ReactJS Flashcards
to learn reactjs
What is React?
React is an open-source frontend JavaScript library which is used for building user interfaces especially for single page applications. It is used for handling view layer for web and mobile apps. React was created by Jordan Walke, a software engineer working for Facebook. React was first deployed on Facebook’s News Feed in 2011 and on Instagram in 2012.
What are the major features of React?
The major features of React are:
It uses VirtualDOM instead RealDOM considering that RealDOM manipulations are expensive.
Supports server-side rendering.
Follows Unidirectional* data flow or data binding.
Uses reusable/composable UI components to develop the view.
What is JSX?
JSX is a XML-like syntax extension to ECMAScript (the acronym stands for JavaScript XML). Basically it just provides syntactic sugar for the React.createElement() function, giving us expressiveness of JavaScript along with HTML like template syntax.
In the example below text inside <h1> tag return as JavaScript function to the render function.
class App extends React.Component {
render() {
return(
<div>
<h1>{'Welcome to React world!'}</h1>
</div>
)
}
}</h1>
What is the difference between Element and Component?
An Element is a plain object describing what you want to appear on the screen in terms of the DOM nodes or other components. Elements can contain other Elements in their props. Creating a React element is cheap. Once an element is created, it is never mutated.
The object representation of React Element would be as follows:
const element = React.createElement(
'div',
{id: 'login-btn'},
'Login'
)
The above React.createElement() function returns an object:
{
type: 'div',
props: {
children: 'Login',
id: 'login-btn'
}
}
And finally it renders to the DOM using ReactDOM.render():
<div>Login</div> Whereas a component can be declared in several different ways. It can be a class with a render() method. Alternatively, in simple cases, it can be defined as a function. In either case, it takes props as an input, and returns a JSX tree as the output:
const Button = ({ onLogin }) =>
<div>Login</div>
Then JSX gets transpiled to a React.createElement() function tree:
const Button = ({ onLogin }) => React.createElement(
'div',
{ id: 'login-btn', onClick: onLogin },
'Login'
)
How to create components in React?
There are two possible ways to create a component.
Function Components: This is the simplest way to create a component. Those are pure JavaScript functions that accept props object as first parameter and return React elements:
function Greeting({ message }) {
return <h1>{`Hello, ${message}`}</h1>
}
Class Components: You can also use ES6 class to define a component. The above function component can be written as:
class Greeting extends React.Component {
render() {
return <h1>{`Hello, ${this.props.message}`}</h1>
}
}
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.
What are Pure Components?
React.PureComponent is exactly the same as React.Component except that it handles the shouldComponentUpdate() method for you. When props or state changes, PureComponent will do a shallow comparison on both props and state. Component on the other hand won’t compare current props and state to next out of the box. Thus, the component will re-render by default whenever shouldComponentUpdate is called.
What is state in React?
State of a component is an object that holds some information that may change over the lifetime of the component. We should always try to make our state as simple as possible and minimize the number of stateful components. Let’s create an user component with message state,
class User extends React.Component {
constructor(props) {
super(props)
this.state = {
message: 'Welcome to React world'
} }
render() {
return (
<div>
<h1>{this.state.message}</h1>
</div>
)
}
}
Difference between state and props
State is similar to props, but it is private and fully controlled by the component. i.e, It is not accessible to any component other than the one that owns and sets it.
What are props in React?
Props are inputs to components. They are single values or objects containing a set of values that are passed to components on creation using a naming convention similar to HTML-tag attributes. They are data passed down from a parent component to a child component.
The primary purpose of props in React is to provide following component functionality:
Pass custom data to your component.
Trigger state changes.
Use via this.props.reactProp inside component’s render() method.
For example, let us create an element with reactProp property:
This reactProp (or whatever you came up with) name then becomes a property attached to React’s native props object which originally already exists on all components created using React library.
props.reactProp
What is the difference between state and props?
Both props and state are plain JavaScript objects. While both of them hold information that influences the output of render, they are different in their functionality with respect to component. Props get passed to the component similar to function parameters whereas state is managed within the component similar to variables declared within a function.
Why should we not update the state directly?
If you try to update state directly then it won’t re-render the component.
//Wrong this.state.message = 'Hello world' Instead use setState() method. It schedules an update to a component's state object. When state changes, the component responds by re-rendering.
//Correct
this.setState({ message: 'Hello World' })
Note: You can directly assign to the state object either in constructor or using latest javascript's class field declaration syntax.
What is the purpose of callback function as an argument of setState()?
The callback function is invoked when setState finished and the component gets rendered. Since setState() is asynchronous the callback function is used for any post action.
Note: It is recommended to use lifecycle method rather than this callback function.
setState({ name: ‘John’ }, () => console.log(‘The name has updated and component re-rendered’))
What is the difference between HTML and React event handling?
In HTML, the event name should be in lowercase:
Whereas in React it follows camelCase convention:
In HTML, you can return false to prevent default behavior:
<a></a>
Whereas in React you must call preventDefault() explicitly:
function handleClick(event) {
event.preventDefault()
console.log(‘The link was clicked.’)
}
How React works? How Virtual-DOM works in React?
React creates a virtual DOM. When state changes in a component it firstly runs a “diffing” algorithm, which identifies what has changed in the virtual DOM. The second step is reconciliation, where it updates the DOM with the results of diff.
The HTML DOM is always tree-structured — which is allowed by the structure of HTML document. The DOM trees are huge nowadays because of large apps. Since we are more and more pushed towards dynamic web apps (Single Page Applications — SPAs), we need to modify the DOM tree incessantly and a lot. And this is a real performance and development pain.
