpdate(object prevProps, object prevState)
```
**Note:** The previous releases of ReactJS also uses `componentWillUpdate(object nextProps, object nextState)` for state changes. It has been deprecated in latest releases.
**[⬆ Back to Top](#table-of-contents)**
103. ### What is the recommended approach of removing an array element in React state?
The better approach is to use `Array.prototype.filter()` method.
For example, let's create a `removeItem()` method for updating the state.
```javascript
removeItem(index) {
this.setState({
data: this.state.data.filter((item, i) => i !== index)
})
}
```
**[⬆ Back to Top](#table-of-contents)**
104. ### Is it possible to use React without rendering HTML?
It is possible with latest version (>=16.2). Below are the possible options:
```jsx harmony
render() {
return false
}
```
```jsx harmony
render() {
return null
}
```
```jsx harmony
render() {
return []
}
```
```jsx harmony
render() {
return ` tag so that the formatting of the `JSON.stringify()` is retained:
```jsx harmony
const data = { name: "John", age: 42 };
class User extends React.Component {
render() {
return {JSON.stringify(data, null, 2)} ;
}
}
React.render(
);
}
}
ReactDOM.render(
);
};
ReactDOM.render({`React version: ${REACT_VERSION}`}
,
document.getElementById("app")
);
```
**[⬆ Back to Top](#table-of-contents)**
111. ### What are the approaches to include polyfills in your `create-react-app`?
There are approaches to include polyfills in create-react-app,
1. **Manual import from `core-js`:**
Create a file called (something like) `polyfills.js` and import it into root `index.js` file. Run `npm install core-js` or `yarn add core-js` and import your specific required features.
```javascript
import "core-js/fn/array/find";
import "core-js/fn/array/includes";
import "core-js/fn/number/is-nan";
```
2. **Using Polyfill service:**
Use the polyfill.io CDN to retrieve custom, browser-specific polyfills by adding this line to `index.html`:
```html
```
In the above script we had to explicitly request the `Array.prototype.includes` feature as it is not included in the default feature set.
**[⬆ Back to Top](#table-of-contents)**
112. ### How to use https instead of http in create-react-app?
You just need to use `HTTPS=true` configuration. You can edit your `package.json` scripts section:
```json
"scripts": {
"start": "set HTTPS=true && react-scripts start"
}
```
or just run `set HTTPS=true && npm start`
**[⬆ Back to Top](#table-of-contents)**
113. ### How to avoid using relative path imports in create-react-app?
Create a file called `.env` in the project root and write the import path:
```
NODE_PATH=src/app
```
After that restart the development server. Now you should be able to import anything inside `src/app` without relative paths.
**[⬆ Back to Top](#table-of-contents)**
114. ### How to add Google Analytics for React Router?
Add a listener on the `history` object to record each page view:
```javascript
history.listen(function (location) {
window.ga("set", "page", location.pathname + location.search);
window.ga("send", "pageview", location.pathname + location.search);
});
```
**[⬆ Back to Top](#table-of-contents)**
115. ### How to update a component every second?
You need to use `setInterval()` to trigger the change, but you also need to clear the timer when the component unmounts to prevent errors and memory leaks.
```javascript
componentDidMount() {
this.interval = setInterval(() => this.setState({ time: Date.now() }), 1000)
}
componentWillUnmount() {
clearInterval(this.interval)
}
```
**[⬆ Back to Top](#table-of-contents)**
116. ### How do you apply vendor prefixes to inline styles in React?
React _does not_ apply _vendor prefixes_ automatically. You need to add vendor prefixes manually.
```jsx harmony
```
**[⬆ Back to Top](#table-of-contents)**
117. ### How to import and export components using React and ES6?
You should use default for exporting the components
```jsx harmony
import React from "react";
import User from "user";
export default class MyProfile extends React.Component {
render() {
return //... ;
}
}
```
With the export specifier, the MyProfile is going to be the member and exported to this module and the same can be imported without mentioning the name in other components.
**[⬆ Back to Top](#table-of-contents)**
119. ### Why is a component constructor called only once?
React's _reconciliation_ algorithm assumes that without any information to the contrary, if a custom component appears in the same place on subsequent renders, it's the same component as before, so reuses the previous instance rather than creating a new one.
**[⬆ Back to Top](#table-of-contents)**
120. ### How to define constants in React?
You can use ES7 `static` field to define constant.
```javascript
class MyComponent extends React.Component {
static DEFAULT_PAGINATION = 10;
}
```
**[⬆ Back to Top](#table-of-contents)**
121. ### How to programmatically trigger click event in React?
You could use the ref prop to acquire a reference to the underlying `HTMLInputElement` object through a callback, store the reference as a class property, then use that reference to later trigger a click from your event handlers using the `HTMLElement.click` method.
This can be done in two steps:
1. Create ref in render method:
```jsx harmony
Error: {error.message}
;
} else {
return (
{employees.map((employee) => (
{employee.name}-{employee.experience}
))}
);
}
}
}
```
**[⬆ Back to Top](#table-of-contents)**
128. ### What are render props?
**Render Props** is a simple technique for sharing code between components using a prop whose value is a function. The below component uses render prop which returns a React element.
```jsx harmony
` components of React Router v4?
React Router v4 provides below 3 `` components:
1. ``
2. ``
3. ``
The above components will create _browser_, _hash_, and _memory_ history instances. React Router v4 makes the properties and methods of the `history` instance associated with your router available through the context in the `router` object.
**[⬆ Back to Top](#table-of-contents)**
132. ### What is the purpose of `push()` and `replace()` methods of `history`?
A history instance has two methods for navigation purpose.
1. `push()`
2. `replace()`
If you think of the history as an array of visited locations, `push()` will add a new location to the array and `replace()` will replace the current location in the array with the new one.
**[⬆ Back to Top](#table-of-contents)**
133. ### How do you programmatically navigate using React Router v4?
There are three different ways to achieve programmatic routing/navigation within components.
1. **Using the `withRouter()` higher-order function:**
The `withRouter()` higher-order function will inject the history object as a prop of the component. This object provides `push()` and `replace()` methods to avoid the usage of context.
```jsx harmony
import { withRouter } from "react-router-dom"; // this also works with 'react-router-native'
const Button = withRouter(({ history }) => (
{
history.push("/new-location");
}}
>
{"Click Me!"}
));
```
2. **Using `` component and render props pattern:**
The `` component passes the same props as `withRouter()`, so you will be able to access the history methods through the history prop.
```jsx harmony
import { Route } from "react-router-dom";
const Button = () => (
(
{
history.push("/new-location");
}}
>
{"Click Me!"}
)}
/>
);
```
3. **Using context:**
This option is not recommended and treated as unstable API.
```jsx harmony
const Button = (props, context) => (
{
context.history.push("/new-location");
}}
>
{"Click Me!"}
);
Button.contextTypes = {
history: React.PropTypes.shape({
push: React.PropTypes.func.isRequired,
}),
};
```
**[⬆ Back to Top](#table-of-contents)**
134. ### How to get query parameters in React Router v4?
The ability to parse query strings was taken out of React Router v4 because there have been user requests over the years to support different implementation. So the decision has been given to users to choose the implementation they like. The recommended approach is to use query strings library.
```javascript
const queryString = require("query-string");
const parsed = queryString.parse(props.location.search);
```
You can also use `URLSearchParams` if you want something native:
```javascript
const params = new URLSearchParams(props.location.search);
const foo = params.get("name");
```
You should use a _polyfill_ for IE11.
**[⬆ Back to Top](#table-of-contents)**
135. ### Why you get "Router may have only one child element" warning?
You have to wrap your Route's in a `` block because `` is unique in that it renders a route exclusively.
At first you need to add `Switch` to your imports:
```javascript
import { Switch, Router, Route } from "react-router";
```
Then define the routes within `` block:
```jsx harmony
```
**[⬆ Back to Top](#table-of-contents)**
136. ### How to pass params to `history.push` method in React Router v4?