The Virtual DOM is an abstraction of the HTML DOM. It is lightweight and detached from the browser-specific implementation details. It is not invented by React but it uses it and provides it for free. ReactElements lives in the virtual DOM. They make the basic nodes here. Once we defined the elements, ReactElements can be render into the “real” DOM.
Whenever a ReactComponent is changing the state, diff algorithm in React runs and identifies what has changed. And then it updates the DOM with the results of diff. The point is - it’s done faster than it would be in the regular DOM.
What is JSX?
JSX is a syntax extension to JavaScript and comes with the full power of JavaScript. JSX produces React “elements”. You can embed any JavaScript expression in JSX by wrapping it in curly braces. After compilation, JSX expressions become regular JavaScript objects. This means that you can use JSX inside of if statements and for loops, assign it to variables, accept it as arguments, and return it from functions. Eventhough React does not require JSX, it is the recommended way of describing our UI in React app.
What is React.createClass?
React.createClass allows us to generate component "classes." But with ES6, React allows us to implement component classes that use ES6 JavaScript classes. The end result is the same -- we have a component class. But the style is different. And one is using a "custom" JavaScript class system (createClass) while the other is using a "native" JavaScript class system. When using React’s createClass() method, we pass in an object as an argument.
What is ReactDOM and what is the difference between ReactDOM and React?
Prior to v0.14, all ReactDOM functionality was part of React. But later, React and ReactDOM were split into two different libraries.
As the name implies, ReactDOM is the glue between React and the DOM. Often, we will only use it for one single thing: mounting with ReactDOM. Another useful feature of ReactDOM is ReactDOM.findDOMNode() which we can use to gain direct access to a DOM element.
For everything else, there’s React. We use React to define and create our elements, for lifecycle hooks, etc. i.e. the guts of a React application.
What are the differences between a class component and functional component?
Class components allows us to use additional features such as local state and lifecycle hooks. Also, to enable our component to have direct access to our store and thus holds state.
When our component just receives props and renders them to the page, this is a ‘stateless component’, for which a pure function can be used. These are also called dumb components or presentational components.
From the previous question, we can say that our Booklist component is functional components and are stateless.
Q6. What is the difference between state and props?
The state is a data structure that starts with a default value when a Component mounts. It may be mutated across time, mostly as a result of user events.
Props (short for properties) are a Component’s configuration. Props are how components talk to each other. They are received from above component and immutable as far as the Component receiving them is concerned. A Component cannot change its props, but it is responsible for putting together the props of its child Components. Props do not have to just be data — callback functions may be passed in as props.
There is also the case that we can have default props so that props are set even if a parent component doesn’t pass props down.
Props and State do similar things but are used in different ways. The majority of our components will probably be stateless. Props are used to pass data from parent to child or by the component itself. They are immutable and thus will not be changed. State is used for mutable data, or data that will change. This is particularly useful for user input.
What are controlled components?
In HTML, form elements such as , , and typically maintain their own state and update it based on user input. When a user submits a form the values from the aforementioned elements are sent with the form. With React it works differently. The component containing the form will keep track of the value of the input in it’s state and will re-render the component each time the callback function e.g. onChange is fired as the state will be updated. A form element whose value is controlled by React in this way is called a “controlled component”.
With a controlled component, every state mutation will have an associated handler function. This makes it straightforward to modify or validate user input.
What is a higher order component?
A higher-order component (HOC) is an advanced technique in React for reusing component logic. HOCs are not part of the React API. They are a pattern that emerges from React’s compositional nature.
A higher-order component is a function that takes a component and returns a new component. HOC’s allow you to reuse code, logic and bootstrap abstraction. HOCs are common in third-party React libraries. The most common is probably Redux’s connect function. Beyond simply sharing utility libraries and simple composition, HOCs are the best way to share behavior between React Components. If you find yourself writing a lot of code in different places that does the same thing, you may be able to refactor that code into a reusable HOC.
What is create-react-app?
create-react-app is the official CLI (Command Line Interface) for React to create React apps with no build configuration.
We don’t need to install or configure tools like Webpack or Babel. They are preconfigured and hidden so that we can focus on the code. We can install easily just like any other node modules. Then it is just one command to start the React project.
It includes everything we need to build a React app:
React, JSX, ES6, and Flow syntax support.
Language extras beyond ES6 like the object spread operator.
Autoprefixed CSS, so you don’t need -webkit- or other prefixes.
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 Redux?
The basic idea of Redux is that the entire application state is kept in a single store. The store is simply a javascript object. The only way to change the state is by firing actions from your application and then writing reducers for these actions that modify the state. The entire state transition is kept inside reducers and should not have any side-effects.
Redux is based on the idea that there should be only a single source of truth for your application state, be it UI state like which tab is active or Data state like the user profile details.
All of these data is retained by redux in a closure that redux calls a store . It also provides us a recipe of creating the said store, namely createStore(x).
The createStore function accepts another function, x as an argument. The passed in function is responsible for returning the state of the application at that point in time, which is then persisted in the store. This passed in function is known as the reducer.
This is a valid example reducer function:
This store can only be updated by dispatching an action. Our App dispatches an action, it is passed into reducer; the reducer returns a fresh instance of the state; the store notifies our App and it can begin it’s re render as required.
What is Redux Thunk used for?
Redux thunk is middleware that allows us to write action creators that return a function instead of an action. The thunk can then be used to delay the dispatch of an action if a certain condition is met. This allows us to handle the asyncronous dispatching of actions. The inner function receives the store methods dispatch and getState as parameters.
To enable Redux Thunk, we need to use applyMiddleware()
What is PureComponent? When to use PureComponent over Component?