While navigating you can pass props to the `history` object:
```javascript
this.props.history.push({
pathname: "/template",
search: "?name=sudheer",
state: { detail: response.data },
});
```
The `search` property is used to pass query params in `push()` method.
**[⬆ Back to Top](#table-of-contents)**
137. ### How to implement _default_ or _NotFound_ page?
A `` renders the first child `` that matches. A `` with no path always matches. So you just need to simply drop path attribute as below
```jsx harmony
```
**[⬆ Back to Top](#table-of-contents)**
138. ### How to get history on React Router v4?
Below are the list of steps to get history object on React Router v4,
1. Create a module that exports a `history` object and import this module across the project.
For example, create `history.js` file:
```javascript
import { createBrowserHistory } from "history";
export default createBrowserHistory({
/* pass a configuration object here if needed */
});
```
2. You should use the `` component instead of built-in routers. Import the above `history.js` inside `index.js` file:
```jsx harmony
import { Router } from "react-router-dom";
import history from "./history";
import App from "./App";
ReactDOM.render(
,
holder
);
```
3. You can also use push method of `history` object similar to built-in history object:
```javascript
// some-other-file.js
import history from "./history";
history.push("/go-here");
```
**[⬆ Back to Top](#table-of-contents)**
139. ### How to perform automatic redirect after login?
The `react-router` package provides `` component in React Router. Rendering a `` will navigate to a new location. Like server-side redirects, the new location will override the current location in the history stack.
```javascript
import React, { Component } from "react";
import { Redirect } from "react-router";
export default class LoginComponent extends Component {
render() {
if (this.state.isLoggedIn === true) {
return {"Login Please"}
;
}
}
}
```
## React Internationalization
**[⬆ Back to Top](#table-of-contents)**
140. ### What is React Intl?
The _React Intl_ library makes internationalization in React straightforward, with off-the-shelf components and an API that can handle everything from formatting strings, dates, and numbers, to pluralization. React Intl is part of _FormatJS_ which provides bindings to React via its components and API.
**[⬆ Back to Top](#table-of-contents)**
141. ### What are the main features of React Intl?
Below are the main features of React Intl,
1. Display numbers with separators.
2. Display dates and times correctly.
3. Display dates relative to "now".
4. Pluralize labels in strings.
5. Support for 150+ languages.
6. Runs in the browser and Node.
7. Built on standards.
**[⬆ Back to Top](#table-of-contents)**
142. ### What are the two ways of formatting in React Intl?
The library provides two ways to format strings, numbers, and dates:
1. **Using react components:**
```jsx harmony
` as placeholder using React Intl?
The `{`The current locale is ${intl.locale}`}
);
MyComponent.propTypes = {
intl: intlShape.isRequired,
};
export default injectIntl(MyComponent);
```
**[⬆ Back to Top](#table-of-contents)**
145. ### How to format date using React Intl?
The `injectIntl()` higher-order component will give you access to the `formatDate()` method via the props in your component. The method is used internally by instances of `FormattedDate` and it returns the string representation of the formatted date.
```jsx harmony
import { injectIntl, intlShape } from "react-intl";
const stringDate = this.props.intl.formatDate(date, {
year: "numeric",
month: "numeric",
day: "numeric",
});
const MyComponent = ({ intl }) => (
{`The formatted date is ${stringDate}`}
);
MyComponent.propTypes = {
intl: intlShape.isRequired,
};
export default injectIntl(MyComponent);
```
## React Testing
**[⬆ Back to Top](#table-of-contents)**
146. ### What is Shallow Renderer in React testing?
_Shallow rendering_ is useful for writing unit test cases in React. It lets you render a component _one level deep_ and assert facts about what its render method returns, without worrying about the behavior of child components, which are not instantiated or rendered.
For example, if you have the following component:
```javascript
function MyComponent() {
return (
{"Title"}
{"Description"}
);
}
```
Then you can assert as follows:
```jsx harmony
import ShallowRenderer from "react-test-renderer/shallow";
// in your test
const renderer = new ShallowRenderer();
renderer.render({"Title"} ,
{"Description"} ,
]);
```
**[⬆ Back to Top](#table-of-contents)**
147. ### What is `TestRenderer` package in React?
This package provides a renderer that can be used to render components to pure JavaScript objects, without depending on the DOM or a native mobile environment. This package makes it easy to grab a snapshot of the platform view hierarchy (similar to a DOM tree) rendered by a ReactDOM or React Native without using a browser or `jsdom`.
```jsx harmony
import TestRenderer from "react-test-renderer";
const Link = ({ page, children }) => {children} ;
const testRenderer = TestRenderer.create(
{"Loaded"}
) : (
{"Not Loaded"}
);
}
}
const mapStateToProps = (state) => ({
isLoaded: state.isLoaded,
});
const mapDispatchToProps = { fetchData };
export default connect(mapStateToProps, mapDispatchToProps)(App);
```
**[⬆ Back to Top](#table-of-contents)**
162. ### How to use `connect()` from React Redux?
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` function is connected to the container. You can import `connect()` from `react-redux`.
```jsx harmony
import React from "react";
import { connect } from "react-redux";
class App extends React.Component {
render() {
return {this.props.containerData}
;
}
}
function mapStateToProps(state) {
return { containerData: state.data };
}
export default connect(mapStateToProps)(App);
```
**[⬆ Back to Top](#table-of-contents)**
163. ### How to reset state in Redux?
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.
```javascript
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-persist` 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.
```javascript
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);
};
```
**[⬆ Back to Top](#table-of-contents)**
164. ### Whats the purpose of `at` symbol in the Redux connect decorator?
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:**
```javascript
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:**
```javascript
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.
**[⬆ Back to Top](#table-of-contents)**
165. ### What is the difference between React context and React Redux?
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.
**[⬆ Back to Top](#table-of-contents)**
166. ### Why are Redux state functions called reducers?
Reducers always return the accumulation of the state (based on all previous and current actions). Therefore, they act as a reducer of state. Each time a Redux reducer is called, the state and action are passed as parameters. This state is then reduced (or accumulated) based on the action, and then the next state is returned. 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.
**[⬆ Back to Top](#table-of-contents)**
167. ### How to make AJAX request in Redux?
You can use `redux-thunk` middleware which allows you to define async actions.
Let's take an example of fetching specific account as an AJAX call using _fetch API_:
```javascript
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 };
}
```
**[⬆ Back to Top](#table-of-contents)**
168. ### Should I keep all component's state in Redux store?
Keep your data in the Redux store, and the UI related state internally in the component.
**[⬆ Back to Top](#table-of-contents)**
169. ### What is the proper way to access Redux store?
The best way to access your store in a component is to use the `connect()` function, that 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 `` component using connect:
```javascript
import { connect } from "react-redux";
import { setVisibilityFilter } from "../actions";
import Link from "../components/Link";
const mapStateToProps = (state, ownProps) => ({
active: ownProps.filter === state.visibilityFilter,
});
const mapDispatchToProps = (dispatch, ownProps) => ({
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 developers almost always recommend using `connect()` over accessing the store directly (using context API).
```javascript
class MyComponent {
someMethod() {
doSomethingWith(this.context.store);
}
}
```
**[⬆ Back to Top](#table-of-contents)**
170. ### What is the difference between component and container in React Redux?
**Component** is a class or function component that describes the presentational part of your application.
**Container** is an informal term for a component that is connected to a Redux store. Containers _subscribe_ to Redux state updates and _dispatch_ actions, and they usually don't render DOM elements; they delegate rendering to presentational child components.
**[⬆ Back to Top](#table-of-contents)**
171. ### What is the purpose of the constants in Redux?
Constants allows 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`).
```javascript
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 action creation:**
Let's take `actions.js`:
```javascript
import { ADD_TODO } from "./actionTypes";
export function addTodo(text) {
return { type: ADD_TODO, text };
}
```
2. **In reducers:**
Let's create `reducer.js`:
```javascript
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;
}
};
```
**[⬆ Back to Top](#table-of-contents)**
172. ### What are the different ways to write `mapDispatchToProps()`?
There are a few ways of binding _action creators_ to `dispatch()` in `mapDispatchToProps()`.