PureComponent is exactly the same as Component except that it handles the shouldComponentUpdate method for us. When props or state changes, PureComponent will do a shallow comparison on both props and state. Component on the other hand won’t compare current props and state to next out of the box. Thus, the component will re-render by default whenever shouldComponentUpdate is called.
When comparing previous props and state to next, a shallow comparison will check that primitives have the same value (eg, 1 equals 1 or that true equals true) and that the references are the same between more complex javascript values like objects and arrays.
It is good to prefer PureComponent over Component whenever we never mutate our objects.
How Virtual-DOM is more efficient than Dirty checking?
In React, each of our components have a state. This state is like an observable. Essentially, React knows when to re-render the scene because it is able to observe when this data changes. Dirty checking is slower than observables because we must poll the data at a regular interval and check all of the values in the data structure recursively. By comparison, setting a value on the state will signal to a listener that some state has changed, so React can simply listen for change events on the state and queue up re-rendering.
The virtual DOM is used for efficient re-rendering of the DOM. This isn’t really related to dirty checking your data. We could re-render using a virtual DOM with or without dirty checking. In fact, the diff algorithm is a dirty checker itself.
We aim to re-render the virtual tree only when the state changes. So using an observable to check if the state has changed is an efficient way to prevent unnecessary re-renders, which would cause lots of unnecessary tree diffs. If nothing has changed, we do nothing.
Is setState() is async? Why is setState() in React Async instead of Sync?
setState() actions are asynchronous and are batched for performance gains. This is explained in documentation as below.
setState() does not immediately mutate this.state but creates a pending state transition. Accessing this.state after calling this method can potentially return the existing value. There is no guarantee of synchronous operation of calls to setState and calls may be batched for performance gains.
This is because setState alters the state and causes rerendering. This can be an expensive operation and making it synchronous might leave the browser unresponsive. Thus the setState calls are asynchronous as well as batched for better UI experience and performance.
What is render() in React? And explain its purpose?
Each React component must have a render() mandatorily. It returns a single React element which is the representation of the native DOM component. If more than one HTML element needs to be rendered, then they must be grouped together inside one enclosing tag such as , , <div> etc. This function must be kept pure i.e., it must return the same result each time it is invoked.</div>
What are controlled and uncontrolled components in React?
This relates to stateful DOM components (form elements) and the difference: A Controlled Component is one that takes its current value through props and notifies changes through callbacks like onChange. A parent component “controls” it by handling the callback and managing its own state and passing the new values as props to the controlled component. You could also call this a “dumb component”. A Uncontrolled Component is one that stores its own state internally, and you query the DOM using a ref to find its current value when you need it. This is a bit more like traditional HTML. In most (or all) cases we should use controlled components.
Explain the components of Redux.
Redux is composed of the following components:
Action — Actions are payloads of information that send data from our application to our store. They are the only source of information for the store. We send them to the store using store.dispatch(). Primarly, they are just an object describes what happened in our app.
Reducer — Reducers specify how the application’s state changes in response to actions sent to the store. Remember that actions only describe what happened, but don’t describe how the application’s state changes. So this place determines how state will change to an action.
Store — The Store is the object that brings Action and Reducer together. The store has the following responsibilities: Holds application state; Allows access to state via getState(); Allows state to be updated via dispatch(action); Registers listeners via subscribe(listener); Handles unregistering of listeners via the function returned by subscribe(listener).
It’s important to note that we’ll only have a single store in a Redux application. When we want to split your data handling logic, we’ll use reducer composition instead of many stores.
What is React.cloneElement? And the difference with this.props.children?
React.cloneElement clone and return a new React element using using the passed element as the starting point. The resulting element will have the original element’s props with the new props merged in shallowly. New children will replace existing children. key and ref from the original element will be preserved.
React.cloneElement only works if our child is a single React element. For almost everything {this.props.children} is the better solution. Cloning is useful in some more advanced scenarios, where a parent send in an element and the child component needs to change some props on that element or add things like ref for accessing the actual DOM element.
What is the second argument that can optionally be passed to setState and what is its purpose?
A callback function which will be invoked when setState has finished and the component is re-rendered.
Since the setState is asynchronous, which is why it takes in a second callback function. With this function, we can do what we want immediately after state has been updated.
What is the difference between React Native and React?
React is a JavaScript library, supporting both front end web and being run on the server, for building user interfaces and web applications.
On the other hand, React Native is a mobile framework that compiles to native app components, allowing us to build native mobile applications (iOS, Android, and Windows) in JavaScript that allows us to use ReactJS to build our components, and implements ReactJS under the hood.
With React Native it is possible to mimic the behavior of the native app in JavaScript and at the end, we will get platform specific code as the output. We may even mix the native code with the JavaScript if we need to optimize our application further.
How to bind methods or event handlers in JSX callbacks?
There are 3 possible ways to achieve this:
Binding in Constructor: In JavaScript classes, the methods are not bound by default. The same thing applies for React event handlers defined as class methods. Normally we bind them in constructor.
class Component extends React.Componenet {
constructor(props) {
super(props)
this.handleClick = this.handleClick.bind(this)
}
handleClick() {
// ...
}
}
Public class fields syntax: If you don't like to use bind approach then public class fields syntax can be used to correctly bind callbacks.
handleClick = () => {
console.log('this is:', this)
}
{‘Click me’}
Arrow functions in callbacks: You can use arrow functions directly in the callbacks.
this.handleClick(event)}>
{‘Click me’}
Note: If the callback is passed as prop to child components, those components might do an extra re-rendering. In those cases, it is preferred to go with .bind() or public class fields syntax approach considering performance.
How to pass a parameter to an event handler or callback?
You can use an arrow function to wrap around an event handler and pass parameters:
this.handleClick(id)} />
This is an equivalent to calling .bind:
Apart from these two approaches, you can also pass arguments to a function which is defined as array function
handleClick = (id) => () => {
console.log(“Hello, your ticket number is”, id)
};
What are synthetic events in React?
SyntheticEvent is a cross-browser wrapper around the browser’s native event. It’s API is same as the browser’s native event, including stopPropagation() and preventDefault(), except the events work identically across all browsers.
What is inline conditional expressions?
You can use either if statements or ternary expressions which are available from JS to conditionally render expressions. Apart from these approaches, you can also embed any expressions in JSX by wrapping them in curly braces and then followed by JS logical operator &&.
<h1>Hello!</h1>
{
messages.length > 0 && !isLogin?
<h2>
You have {messages.length} unread messages.
</h2>
:
<h2>
You don’t have unread messages.
</h2>
}
What are “key” props and what is the benefit of using them in arrays of elements?
A key is a special string attribute you should include when creating arrays of elements. Keys help React identify which items have changed, are added, or are removed.
Most often we use IDs from our data as keys:
const todoItems = todos.map((todo) =>
<li>
{todo.text}
</li>
)
When you don't have stable IDs for rendered items, you may use the item index as a key as a last resort:
const todoItems = todos.map((todo, index) =>
<li>
{todo.text}
</li>
)
Note:
Using indexes for keys is not recommended if the order of items may change. This can negatively impact performance and may cause issues with component state.
If you extract list item as separate component then apply keys on list component instead of li tag.
There will be a warning message in the console if the key prop is not present on list items.
How to create refs?
There are two approaches
This is a recently added approach. Refs are created using React.createRef() method and attached to React elements via the ref attribute. In order to use refs throughout the component, just assign the ref to the instance property within constructor.
class MyComponent extends React.Component {
constructor(props) {
super(props)
this.myRef = React.createRef()
}
render() {
return <div></div>
}
}
You can also use ref callbacks approach regardless of React version. For example, the search bar component's input element accessed as follows,
class SearchBar extends Component {
constructor(props) {
super(props);
this.txtSearch = null;
this.state = { term: '' };
this.setInputSearchRef = e => {
this.txtSearch = e;
}
}
onInputChange(event) {
this.setState({ term: this.txtSearch.value });
}
render() {
return (
); } } You can also use refs in function components using closures. Note: You can also use inline ref callbacks even though it is not a recommended approach
What are forward refs?
Ref forwarding is a feature that lets some components take a ref they receive, and pass it further down to a child.
const ButtonElement = React.forwardRef((props, ref) => (
{props.children}
));
// Create ref to the DOM button:
const ref = React.createRef();
{'Forward Ref'}
Which is preferred option with in callback refs and findDOMNode()?
It is preferred to use callback refs over findDOMNode() API. Because findDOMNode() prevents certain improvements in React in the future.
The legacy approach of using findDOMNode:
class MyComponent extends Component {
componentDidMount() {
findDOMNode(this).scrollIntoView()
}
render() {
return <div></div>
}
}
The recommended approach is:
class MyComponent extends Component {
constructor(props){
super(props);
this.node = createRef();
}
componentDidMount() {
this.node.current.scrollIntoView();
}
render() {
return <div></div>
}
}
Why are String Refs legacy?
If you worked with React before, you might be familiar with an older API where the ref attribute is a string, like ref={‘textInput’}, and the DOM node is accessed as this.refs.textInput. We advise against it because string refs have below issues, and are considered legacy. String refs were removed in React v16.
They force React to keep track of currently executing component. This is problematic because it makes react module stateful, and thus causes weird errors when react module is duplicated in the bundle.
They are not composable — if a library puts a ref on the passed child, the user can't put another ref on it. Callback refs are perfectly composable.
They don't work with static analysis like Flow. Flow can't guess the magic that framework does to make the string ref appear on this.refs, as well as its type (which could be different). Callback refs are friendlier to static analysis.
It doesn't work as most people would expect with the "render callback" pattern (e.g. )
class MyComponent extends Component {
renderRow = (index) => {
// This won't work. Ref will get attached to DataTable rather than MyComponent:
return ;
// This would work though! Callback refs are awesome.
return this['input-' + index] = input} />;
}
render() {
return
}
}
What is Virtual DOM?
The Virtual DOM works in three simple steps.
Whenever any underlying data changes, the entire UI is re-rendered in Virtual DOM representation.
Then the difference between the previous DOM representation and the new one is calculated.
Once the calculations are done, the real DOM will be updated with only the things that have actually changed.
What is the difference between Shadow DOM and Virtual DOM?
The Shadow DOM is a browser technology designed primarily for scoping variables and CSS in web components. The Virtual DOM is a concept implemented by libraries in JavaScript on top of browser APIs.
What is React Fiber?
Fiber is the new reconciliation engine or reimplementation of core algorithm in React v16. The goal of React Fiber is to increase its suitability for areas like animation, layout, gestures, ability to pause, abort, or reuse work and assign priority to different types of updates; and new concurrency primitives.
The goal of React Fiber is to increase its suitability for areas like animation, layout, and gestures. Its headline feature is incremental rendering: the ability to split rendering work into chunks and spread it out over multiple frames.
What are controlled components?
A component that controls the input elements within the forms on subsequent user input is called Controlled Component, i.e, every state mutation will have an associated handler function.
For example, to write all the names in uppercase letters, we use handleChange as below,
handleChange(event) {
this.setState({value: event.target.value.toUpperCase()})
}
What are uncontrolled components?
The Uncontrolled Components are the ones that store their own state internally, and you query the DOM using a ref to find its current value when you need it. This is a bit more like traditional HTML.