Below are the possible options:
```javascript
const mapDispatchToProps = (dispatch) => ({
action: () => dispatch(action()),
});
```
```javascript
const mapDispatchToProps = (dispatch) => ({
action: bindActionCreators(action, dispatch),
});
```
```javascript
const mapDispatchToProps = { action };
```
The third option is just a shorthand for the first one.
**[⬆ Back to Top](#table-of-contents)**
173. ### What is the use of the `ownProps` parameter in `mapStateToProps()` and `mapDispatchToProps()`?
If the `ownProps` parameter is specified, React Redux will pass the props that were passed to the component into your _connect_ functions. So, if you use a connected component:
```jsx harmony
import ConnectedComponent from "./containers/ConnectedComponent";
}
```
**[⬆ Back to Top](#table-of-contents)**
196. ### What is React Dev Tools?
_React Developer Tools_ let you inspect the component hierarchy, including component props and state. It exists both as a browser extension (for Chrome and Firefox), and as a standalone app (works with other environments including Safari, IE, and React Native).
The official extensions available for different browsers or environments.
1. **Chrome extension**
2. **Firefox extension**
3. **Standalone app** (Safari, React Native, etc)
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197. ### Why is DevTools not loading in Chrome for local files?
If you opened a local HTML file in your browser (`file://...`) then you must first open _Chrome Extensions_ and check `Allow access to file URLs`.
**[⬆ Back to Top](#table-of-contents)**
198. ### How to use Polymer in React?
You need to follow below steps to use Polymer in React,
1. Create a Polymer element:
```jsx harmony
` and `<Wrapper>` components with specific styles for each.
```javascript
import React from "react";
import styled from "styled-components";
// Create a <Title> component that renders an <h1> which is centered, red and sized at 1.5em
const Title = styled.h1`
font-size: 1.5em;
text-align: center;
color: palevioletred;
`;
// Create a <Wrapper> component that renders a <section> with some padding and a papayawhip background
const Wrapper = styled.section`
padding: 4em;
background: papayawhip;
`;
```
These two variables, `Title` and `Wrapper`, are now components that you can render just like any other react component.
```jsx harmony
<Wrapper>
<Title>{"Lets start first styled component!"}
```
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204. ### What is Relay?
Relay is a JavaScript framework for providing a data layer and client-server communication to web applications using the React view layer.
**[⬆ Back to Top](#table-of-contents)**
205. ### How to use TypeScript in `create-react-app` application?
Starting from react-scripts@2.1.0 or higher, there is a built-in support for typescript. i.e, `create-react-app` now supports typescript natively. You can just pass `--typescript` option as below
```bash
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. `react-scripts-ts` is a set of adjustments to take the standard `create-react-app` project pipeline and bring TypeScript into the mix.
Now the project layout should look like the following:
```
my-app/
├─ .gitignore
├─ images.d.ts
├─ node_modules/
├─ public/
├─ src/
│ └─ ...
├─ package.json
├─ tsconfig.json
├─ tsconfig.prod.json
├─ tsconfig.test.json
└─ tslint.json
```
## Miscellaneous
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206. ### What are the main features of Reselect library?
Let's see the main features of Reselect library,
1. Selectors can compute derived data, allowing Redux to store the minimal possible state.
2. Selectors are efficient. A selector is not recomputed unless one of its arguments changes.
3. Selectors are composable. They can be used as input to other selectors.
207. #### Give an example of Reselect usage?
Let's take calculations and different amounts of a shipment order with the simplified usage of Reselect:
```javascript
import { createSelector } from "reselect";
const shopItemsSelector = (state) => state.shop.items;
const taxPercentSelector = (state) => state.shop.taxPercent;
const subtotalSelector = createSelector(shopItemsSelector, (items) =>
items.reduce((acc, item) => acc + item.value, 0)
);
const taxSelector = createSelector(
subtotalSelector,
taxPercentSelector,
(subtotal, taxPercent) => subtotal * (taxPercent / 100)
);
export const totalSelector = createSelector(
subtotalSelector,
taxSelector,
(subtotal, tax) => ({ total: subtotal + tax })
);
let exampleState = {
shop: {
taxPercent: 8,
items: [
{ name: "apple", value: 1.2 },
{ name: "orange", value: 0.95 },
],
},
};
console.log(subtotalSelector(exampleState)); // 2.15
console.log(taxSelector(exampleState)); // 0.172
console.log(totalSelector(exampleState)); // { total: 2.322 }
```
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209. ### Does the statics object work with ES6 classes in React?
No, `statics` only works with `React.createClass()`:
```javascript
someComponent = React.createClass({
statics: {
someMethod: function () {
// ..
},
},
});
```
But you can write statics inside ES6+ classes as below,
```javascript
class Component extends React.Component {
static propTypes = {
// ...
};
static someMethod() {
// ...
}
}
```
or writing them outside class as below,
```javascript
class Component extends React.Component {
....
}
Component.propTypes = {...}
Component.someMethod = function(){....}
```
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210. ### Can Redux only be used with React?
Redux can be used as a data store for any UI layer. The most common usage is with React and React Native, but there are bindings available for Angular, Angular 2, Vue, Mithril, and more. Redux simply provides a subscription mechanism which can be used by any other code.
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211. ### Do you need to have a particular build tool to use Redux?
Redux is originally written in ES6 and transpiled for production into ES5 with Webpack and Babel. You should be able to use it regardless of your JavaScript build process. Redux also offers a UMD build that can be used directly without any build process at all.
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212. ### How Redux Form `initialValues` get updated from state?
You need to add `enableReinitialize : true` setting.
```javascript
const InitializeFromStateForm = reduxForm({
form: "initializeFromState",
enableReinitialize: true,
})(UserEdit);
```
If your `initialValues` prop gets updated, your form will update too.
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213. ### How React PropTypes allow different types for one prop?
You can use `oneOfType()` method of `PropTypes`.
For example, the height property can be defined with either `string` or `number` type as below:
```javascript
Component.propTypes = {
size: PropTypes.oneOfType([PropTypes.string, PropTypes.number]),
};
```
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214. ### Can I import an SVG file as react component?
You can import SVG directly as component instead of loading it as a file. This feature is available with `react-scripts@2.0.0` and higher.
```jsx harmony
import { ReactComponent as Logo } from "./logo.svg";
const App = () => (
{/* Logo is an actual react component */}
);
```
**Note**: Don't forget about the curly braces in the import.
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215. ### 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. This is because a new instance of the function is created with each render, so React needs to clear the old ref and set up the new one.
```jsx
class UserForm extends Component {
handleSubmit = () => {
console.log("Input Value is: ", this.input.value);
};
render() {
return (
);
}
}
```
But our expectation is for the ref callback to get called once, when the component mounts. One quick fix is to use the ES7 class property syntax to define the function
```jsx
class UserForm extends Component {
handleSubmit = () => {
console.log("Input Value is: ", this.input.value);
};
setSearchInput = (input) => {
this.input = input;
};
render() {
return (
);
}
}
```
**Note:** In React v16.3,
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216. ### What is render hijacking in react?
The concept of render hijacking is the ability to control what a component will output from another component. It means that you decorate your component by wrapping it into a Higher-Order component. By wrapping, you can inject additional props or make other changes, which can cause changing logic of rendering. It does not actually enable hijacking, but by using HOC you make your component behave differently.
**[⬆ Back to Top](#table-of-contents)**
217. ### What are HOC factory implementations?
There are two main ways of implementing HOCs in React.
1. Props Proxy (PP) and
2. Inheritance Inversion (II).
But they follow different approaches for manipulating the _WrappedComponent_.
**Props Proxy**
In this approach, the render method of the HOC returns a React Element of the type of the WrappedComponent. We also pass through the props that the HOC receives, hence the name **Props Proxy**.
```jsx
function ppHOC(WrappedComponent) {
return class PP extends React.Component {
render() {
return
);
}
```
**Note:**
`React.lazy` and `Suspense` is not yet available for server-side rendering. If you want to do code-splitting in a server rendered app, we still recommend React Loadable.
**[⬆ Back to Top](#table-of-contents)**
223. ### 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. If the values are the same then you need to return **null** to stop re-rendering otherwise return the latest state value.
For example, the user profile information is conditionally rendered as follows,
```jsx
getUserProfile = (user) => {
const latestAddress = user.address;
this.setState((state) => {
if (state.address === latestAddress) {
return null;
} else {
return { title: latestAddress };
}
});
};
```
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224. ### 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. You are able to return multiple sibling elements without a wrapping element by returning an array.
For example, let us take the below list of developers,
```jsx
const ReactJSDevs = () => {
return [
John ,
Jackie ,
Jordan ,
];
};
```
You can also merge this array of items in another array component.
```jsx
const JSDevs = () => {
return (
);
};
```
**Strings and Numbers:** You can also return string and number type from the render method.
```jsx
render() {
return 'Welcome to ReactJS questions';
}
// Number
render() {
return 2018;
}
```
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225. ### How to use class field declarations syntax in React classes?
React Class Components can be made much more concise using the class field declarations. 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.
Let's take a counter example to demonstrate class field declarations for state without using constructor and methods without binding,
```jsx
class Counter extends Component {
state = { value: 0 };
handleIncrement = () => {
this.setState((prevState) => ({
value: prevState.value + 1,
}));
};
handleDecrement = () => {
this.setState((prevState) => ({
value: prevState.value - 1,
}));
};
render() {
return (
{this.state.value}
+
-
);
}
}
```
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226. ### What are hooks?
Hooks is a special function (introduced as a new feature in React 16.8) that lets you use state and other React features without writing a class.
Let's see an example of useState hook:
```jsx
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
);
}
```
**Note:** Hooks can be used inside an existing function component.
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227. ### What rules need to be followed for hooks?
You need to follow two rules in order to use hooks,
1. Call Hooks only at the top level of your react functions. i.e, You shouldn’t call Hooks inside loops, conditions, or nested functions. This will ensure that Hooks are called in the same order each time a component renders and it preserves the state of Hooks between multiple useState and useEffect calls.
2. Call Hooks from React Functions only. i.e, You shouldn’t call Hooks from regular JavaScript functions.
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228. ### How to ensure hooks followed the rules in your project?
React team released an ESLint plugin called **eslint-plugin-react-hooks** that enforces these two rules. You can add this plugin to your project using the below command,
```javascript
npm install eslint-plugin-react-hooks@next
```
And apply the below config in your ESLint config file,
```javascript
// Your ESLint configuration
{
"plugins": [
// ...
"react-hooks"
],
"rules": {
// ...
"react-hooks/rules-of-hooks": "error"
}
}
```
**Note:** This plugin is intended to use in Create React App by default.
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229. ### What are the differences between Flux and Redux?
Below are the major differences between Flux and Redux
| Flux | Redux |
| ---------------------------------------------- | ------------------------------------------ |
| State is mutable | State is immutable |
| The Store contains both state and change logic | The Store and change logic are separate |
| There are multiple stores exist | There is only one store exist |
| All the stores are disconnected and flat | Single store with hierarchical reducers |
| It has a singleton dispatcher | There is no concept of dispatcher |
| React components subscribe to the store | Container components uses connect function |
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230. ### What are the benefits of React Router V4?
Below are the main benefits of React Router V4 module,
1. In React Router v4(version 4), the API is completely about components. A router can be visualized as a single component(``) which wraps specific child router components(``).
2. You don't need to manually set history. The router module will take care history by wrapping routes with `` component.
3. The application size is reduced by adding only the specific router module(Web, core, or native)
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231. ### Can you describe about componentDidCatch lifecycle method signature?
The **componentDidCatch** lifecycle method is invoked after an error has been thrown by a descendant component. The method receives two parameters,
1. error: - The error object which was thrown
2. info: - An object with a componentStack key contains the information about which component threw the error.
The method structure would be as follows
```javascript
componentDidCatch(error, info);
```
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232. ### In which scenarios error boundaries do not catch errors?
Below are the cases in which error boundaries doesn't work,
1. Inside Event handlers
2. Asynchronous code using **setTimeout or requestAnimationFrame** callbacks
3. During Server side rendering
4. When errors thrown in the error boundary code itself
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233. ### Why do you not need error boundaries for event handlers?
Error boundaries do not catch errors inside event handlers.
React doesn’t need error boundaries to recover from errors in event handlers. Unlike the render method and lifecycle methods, the event handlers don’t happen during rendering. So if they throw, React still knows what to display on the screen.
If you need to catch an error inside an event handler, use the regular JavaScript try / catch statement:
```javascript
class MyComponent extends React.Component {
constructor(props) {
super(props);
this.state = { error: null };
this.handleClick = this.handleClick.bind(this);
}
handleClick() {
try {
// Do something that could throw
} catch (error) {
this.setState({ error });
}
}
render() {
if (this.state.error) {
return Click Me ;
}
}
```
Note that the above example is demonstrating regular JavaScript behavior and doesn’t use error boundaries.
**[⬆ Back to Top](#table-of-contents)**
234. ### 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.
For example, the try catch block used for below imperative code
```javascript
try {
showButton();
} catch (error) {
// ...
}
```
Whereas error boundaries wrap declarative code as below,
```javascript
```
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.
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235. ### 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. The reason behind this decision is that it is worse to leave corrupted UI in place than to completely remove it. For example, it is worse for a payments app to display a wrong amount than to render nothing.
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236. ### 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 follow either of these approaches,
1. You can wrap top-level route components to display a generic error message for the entire application.
2. You can also wrap individual components in an error boundary to protect them from crashing the rest of the application.
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237. ### 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.
For example, BuggyCounter component displays the component stack trace as below,
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238. ### What is the required method to be defined for a class component?
The `render()` method is the only required method in a class component. i.e, All methods other than render method are optional for a class component.
**[⬆ Back to Top](#table-of-contents)**
239. ### What are the possible return types of render method?
Below are the list of following types used and return from render method,
1. **React elements:** Elements that instruct React to render a DOM node. It includes html elements such as `
` and user defined elements.
2. **Arrays and fragments:** Return multiple elements to render as Arrays and Fragments to wrap multiple elements
3. **Portals:** Render children into a different DOM subtree.
4. **String and numbers:** Render both Strings and Numbers as text nodes in the DOM
5. **Booleans or null:** Doesn't render anything but these types are used to conditionally render content.
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240. ### What is the main purpose of constructor?
The constructor is mainly used for two purposes,
1. To initialize local state by assigning object to this.state
2. For binding event handler methods to the instance
For example, the below code covers both the above cases,
```javascript
constructor(props) {
super(props);
// Don't call this.setState() here!
this.state = { counter: 0 };
this.handleClick = this.handleClick.bind(this);
}
```
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241. ### Is it mandatory to define constructor for React component?
No, it is not mandatory. i.e, If you don’t initialize state and you don’t bind methods, you don’t need to implement a constructor for your React component.
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242. ### What are default props?
The defaultProps are defined as a property on the component class to set the default props for the class. This is used for undefined props, but not for null props.
For example, let us create color default prop for the button component,
```javascript
class MyButton extends React.Component {
// ...
}
MyButton.defaultProps = {
color: "red",
};
```
If `props.color` is not provided then it will set the default value to 'red'. i.e, Whenever you try to access the color prop it uses default value
```javascript
render() {
return
Load
);
}
}
export default App;
```
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251. ### What is the benefit of strict mode?
The will be helpful in the below cases
1. Identifying components with **unsafe lifecycle methods**.
2. Warning about **legacy string ref** API usage.
3. Detecting unexpected **side effects**.
4. Detecting **legacy context** API.
5. Warning about deprecated findDOMNode usage
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252. ### What are Keyed Fragments?
The Fragments declared with the explicit syntax may have keys. The general use case is mapping a collection to an array of fragments as below,
```javascript
function Glossary(props) {
return (
{props.items.map((item) => (
// Without the `key`, React will fire a key warning
{item.term}
{item.description}
))}
);
}
```
**Note:** key is the only attribute that can be passed to Fragment. In the future, there might be a support for additional attributes, such as event handlers.