In the below UserProfile component, the name input is accessed using ref.
class UserProfile extends React.Component {
constructor(props) {
super(props)
this.handleSubmit = this.handleSubmit.bind(this)
this.input = React.createRef()
}
handleSubmit(event) {
alert(‘A name was submitted: ‘ + this.input.current.value)
event.preventDefault()
}
render() {
return (
{'Name:'}
); } } In most cases, it's recommend to use controlled components to implement forms.
What is the difference between createElement and cloneElement?
JSX elements will be transpiled to React.createElement() functions to create React elements which are going to be used for the object representation of UI. Whereas cloneElement is used to clone an element and pass it new props.
What is Lifting State Up in React?
When several components need to share the same changing data then it is recommended to lift the shared state up to their closest common ancestor. That means if two child components share the same data from its parent, then move the state to parent instead of maintaining local state in both of the child components.
What are the different phases of component lifecycle?
The component lifecycle has three distinct lifecycle phases:
Mounting: The component is ready to mount in the browser DOM. This phase covers initialization from constructor(), getDerivedStateFromProps(), render(), and componentDidMount() lifecycle methods.
Updating: In this phase, the component get updated in two ways, sending the new props and updating the state either from setState() or forceUpdate(). This phase covers getDerivedStateFromProps(), shouldComponentUpdate(), render(), getSnapshotBeforeUpdate() and componentDidUpdate() lifecycle methods.
Unmounting: In this last phase, the component is not needed and get unmounted from the browser DOM. This phase includes componentWillUnmount() lifecycle method.
It’s worth mentioning that React internally has a concept of phases when applying changes to the DOM. They are separated as follows
Render The component will render without any side-effects. This applies for Pure components and in this phase, React can pause, abort, or restart the render.
Pre-commit Before the component actually applies the changes to the DOM, there is a moment that allows React to read from the DOM through the getSnapshotBeforeUpdate().
Commit React works with the DOM and executes the final lifecycles respectively componentDidMount() for mounting, componentDidUpdate() for updating, and componentWillUnmount() for unmounting.
What are the lifecycle methods of React?
getDerivedStateFromProps: Invoked right before calling render() and is invoked on every render. This exists for rare use cases where you need derived state. Worth reading if you need derived state.
componentDidMount: Executed after first rendering and here all AJAX requests, DOM or state updates, and set up event listeners should occur.
shouldComponentUpdate: Determines if the component will be updated or not. By default it returns true. If you are sure that the component doesn’t need to render after state or props are updated, you can return false value. It is a great place to improve performance as it allows you to prevent a re-render if component receives new prop.
getSnapshotBeforeUpdate: Executed right before rendered output is committed to the DOM. Any value returned by this will be passed into componentDidUpdate(). This is useful to capture information from the DOM i.e. scroll position.
componentDidUpdate: Mostly it is used to update the DOM in response to prop or state changes. This will not fire if shouldComponentUpdate() returns false.
componentWillUnmount It will be used to cancel any outgoing network requests, or remove all event listeners associated with the component.
What are Higher-Order Components?
A higher-order component (HOC) is a function that takes a component and returns a new component. Basically, it’s a pattern that is derived from React’s compositional nature.
We call them pure components because they can accept any dynamically provided child component but they won’t modify or copy any behavior from their input components.
const EnhancedComponent = higherOrderComponent(WrappedComponent) HOC can be used for many use cases:
Code reuse, logic and bootstrap abstraction.
Render hijacking.
State abstraction and manipulation.
Props manipulation.
How to create props proxy for HOC component?
You can add/edit props passed to the component using props proxy pattern like this:
function HOC(WrappedComponent) {
return class Test extends Component {
render() {
const newProps = {
title: 'New Header',
footer: false,
showFeatureX: false,
showFeatureY: true
}
return } } }
What is context?
Context provides a way to pass data through the component tree without having to pass props down manually at every level. For example, authenticated user, locale preference, UI theme need to be accessed in the application by many components.
const {Provider, Consumer} = React.createContext(defaultValue)
What is children prop?
Children is a prop (this.prop.children) that allow you to pass components as data to other components, just like any other prop you use. Component tree put between component’s opening and closing tag will be passed to that component as children prop.
There are a number of methods available in the React API to work with this prop. These include React.Children.map, React.Children.forEach, React.Children.count, React.Children.only, React.Children.toArray. A simple usage of children prop looks as below,
const MyDiv = React.createClass({
render: function() {
return <div>{this.props.children}</div>
}
})
ReactDOM.render(
<span>{'Hello'}</span>
<span>{'World'}</span>
,
node
)
What is the purpose of using super constructor with props argument?
A child class constructor cannot make use of this reference until super() method has been called. The same applies for ES6 sub-classes as well. The main reason of passing props parameter to super() call is to access this.props in your child constructors.
Passing props:
class MyComponent extends React.Component {
constructor(props) {
super(props)
console.log(this.props) // prints { name: 'John', age: 42 } } } Not passing props:
class MyComponent extends React.Component {
constructor(props) {
super()
console.log(this.props) // prints undefined
// but props parameter is still available
console.log(props) // prints { name: 'John', age: 42 }
}
render() {
// no difference outside constructor
console.log(this.props) // prints { name: 'John', age: 42 }
}
}
The above code snippets reveals that this.props is different only within the constructor. It would be the same outside the constructor.
What is reconciliation?
When a component’s props or state change, React decides whether an actual DOM update is necessary by comparing the newly returned element with the previously rendered one. When they are not equal, React will update the DOM. This process is called reconciliation.
How to set state with a dynamic key name?
If you are using ES6 or the Babel transpiler to transform your JSX code then you can accomplish this with computed property names.
handleInputChange(event) {
this.setState({ [event.target.id]: event.target.value })
}
What would be the common mistake of function being called every time the component renders?