**[⬆ Back to Top](#table-of-contents)**
253. ### Does React support all HTML attributes?
As of React 16, both standard or custom DOM attributes are fully supported. Since React components often take both custom and DOM-related props, React uses the camelCase convention just like the DOM APIs.
Let us take few props with respect to standard HTML attributes,
```javascript
// Just like node.tabIndex DOM API
// Just like node.className DOM API
this.handleClick()}>Click Me ;
}
}
```
**Note:** Using an arrow function in render method creates a new function each time the component renders, which may have performance implications
**[⬆ Back to Top](#table-of-contents)**
260. ### How to prevent a function from being called multiple times?
If you use an event handler such as **onClick or onScroll** and want to prevent the callback from being fired too quickly, then you can limit the rate at which callback is executed. This can be achieved in the below possible ways,
1. **Throttling:** Changes based on a time based frequency. For example, it can be used using \_.throttle lodash function
2. **Debouncing:** Publish changes after a period of inactivity. For example, it can be used using \_.debounce lodash function
3. **RequestAnimationFrame throttling:** Changes based on requestAnimationFrame. For example, it can be used using raf-schd lodash function
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261. ### How JSX prevents Injection Attacks?
React DOM escapes any values embedded in JSX before rendering them. Thus it ensures that you can never inject anything that’s not explicitly written in your application. Everything is converted to a string before being rendered.
For example, you can embed user input as below,
```javascript
const name = response.potentiallyMaliciousInput;
const element =
this.updateUser(userId, e)}>Update User details
Update User details
```
In the both approaches, the synthetic argument `e` is passed as a second argument. You need to pass it explicitly for arrow functions and it will be passed automatically for `bind` method.
**[⬆ Back to Top](#table-of-contents)**
266. ### How to prevent component from rendering?
You can prevent component from rendering by returning null based on specific condition. This way it can conditionally render component.
```javascript
function Greeting(props) {
if (!props.loggedIn) {
return null;
}
return welcome, {props.name}
;
}
```
```javascript
class User extends React.Component {
constructor(props) {
super(props);
this.state = {loggedIn: false, name: 'John'};
}
render() {
return (
//Prevent component render if it is not loggedIn
);
}
```
In the above example, the `greeting` component skips its rendering section by applying condition and returning null value.
**[⬆ Back to Top](#table-of-contents)**
267. ### What are the conditions to safely use the index as a key?
There are three conditions to make sure, it is safe use the index as a key.
1. The list and items are static– they are not computed and do not change
2. The items in the list have no ids
3. The list is never reordered or filtered.
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268. ### Should keys be globally unique?
The keys used within arrays should be unique among their siblings but they don’t need to be globally unique. i.e, You can use the same keys with two different arrays.
For example, the below `Book` component uses two arrays with different arrays,
```javascript
function Book(props) {
const index = (
{props.pages.map((page) => (
{page.title}
))}
);
const content = props.pages.map((page) => (
{page.content}
{page.pageNumber}
));
return (
{index}
);
}
}
export default App;
```
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273. ### What is dynamic import?
You can achieve code-splitting in your app using dynamic import.
Let's take an example of addition,
1. **Normal Import**
```javascript
import { add } from "./math";
console.log(add(10, 20));
```
2. **Dynamic Import**
```javascript
import("./math").then((math) => {
console.log(math.add(10, 20));
});
```
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274. ### What are loadable components?
If you want to do code-splitting in a server rendered app, it is recommend to use Loadable Components because React.lazy and Suspense is not yet available for server-side rendering. Loadable lets you render a dynamic import as a regular component.
Lets take an example,
```javascript
import loadable from "@loadable/component";
const OtherComponent = loadable(() => import("./OtherComponent"));
function MyComponent() {
return (
);
}
```
Now OtherComponent will be loaded in a separated bundle
**[⬆ Back to Top](#table-of-contents)**
275. ### What is suspense component?
If the module containing the dynamic import is not yet loaded by the time parent component renders, you must show some fallback content while you’re waiting for it to load using a loading indicator. This can be done using **Suspense** component.
For example, the below code uses suspense component,
```javascript
const OtherComponent = React.lazy(() => import("./OtherComponent"));
function MyComponent() {
return (
Loading...
}>
);
}
```
As mentioned in the above code, Suspense is wrapped above the lazy component.
**[⬆ Back to Top](#table-of-contents)**
276. ### What is route based code splitting?
One of the best place to do code splitting is with routes. The entire page is going to re-render at once so users are unlikely to interact with other elements in the page at the same time. Due to this, the user experience won't be disturbed.
Let us take an example of route based website using libraries like React Router with React.lazy,
```javascript
import { BrowserRouter as Router, Route, Switch } from "react-router-dom";
import React, { Suspense, lazy } from "react";
const Home = lazy(() => import("./routes/Home"));
const About = lazy(() => import("./routes/About"));
const App = () => (
Loading... }>
);
```
In the above code, the code splitting will happen at each route level.
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277. ### Give an example on How to use context?
**Context** is designed to share data that can be considered **global** for a tree of React components.
For example, in the code below lets manually thread through a “theme” prop in order to style the Button component.
```javascript
//Lets create a context with a default theme value "luna"
const ThemeContext = React.createContext("luna");
// Create App component where it uses provider to pass theme value in the tree
class App extends React.Component {
render() {
return (
);
}
}
// A middle component where you don't need to pass theme prop anymore
function Toolbar(props) {
return (
);
}
// Lets read theme value in the button component to use
class ThemedButton extends React.Component {
static contextType = ThemeContext;
render() {
return
;
}
}
```
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278. ### What is the purpose of default value in context?
The defaultValue argument is only used when a component does not have a matching Provider above it in the tree. This can be helpful for testing components in isolation without wrapping them.
Below code snippet provides default theme value as Luna.
```javascript
const MyContext = React.createContext(defaultValue);
```
**[⬆ Back to Top](#table-of-contents)**
279. ### How do you use contextType?
ContextType is used to consume the context object. The contextType property can be used in two ways,
1. **contextType as property of class:**
The contextType property on a class can be assigned a Context object created by React.createContext(). After that, you can consume the nearest current value of that Context type using this.context in any of the lifecycle methods and render function.
Lets assign contextType property on MyClass as below,
```javascript
class MyClass extends React.Component {
componentDidMount() {
let value = this.context;
/* perform a side-effect at mount using the value of MyContext */
}
componentDidUpdate() {
let value = this.context;
/* ... */
}
componentWillUnmount() {
let value = this.context;
/* ... */
}
render() {
let value = this.context;
/* render something based on the value of MyContext */
}
}
MyClass.contextType = MyContext;
```
2. **Static field**
You can use a static class field to initialize your contextType using public class field syntax.
```javascript
class MyClass extends React.Component {
static contextType = MyContext;
render() {
let value = this.context;
/* render something based on the value */
}
}
```
**[⬆ Back to Top](#table-of-contents)**
280. ### What is a consumer?
A Consumer is a React component that subscribes to context changes. It requires a function as a child which receives current context value as argument and returns a react node. The value argument passed to the function will be equal to the value prop of the closest Provider for this context above in the tree.
Lets take a simple example,
```javascript
{value => /* render something based on the context value */}
```
**[⬆ Back to Top](#table-of-contents)**
281. ### How do you solve performance corner cases while using context?
The context uses reference identity to determine when to re-render, there are some gotchas that could trigger unintentional renders in consumers when a provider’s parent re-renders.
For example, the code below will re-render all consumers every time the Provider re-renders because a new object is always created for value.
```javascript
class App extends React.Component {
render() {
return (
);
}
}
```
This can be solved by lifting up the value to parent state,
```javascript
class App extends React.Component {
constructor(props) {
super(props);
this.state = {
value: { something: "something" },
};
}
render() {
return (
);
}
}
```
**[⬆ Back to Top](#table-of-contents)**
282. ### What is the purpose of forward ref in HOCs?
Refs will not get passed through because ref is not a prop. It is handled differently by React just like **key**. If you add a ref to a HOC, the ref will refer to the outermost container component, not the wrapped component. In this case, you can use Forward Ref API. For example, we can explicitly forward refs to the inner FancyButton component using the React.forwardRef API.
The below HOC logs all props,
```javascript
function logProps(Component) {
class LogProps extends React.Component {
componentDidUpdate(prevProps) {
console.log("old props:", prevProps);
console.log("new props:", this.props);
}
render() {
const { forwardedRef, ...rest } = this.props;
// Assign the custom prop "forwardedRef" as a ref
return
;
}
}
return React.forwardRef((props, ref) => {
return
;
});
}
```
Let's use this HOC to log all props that get passed to our “fancy button” component,
```javascript
class FancyButton extends React.Component {
focus() {
// ...
}
// ...
}
export default logProps(FancyButton);
```
Now let's create a ref and pass it to FancyButton component. In this case, you can set focus to button element.
```javascript
import FancyButton from "./FancyButton";
const ref = React.createRef();
ref.current.focus();
;
```
**[⬆ Back to Top](#table-of-contents)**
283. ### Is ref argument available for all functions or class components?
Regular function or class components don’t receive the ref argument, and ref is not available in props either. The second ref argument only exists when you define a component with React.forwardRef call.
**[⬆ Back to Top](#table-of-contents)**
284. ### Why do you need additional care for component libraries while using forward refs?
When you start using forwardRef in a component library, you should treat it as a breaking change and release a new major version of your library. This is because your library likely has a different behavior such as what refs get assigned to, and what types are exported. These changes can break apps and other libraries that depend on the old behavior.
**[⬆ Back to Top](#table-of-contents)**
285. ### How to create react class components without ES6?
If you don’t use ES6 then you may need to use the create-react-class module instead. For default props, you need to define getDefaultProps() as a function on the passed object. Whereas for initial state, you have to provide a separate getInitialState method that returns the initial state.
```javascript
var Greeting = createReactClass({
getDefaultProps: function () {
return {
name: "Jhohn",
};
},
getInitialState: function () {
return { message: this.props.message };
},
handleClick: function () {
console.log(this.state.message);
},
render: function () {
return
;
},
});
```
**Note:** If you use createReactClass then auto binding is available for all methods. i.e, You don't need to use `.bind(this)` with in constructor for event handlers.
**[⬆ Back to Top](#table-of-contents)**
286. ### Is it possible to use react without JSX?
Yes, JSX is not mandatory for using React. Actually it is convenient when you don’t want to set up compilation in your build environment. Each JSX element is just syntactic sugar for calling `React.createElement(component, props, ...children)`.
For example, let us take a greeting example with JSX,
```javascript
class Greeting extends React.Component {
render() {
return
Hello {this.props.message}
;
}
}
ReactDOM.render(
,
document.getElementById("root")
);
```
You can write the same code without JSX as below,
```javascript
class Greeting extends React.Component {
render() {
return React.createElement("div", null, `Hello ${this.props.message}`);
}
}
ReactDOM.render(
React.createElement(Greeting, { message: "World" }, null),
document.getElementById("root")
);
```
**[⬆ Back to Top](#table-of-contents)**
287. ### What is diffing algorithm?
React needs to use algorithms to find out how to efficiently update the UI to match the most recent tree. The diffing algorithms is generating the minimum number of operations to transform one tree into another. However, the algorithms have a complexity in the order of O(n³) where n is the number of elements in the tree.
In this case, displaying 1000 elements would require in the order of one billion comparisons. This is far too expensive. Instead, React implements a heuristic O(n) algorithm based on two assumptions:
1. Two elements of different types will produce different trees.
2. The developer can hint at which child elements may be stable across different renders with a key prop.
**[⬆ Back to Top](#table-of-contents)**
288. ### What are the rules covered by diffing algorithm?
When diffing two trees, React first compares the two root elements. The behavior is different depending on the types of the root elements. It covers the below rules during reconciliation algorithm,
1. **Elements Of Different Types:**
Whenever the root elements have different types, React will tear down the old tree and build the new tree from scratch. For example, elements
to to of different types lead a full rebuild.
2. **DOM Elements Of The Same Type:**
When comparing two React DOM elements of the same type, React looks at the attributes of both, keeps the same underlying DOM node, and only updates the changed attributes. Lets take an example with same DOM elements except className attribute,
```javascript
```
3. **Component Elements Of The Same Type:**
When a component updates, the instance stays the same, so that state is maintained across renders. React updates the props of the underlying component instance to match the new element, and calls componentWillReceiveProps() and componentWillUpdate() on the underlying instance. After that, the render() method is called and the diff algorithm recurses on the previous result and the new result.
4. **Recursing On Children:**
when recursing on the children of a DOM node, React just iterates over both lists of children at the same time and generates a mutation whenever there’s a difference. For example, when adding an element at the end of the children, converting between these two trees works well.
```javascript
```
5. **Handling keys:**
React supports a key attribute. When children have keys, React uses the key to match children in the original tree with children in the subsequent tree. For example, adding a key can make the tree conversion efficient,
```javascript
Connecticut
Duke
Villanova
```
**[⬆ Back to Top](#table-of-contents)**
289. ### When do you need to use refs?
There are few use cases to go for refs,
1. Managing focus, text selection, or media playback.
2. Triggering imperative animations.
3. Integrating with third-party DOM libraries.
**[⬆ Back to Top](#table-of-contents)**
290. ### Must prop be named as render for render props?
Even though the pattern named render props, you don’t have to use a prop named render to use this pattern. i.e, Any prop that is a function that a component uses to know what to render is technically a “render prop”. Lets take an example with the children prop for render props,
```javascript
(
The mouse position is {mouse.x}, {mouse.y}
)}
/>
```
Actually children prop doesn’t need to be named in the list of “attributes” in JSX element. Instead, you can keep it directly inside element,
```javascript
{(mouse) => (
The mouse position is {mouse.x}, {mouse.y}
)}
```
While using this above technique(without any name), explicitly state that children should be a function in your propTypes.
```javascript
Mouse.propTypes = {
children: PropTypes.func.isRequired,
};
```
**[⬆ Back to Top](#table-of-contents)**
291. ### What are the problems of using render props with pure components?
If you create a function inside a render method, it negates the purpose of pure component. Because the shallow prop comparison will always return false for new props, and each render in this case will generate a new value for the render prop. You can solve this issue by defining the render function as instance method.
**[⬆ Back to Top](#table-of-contents)**
292. ### How do you create HOC using render props?
You can implement most higher-order components (HOC) using a regular component with a render prop. For example, if you would prefer to have a withMouse HOC instead of a component, you could easily create one using a regular with a render prop.
```javascript
function withMouse(Component) {
return class extends React.Component {
render() {
return (
My JavaScript variable is {String(myVariable)}.
```
**[⬆ Back to Top](#table-of-contents)**
295. ### What is the typical use case of portals?
React portals are very useful when a parent component has overflow: hidden or has properties that affect the stacking context (e.g. z-index, position, opacity) and you need to visually “break out” of its container.
For example, dialogs, global message notifications, hovercards, and tooltips.
**[⬆ Back to Top](#table-of-contents)**
296. ### How do you set default value for uncontrolled component?
In React, the value attribute on form elements will override the value in the DOM. With an uncontrolled component, you might want React to specify the initial value, but leave subsequent updates uncontrolled. To handle this case, you can specify a **defaultValue** attribute instead of **value**.
```javascript
render() {
return (
);
}
```
The same applies for `select` and `textArea` inputs. But you need to use **defaultChecked** for `checkbox` and `radio` inputs.
**[⬆ Back to Top](#table-of-contents)**
297. ### What is your favorite React stack?
Even though the tech stack varies from developer to developer, the most popular stack is used in react boilerplate project code. It mainly uses Redux and redux-saga for state management and asynchronous side-effects, react-router for routing purpose, styled-components for styling react components, axios for invoking REST api, and other supported stack such as webpack, reselect, ESNext, Babel.