You need to make sure that function is not being called while passing the function as a parameter.
render() {
// Wrong: handleClick is called instead of passed as a reference!
return {'Click Me'}
}
Instead, pass the function itself without parenthesis:
render() {
// Correct: handleClick is passed as a reference!
return {'Click Me'}
}
Is lazy function supports named exports?
No, currently React.lazy function supports default exports only. If you would like to import modules which are named exports, you can create an intermediate module that reexports it as the default. It also ensures that tree shaking keeps working and don’t pull unused components. Let’s take a component file which exports multiple named components,
// MoreComponents.js export const SomeComponent = /* ... */; export const UnusedComponent = /* ... */; and reexport MoreComponents.js components in an intermediate file IntermediateComponent.js
// IntermediateComponent.js
export { SomeComponent as default } from "./MoreComponents.js";
Now you can import the module using lazy function as below,
import React, { lazy } from 'react';
const SomeComponent = lazy(() => import("./IntermediateComponent.js"));
Why React uses className over class attribute?
class is a keyword in JavaScript, and JSX is an extension of JavaScript. That’s the principal reason why React uses className instead of class. Pass a string as the className prop.
render() {
return <span>{‘Menu’}</span>
}
What are fragments?
It’s common pattern in React which is used for a component to return multiple elements. Fragments let you group a list of children without adding extra nodes to the DOM.
render() {
return (
)
}
There is also a shorter syntax, but it’s not supported in many tools:
render() {
return (
<>
> ) }
Why fragments are better than container divs?
Fragments are a bit faster and use less memory by not creating an extra DOM node. This only has a real benefit on very large and deep trees.
Some CSS mechanisms like Flexbox and CSS Grid have a special parent-child relationships, and adding divs in the middle makes it hard to keep the desired layout.
The DOM Inspector is less cluttered.
What are portals in React?
Portal is a recommended way to render children into a DOM node that exists outside the DOM hierarchy of the parent component.
ReactDOM.createPortal(child, container)
The first argument is any render-able React child, such as an element, string, or fragment. The second argument is a DOM element.
What are stateless components?
If the behaviour is independent of its state then it can be a stateless component. You can use either a function or a class for creating stateless components. But unless you need to use a lifecycle hook in your components, you should go for function components. There are a lot of benefits if you decide to use function components here; they are easy to write, understand, and test, a little faster, and you can avoid the this keyword altogether.
What are stateful components?
If the behaviour of a component is dependent on the state of the component then it can be termed as stateful component. These stateful components are always class components and have a state that gets initialized in the constructor.
class App extends Component {
constructor(props) {
super(props)
this.state = { count: 0 }
}
render() {
// ...
}
}
How to apply validation on props in React?
When the application is running in development mode, React will automatically check all props that we set on components to make sure they have correct type. If the type is incorrect, React will generate warning messages in the console. It’s disabled in production mode due performance impact. The mandatory props are defined with isRequired.
The set of predefined prop types:
PropTypes.number PropTypes.string PropTypes.array PropTypes.object PropTypes.func PropTypes.node PropTypes.element PropTypes.bool PropTypes.symbol PropTypes.any We can define propTypes for User component as below:
import React from ‘react’
import PropTypes from ‘prop-types’
class User extends React.Component {
static propTypes = {
name: PropTypes.string.isRequired,
age: PropTypes.number.isRequired
}
render() {
return (
<>
<h1>{Welcome, ${this.props.name}}</h1>
<h2>{Age, ${this.props.age}}</h2>
>
)
}
}
Note: In React v15.5 PropTypes were moved from React.PropTypes to prop-types library.
What are the advantages of React?
Increases the application’s performance with Virtual DOM.
JSX makes code easy to read and write.
It renders both on client and server side (SSR).
Easy to integrate with frameworks (Angular, Backbone) since it is only a view library.
Easy to write unit and integration tests with tools such as Jest.
What are the limitations of React?
React is just a view library, not a full framework.
There is a learning curve for beginners who are new to web development.
Integrating React into a traditional MVC framework requires some additional configuration.
The code complexity increases with inline templating and JSX.
Too many smaller components leading to over engineering or boilerplate.
What are error boundaries in React v16?
Error boundaries are components that catch JavaScript errors anywhere in their child component tree, log those errors, and display a fallback UI instead of the component tree that crashed.
A class component becomes an error boundary if it defines a new lifecycle method called componentDidCatch(error, info) or static getDerivedStateFromError() :
class ErrorBoundary extends React.Component {
constructor(props) {
super(props)
this.state = { hasError: false }
}
componentDidCatch(error, info) {
// You can also log the error to an error reporting service
logErrorToMyService(error, info)
}
static getDerivedStateFromError(error) {
// Update state so the next render will show the fallback UI.
return { hasError: true };
}
render() {
if (this.state.hasError) {
// You can render any custom fallback UI
return <h1>{'Something went wrong.'}</h1>
}
return this.props.children
}
}
After that use it as a regular component:
How error boundaries handled in React v15?
React v15 provided very basic support for error boundaries using unstable_handleError method. It has been renamed to componentDidCatch in React v16.
What are the recommended ways for static type checking?
Normally we use PropTypes library (React.PropTypes moved to a prop-types package since React v15.5) for type checking in the React applications. For large code bases, it is recommended to use static type checkers such as Flow or TypeScript, that perform type checking at compile time and provide auto-completion features.
What is the use of react-dom package?
The react-dom package provides DOM-specific methods that can be used at the top level of your app. Most of the components are not required to use this module. Some of the methods of this package are:
render() hydrate() unmountComponentAtNode() findDOMNode() createPortal()
What is the purpose of render method of react-dom?