You can clone the project https://github.com/react-boilerplate/react-boilerplate and start working on any new react project.
**[⬆ Back to Top](#table-of-contents)**
298. ### What is the difference between Real DOM and Virtual DOM?
Below are the main differences between Real DOM and Virtual DOM,
| Real DOM | Virtual DOM |
| ------------------------------------ | ------------------------------------ |
| Updates are slow | Updates are fast |
| DOM manipulation is very expensive. | DOM manipulation is very easy |
| You can update HTML directly. | You Can’t directly update HTML |
| It causes too much of memory wastage | There is no memory wastage |
| Creates a new DOM if element updates | It updates the JSX if element update |
**[⬆ Back to Top](#table-of-contents)**
299. ### How to add Bootstrap to a react application?
Bootstrap can be added to your React app in a three possible ways,
1. Using the Bootstrap CDN:
This is the easiest way to add bootstrap. Add both bootstrap CSS and JS resources in a head tag.
2. Bootstrap as Dependency:
If you are using a build tool or a module bundler such as Webpack, then this is the preferred option for adding Bootstrap to your React application
```javascript
npm install bootstrap
```
3. React Bootstrap Package:
In this case, you can add Bootstrap to our React app is by using a package that has rebuilt Bootstrap components to work particularly as React components. Below packages are popular in this category,
1. react-bootstrap
2. reactstrap
**[⬆ Back to Top](#table-of-contents)**
300. ### Can you list down top websites or applications using react as front end framework?
Below are the `top 10 websites` using React as their front-end framework,
1. Facebook
2. Uber
3. Instagram
4. WhatsApp
5. Khan Academy
6. Airbnb
7. Dropbox
8. Flipboard
9. Netflix
10. PayPal
**[⬆ Back to Top](#table-of-contents)**
301. ### Is it recommended to use CSS In JS technique in React?
React does not have any opinion about how styles are defined but if you are a beginner then good starting point is to define your styles in a separate \*.css file as usual and refer to them using className. This functionality is not part of React but came from third-party libraries. But If you want to try a different approach(CSS-In-JS) then styled-components library is a good option.
**[⬆ Back to Top](#table-of-contents)**
302. ### Do I need to rewrite all my class components with hooks?
No. But you can try Hooks in a few components(or new components) without rewriting any existing code. Because there are no plans to remove classes in ReactJS.
**[⬆ Back to Top](#table-of-contents)**
303. ### How to fetch data with React Hooks?
The effect hook called `useEffect` is used to fetch the data with axios from the API and to set the data in the local state of the component with the state hook’s update function.
Let's take an example in which it fetches list of react articles from the API
```javascript
import React, { useState, useEffect } from "react";
import axios from "axios";
function App() {
const [data, setData] = useState({ hits: [] });
useEffect(() => {
(async () => {
const result = await axios(
"http://hn.algolia.com/api/v1/search?query=react"
);
setData(result.data);
})();
}, []);
return (
);
}
export default App;
```
Remember we provided an empty array as second argument to the effect hook to avoid activating it on component updates but only on mounting of the component. i.e, It fetches only on component mount.
**[⬆ Back to Top](#table-of-contents)**
304. ### Is Hooks cover all use cases for classes?
Hooks doesn't cover all use cases of classes but there is a plan to add them soon. Currently there are no Hook equivalents to the uncommon **getSnapshotBeforeUpdate** and **componentDidCatch** lifecycles yet.
**[⬆ Back to Top](#table-of-contents)**
305. ### What is the stable release for hooks support?
React includes a stable implementation of React Hooks in 16.8 release for below packages
1. React DOM
2. React DOM Server
3. React Test Renderer
4. React Shallow Renderer
**[⬆ Back to Top](#table-of-contents)**
306. ### Why do we use array destructuring (square brackets notation) in `useState`?
When we declare a state variable with `useState`, it returns a pair — an array with two items. The first item is the current value, and the second is a function that updates the value. Using [0] and [1] to access them is a bit confusing because they have a specific meaning. This is why we use array destructuring instead.
For example, the array index access would look as follows:
```javascript
var userStateVariable = useState("userProfile"); // Returns an array pair
var user = userStateVariable[0]; // Access first item
var setUser = userStateVariable[1]; // Access second item
```
Whereas with array destructuring the variables can be accessed as follows:
```javascript
const [user, setUser] = useState("userProfile");
```
**[⬆ Back to Top](#table-of-contents)**
307. ### What are the sources used for introducing hooks?
Hooks got the ideas from several different sources. Below are some of them,
1. Previous experiments with functional APIs in the react-future repository
2. Community experiments with render prop APIs such as Reactions Component
3. State variables and state cells in DisplayScript.
4. Subscriptions in Rx.
5. Reducer components in ReasonReact.
**[⬆ Back to Top](#table-of-contents)**
308. ### How do you access imperative API of web components?
Web Components often expose an imperative API to implement its functions. You will need to use a **ref** to interact with the DOM node directly if you want to access imperative API of a web component. But if you are using third-party Web Components, the best solution is to write a React component that behaves as a **wrapper** for your Web Component.
**[⬆ Back to Top](#table-of-contents)**
309. ### What is formik?
Formik is a small react form library that helps you with the three major problems,
1. Getting values in and out of form state
2. Validation and error messages
3. Handling form submission
**[⬆ Back to Top](#table-of-contents)**
310. ### What are typical middleware choices for handling asynchronous calls in Redux?
Some of the popular middleware choices for handling asynchronous calls in Redux eco system are `Redux Thunk, Redux Promise, Redux Saga`.
**[⬆ Back to Top](#table-of-contents)**
311. ### Do browsers understand JSX code?
No, browsers can't understand JSX code. You need a transpiler to convert your JSX to regular Javascript that browsers can understand. The most widely used transpiler right now is Babel.
**[⬆ Back to Top](#table-of-contents)**
312. ### Describe about data flow in react?
React implements one-way reactive data flow using props which reduce boilerplate and is easier to understand than traditional two-way data binding.
**[⬆ Back to Top](#table-of-contents)**
313. ### What is react scripts?
The `react-scripts` package is a set of scripts from the create-react-app starter pack which helps you kick off projects without configuring. The `react-scripts start` command sets up the development environment and starts a server, as well as hot module reloading.
**[⬆ Back to Top](#table-of-contents)**
314. ### What are the features of create react app?
Below are the list of some of the features provided by create react app.
1. React, JSX, ES6, Typescript and Flow syntax support.
2. Autoprefixed CSS
3. CSS Reset/Normalize
4. A live development server
5. A fast interactive unit test runner with built-in support for coverage reporting
6. A build script to bundle JS, CSS, and images for production, with hashes and sourcemaps
7. An offline-first service worker and a web app manifest, meeting all the Progressive Web App criteria.
**[⬆ Back to Top](#table-of-contents)**
315. ### What is the purpose of renderToNodeStream method?
The `ReactDOMServer#renderToNodeStream` method is used to generate HTML on the server and send the markup down on the initial request for faster page loads. It also helps search engines to crawl your pages easily for SEO purposes.
**Note:** Remember this method is not available in the browser but only server.
**[⬆ Back to Top](#table-of-contents)**
316. ### What is MobX?
MobX is a simple, scalable and battle tested state management solution for applying functional reactive programming (TFRP). For reactJs application, you need to install below packages,
```bash
npm install mobx --save
npm install mobx-react --save
```
**[⬆ Back to Top](#table-of-contents)**
317. ### What are the differences between Redux and MobX?
Below are the main differences between Redux and MobX,
| Topic | Redux | MobX |
| ------------- | ------------------------------------------------------------- | ---------------------------------------------------------------------- |
| Definition | It is a javascript library for managing the application state | It is a library for reactively managing the state of your applications |
| Programming | It is mainly written in ES6 | It is written in JavaScript(ES5) |
| Data Store | There is only one large store exist for data storage | There is more than one store for storage |
| Usage | Mainly used for large and complex applications | Used for simple applications |
| Performance | Need to be improved | Provides better performance |
| How it stores | Uses JS Object to store | Uses observable to store the data |
**[⬆ Back to Top](#table-of-contents)**
318. ### Should I learn ES6 before learning ReactJS?
No, you don’t have to learn es2015/es6 to learn react. But you may find many resources or React ecosystem uses ES6 extensively. Let's see some of the frequently used ES6 features,
1. **Destructuring:** To get props and use them in a component
```javascript
// in es 5
var someData = this.props.someData;
var dispatch = this.props.dispatch;
// in es6
const { someData, dispatch } = this.props;
```
2. Spread operator: Helps in passing props down into a component
```javascript
// in es 5
{user.name} ;
});
// es 6
const users = usersList.map((user) => {user.name} );
```
**[⬆ Back to Top](#table-of-contents)**
319. ### What is Concurrent Rendering?
The Concurrent rendering makes React apps to be more responsive by rendering component trees without blocking the main UI thread. It allows React to interrupt a long-running render to handle a high-priority event. i.e, When you enabled concurrent Mode, React will keep an eye on other tasks that need to be done, and if there's something with a higher priority it will pause what it is currently rendering and let the other task finish first. You can enable this in two ways,
```javascript
// 1. Part of an app by wrapping with ConcurrentMode
;
// 2. Whole app using createRoot
ReactDOM.unstable_createRoot(domNode).render(` and create a security hole.
```javascript
const companyProfile = {
website: "javascript: alert('Your website is hacked')",
};
// It will log a warning
More details ;
```
Remember that the future versions will throw an error for javascript URLs.
**[⬆ Back to Top](#table-of-contents)**
322. ### What is the purpose of eslint plugin for hooks?
The ESLint plugin enforces rules of Hooks to avoid bugs. It assumes that any function starting with ”use” and a capital letter right after it is a Hook. In particular, the rule enforces that,
1. Calls to Hooks are either inside a PascalCase function (assumed to be a component) or another useSomething function (assumed to be a custom Hook).
2. Hooks are called in the same order on every render.
**[⬆ Back to Top](#table-of-contents)**
323. ### What is the difference between Imperative and Declarative in React?
Imagine a simple UI component, such as a "Like" button. When you tap it, it turns blue if it was previously grey, and grey if it was previously blue.
The imperative way of doing this would be:
```javascript
if (user.likes()) {
if (hasBlue()) {
removeBlue();
addGrey();
} else {
removeGrey();
addBlue();
}
}
```
Basically, you have to check what is currently on the screen and handle all the changes necessary to redraw it with the current state, including undoing the changes from the previous state. You can imagine how complex this could be in a real-world scenario.
In contrast, the declarative approach would be:
```javascript
if (this.state.liked) {
return
,
document.getElementById("root")
);
```
**App.js**
```js
const authContext = useContext(AuthContext);
const { loadUser } = authContext;
useEffect(() => {
loadUser();
}, []);
```
**loadUser**
```js
const loadUser = async () => {
const token = sessionStorage.getItem("token");
if (!token) {
dispatch({
type: ERROR,
});
}
setAuthToken(token);
try {
const res = await axios("/api/auth");
dispatch({
type: USER_LOADED,
payload: res.data.data,
});
} catch (err) {
console.error(err);
}
};
```
**[⬆ Back to Top](#table-of-contents)**
326. ### What are the benefits of new JSX transform?
There are three major benefits of new JSX transform,
1. It is possible to use JSX without importing React packages
2. The compiled output might improve the bundle size in a small amount
3. The future improvements provides the flexibility to reduce the number of concepts to learn React.
**[⬆ Back to Top](#table-of-contents)**
327. ### How is the new JSX transform different from old transform??
The new JSX transform doesn’t require React to be in scope. i.e, You don't need to import React package for simple scenarios.
Let's take an example to look at the main differences between the old and the new transform,
**Old Transform:**
```js
import React from "react";
function App() {
return ` passing the store to React components
4. Small "counter" example to demo how to add redux logic and React-Redux hooks API to interact with the store from components
The below commands need to be executed along with template option as below,
5. **Javascript template:**
```js
npx create-react-app my-app --template redux
```
2. **Typescript template:**
```js
npx create-react-app my-app --template redux-typescript
```
**[⬆ Back to Top](#table-of-contents)**
329. ### What are React Server components?
React Server Component is a way to write React component that gets rendered in the server-side with the purpose of improving React app performance. These components allow us to load components from the backend.
**Note:** React Server Components is still under development and not recommended for production yet.
**[⬆ Back to Top](#table-of-contents)**
330. ### What is prop drilling?
Prop Drilling is the process by which you pass data from one component of the React Component tree to another by going through other components that do not need the data but only help in passing it around.
**[⬆ Back to Top](#table-of-contents)**
331. ### What is state mutation and how to prevent it?
`State mutation` happens when you try to update the state of a component without actually using `setState` function. This can happen when you are trying to do some computations using a state variable and unknowingly save the result in the same state variable. This is the main reason why it is advised to return new instances of state variables from the reducers by using Object.assign({}, ...) or spread syntax.
This can cause unknown issues in the UI as the value of the state variable got updated without telling React to check what all components were being affected from this update and it can cause UI bugs.
Ex:
```javascript
class A extends React.component {
constructor(props) {
super(props);
this.state = {
loading: false
}
}
componentDidMount() {
let { loading } = this.state;
loading = (() => true)(); // Trying to perform an operation and directly saving in a state variable
}
```
**How to prevent it:** Make sure your state variables are immutable by either enforcing immutability by using plugins like Immutable.js, always using `setState` to make updates, and returning new instances in reducers when sending updated state values.
**[⬆ Back to Top](#table-of-contents)**
332. ### What is the difference between useState and useRef hook?
1. useState causes components to re-render after state updates whereas useRef doesn’t cause a component to re-render when the value or state changes.
Essentially, useRef is like a “box” that can hold a mutable value in its (.current) property.
2. useState allows us to update the state inside components. While useRef allows refrencing DOM elements.
**[⬆ Back to Top](#table-of-contents)**
333. ### What are the Differences Between Functional and Class Component
## Class Component syntax
```
class Example extends Reacts.Component {
render(){
return
A simple example showing how we use the setState() and constructor()
```js
class Example extends Component {
constructor() {
super();
this.state = {
example: "This is a class component",
};
}
changeText() {
this.setState({
example: "implementing the setState() in class component",
});
}
render() {
return (
<>
{
this.changeText();
}}>
Click!!
);
}
}
```
## Using Sates in Functional Components
Initially, we could not use state in functional components because it was only supported in class components. over the years hooks were implemented in functional component. The hooks that enable us to use state in functional component is called useState
The useState( ) hook returns an array of the current state and a function ( setState) that we can use to update the value of the current state. The array is being destructured so we can can see the variables the array holds that is, the initial state and the updated state lets see an example
```js
function App() {
const [country, setCountry] = useState("i'm a Nigerian");
const change = () => {
setCountry("i am a Canadian");
};
return (
Change
);
}
```
);
}
}
```
## Props in Functional Components
Props in functional components are similar to that of the class components, the difference is the absence of the 'this' keyword. We also destructure in a similar way without the 'this' keyword. they are also immutable in the functional component
```js
function Food(props) {
return
);
}
```
> As you can see in the example above there wasn't a need for the 'this' keyword.
```js
function Food(props) {
const { fav } = props;
return
);
}
```
summary:
* The functional components doesn't require the render method
* Class component require the render() method that returns JSX.
To use states in functional component we use the **useState** hook.
To use State in class components we use the constructor method and the setState function.
Functional components use the useEffect hook instead of lifecycle method [useEffect](https://www.w3schools.com/react/react_useeffect.asp )
Class components uses Lifecycle method **componentWillUnmount** etc. [ Lifecycle methods](https://www.w3schools.com/react/react_lifecycle.asp )
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## Disclaimer
The questions provided in this repository are the summary of frequently asked questions across numerous companies. We cannot guarantee that these questions will actually be asked during your interview process, nor should you focus on memorizing all of them. The primary purpose is for you to get a sense of what some companies might ask — do not get discouraged if you don't know the answer to all of them — that is ok!