This method is used to render a React element into the DOM in the supplied container and return a reference to the 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.
ReactDOM.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). This object is mainly used for server-side rendering (SSR). The following methods can be used in both the server and browser environments:
renderToString()
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.
// using Express
import { renderToString } from 'react-dom/server'
import MyPage from './MyPage'
app.get(‘/’, (req, res) => {
res.write(‘My Page’)
res.write(‘<div>’)
res.write(renderToString())
res.write(‘</div>’)
res.end()
})
How to use innerHTML in React?
The dangerouslySetInnerHTML attribute is React’s replacement for using innerHTML in the browser DOM. Just like innerHTML, it is risky to use this attribute considering cross-site scripting (XSS) attacks. You just need to pass a __html object as key and HTML text as value.
In this example MyComponent uses dangerouslySetInnerHTML attribute for setting HTML markup:
function createMarkup() {
return { \_\_html: 'First · Second' }
}
function MyComponent() {
return <div></div>
}
How to use styles in React?
The style attribute accepts a JavaScript object with camelCased properties rather than a CSS string. This is consistent with the DOM style JavaScript property, is more efficient, and prevents XSS security holes.
const divStyle = {
color: 'blue',
backgroundImage: 'url(' + imgUrl + ')'
};
function HelloWorldComponent() {
return <div style="">Hello World!</div>
}
Style keys are camelCased in order to be consistent with accessing the properties on DOM nodes in JavaScript (e.g. node.style.backgroundImage).
How events are different in React?
Handling events in React elements has some syntactic differences:
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. You would get error like this: Can only update a mounted or mounting component. So we need to use this.state to initialize variables inside constructor.
What is the impact of indexes as keys?
Keys should be stable, predictable, and unique so that React can keep track of elements.
In the below code snippet each element’s key will be based on ordering, rather than tied to the data that is being represented. This limits the optimizations that React can do.
{todos.map((todo, index) =>
)}
If you use element data for unique key, assuming todo.id is unique to this list and stable, React would be able to reorder elements without needing to reevaluate them as much.
{todos.map((todo) =>
)}
Is it good to use setState() in componentWillMount() method?
It is recommended to avoid async initialization in componentWillMount() lifecycle method. componentWillMount() is invoked immediately before mounting occurs. It is called before render(), therefore setting state in this method will not trigger a re-render. Avoid introducing any side-effects or subscriptions in this method. We need to make sure async calls for component initialization happened in componentDidMount() instead of componentWillMount().
componentDidMount() {
axios.get(`api/todos`)
.then((result) => {
this.setState({
messages: [...result.data]
})
})
}
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 initialization of state from props only runs when the component is first created.
The below component won’t display the updated input value:
class MyComponent extends React.Component {
constructor(props) {
super(props)
this.state = {
records: [],
inputValue: this.props.inputValue
}; }
render() {
return <div>{this.state.inputValue}</div>
}
}
Using props inside render method will update the value:
class MyComponent extends React.Component {
constructor(props) {
super(props)
this.state = {
record: []
} }
render() {
return <div>{this.props.inputValue}</div>
}
}
How do you conditionally render components?
In some cases you want to render different components depending on some state. 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.
const MyComponent = ({ name, address }) => (
<div>
<h2>{name}</h2>
{address &&
<p>{address}</p>
}
</div>
)
If you need an if-else condition then use ternary operator.
const MyComponent = ({ name, address }) => (
<div>
<h2>{name}</h2>
{address
? <p>{address}</p>
: <p>{'Address is not available'}</p>
}
</div>
)
Why we need to be careful when spreading props on DOM elements?
When we spread props we run into the risk of adding unknown HTML attributes, which is a bad practice. Instead we can use prop destructuring with …rest operator, so it will add only required props. For example,
const ComponentA = () =>
const ComponentB = ({ isDisplay, ...domProps }) =>
<div>{'ComponentB'}</div>
How you use decorators in React?
You can decorate your class components, which is the same as passing the component into a function. Decorators are flexible and readable way of modifying component functionality.
@setTitle('Profile')
class Profile extends React.Component {
//....
}
/*
title is a string that will be set as a document title
WrappedComponent is what our decorator will receive when
put directly above a component class as seen in the example above
*/
const setTitle = (title) => (WrappedComponent) => {
return class extends React.Component {
componentDidMount() {
document.title = title
}
render() {
return
}
}
}
Note: Decorators are a feature that didn't make it into ES7, but are currently a stage 2 proposa
How do you memoize a component?
There are memoize libraries available which can be used on function components. For example moize library can memoize the component in another component.
import moize from 'moize' import Component from './components/Component' // this module exports a non-memoized component
const MemoizedFoo = moize.react(Component)
const Consumer = () => {
<div>
{'I will memoize the following entry:'}
</div>
}
How you implement Server Side Rendering or SSR?
React is already equipped to handle rendering on Node servers. A special version of the DOM renderer is available, which follows the same pattern as on the client side.
import ReactDOMServer from ‘react-dom/server’
import App from ‘./App’
ReactDOMServer.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. On the client side, React detects the pre-rendered content and seamlessly picks up where it left off.
How to enable production mode in React?
You should use Webpack’s DefinePlugin method to set NODE_ENV to production, by which it strip out things like propType validation and extra warnings. 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.
What is CRA and its benefits?
It includes everything we need to build a React app:
React, JSX, ES6, and Flow syntax support.
Language extras beyond ES6 like the object spread operator.
Autoprefixed CSS, so you don’t need -webkit- or other prefixes.
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.
constructor()
static getDerivedStateFromProps()
render()
componentDidMount()
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()
componentWillReceiveProps()
componentWillUpdate()
Starting with React v16.3 these methods are aliased with UNSAFE_ prefix, and the unprefixed version will be removed in React v17.
What is the purpose of getDerivedStateFromProps() lifecycle method?
The new static getDerivedStateFromProps() lifecycle method is invoked after a component is instantiated as well as before it is re-rendered. It can return an object to update state, or null to indicate that the new props do not require any state updates.
class MyComponent extends React.Component {
static getDerivedStateFromProps(props, state) {
// ...
}
}
What is the purpose of getSnapshotBeforeUpdate() lifecycle method?
The new getSnapshotBeforeUpdate() lifecycle method is called right before DOM updates. The return value from this method will be passed as the third parameter to componentDidUpdate().
class MyComponent extends React.Component {
getSnapshotBeforeUpdate(prevProps, prevState) {
// ...
}
}
This lifecycle method along with componentDidUpdate() covers all the use cases of componentWillUpdate().
Do Hooks replace render props and higher order components?
Both render props and higher-order components render only a single child but in most of the cases Hooks are a simpler way to serve this by reducing nesting in your tree.
What is the recommended way for naming components?
It is recommended to name the component by reference instead of using displayName.
Using displayName for naming component:
export default React.createClass({
displayName: 'TodoApp',
// ...
})
The recommended approach:
export default class TodoApp extends React.Component {
// ...
}
What is the recommended ordering of methods in component class?
Recommended ordering of methods from mounting to render stage:
static methods constructor() getChildContext() componentWillMount() componentDidMount() componentWillReceiveProps() shouldComponentUpdate() componentWillUpdate() componentDidUpdate() componentWillUnmount() click handlers or event handlers like onClickSubmit() or onChangeDescription() getter methods for render like getSelectReason() or getFooterContent() optional render methods like renderNavigation() or renderProfilePicture() render()
What is a switching component?
A switching component is a component that renders one of many components. We need to use object to map prop values to components.
For example, a switching component to display different pages based on page prop:
import HomePage from ‘./HomePage’
import AboutPage from ‘./AboutPage’
import ServicesPage from ‘./ServicesPage’
import ContactPage from ‘./ContactPage’
const PAGES = {
home: HomePage,
about: AboutPage,
services: ServicesPage,
contact: ContactPage
}
const Page = (props) => {
const Handler = PAGES[props.page] || ContactPage
return
}
// The keys of the PAGES object can be used in the prop types to catch dev-time errors.
Page.propTypes = {
page: PropTypes.oneOf(Object.keys(PAGES)).isRequired
}
Why we need to pass a function to setState()?
The reason behind for this is that setState() is an asynchronous operation. React batches state changes for performance reasons, so the state may not change immediately after setState() is called. That means you should not rely on the current state when calling setState() since you can’t be sure what that state will be. The solution is to pass a function to setState(), with the previous state as an argument. By doing this you can avoid issues with the user getting the old state value on access due to the asynchronous nature of setState().
Let’s say the initial count value is zero. After three consecutive increment operations, the value is going to be incremented only by one.
// assuming this.state.count === 0
this.setState({ count: this.state.count + 1 })
this.setState({ count: this.state.count + 1 })
this.setState({ count: this.state.count + 1 })
// this.state.count === 1, not 3
If we pass a function to setState(), the count gets incremented correctly.
this.setState((prevState, props) => ({
count: prevState.count + props.increment
}))
// this.state.count === 3 as expected
What is strict mode in React?
React.StrictMode is an useful component for highlighting potential problems in an application. Just like , does not render any extra DOM elements. It activates additional checks and warnings for its descendants. These checks apply for development mode only.
import React from ‘react’
function ExampleApplication() {
return (
<div>
<div>
</div>
</div> ) } In the example above, the strict mode checks apply to and components only.
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.
if (this.isMounted()) {
this.setState({...})
}
Checking isMounted() before calling setState() does eliminate the warning, but it also defeats the purpose of the warning. Using isMounted() is a code smell because the only reason you would check is because you think you might be holding a reference after the component has unmounted.
An optimal solution would be to find places where setState() might be called after a component has unmounted, and fix them. Such situations most commonly occur due to callbacks, when a component is waiting for some data and gets unmounted before the data arrives. 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. In the old days we had a mouse and respective event listeners to handle them but nowadays we have many devices which don’t correlate to having a mouse, like phones with touch surface or pens. We need to remember that these events will only work in browsers that support the Pointer Events specification.
The following event types are now available in React DOM:
onPointerDown onPointerMove onPointerUp onPointerCancel onGotPointerCapture onLostPointerCaptur onPointerEnter onPointerLeave onPointerOver onPointerOut
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 unrecognized tag. This convention is because only HTML elements and SVG tags can begin with a lowercase letter.
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. Here lowercase is fine:
class myComponent extends Component {
render() {
return <div></div>
}
}
export default myComponent
While when imported in another file it should start with capital letter:
import MyComponent from ‘./MyComponent’
Are custom DOM attributes supported in React v16?
Yes. In the past, React used to ignore unknown DOM attributes. If you wrote JSX with an attribute that React doesn’t recognize, React would just skip it. For example, this:
<div></div>
Would render an empty div to the DOM with React v15:
<div></div>
In React v16 any unknown attributes will end up in the DOM:
<div></div>
This is useful for supplying browser-specific non-standard attributes, trying new DOM APIs, and integrating with opinionated third-party libraries.
What is the difference between constructor and getInitialState?
You should initialize state in the constructor when using ES6 classes, and getInitialState() method when using React.createClass().
Using ES6 classes:
class MyComponent extends React.Component {
constructor(props) {
super(props)
this.state = { /* initial state */ }
}
}
Using React.createClass():
const MyComponent = React.createClass({
getInitialState() {
return { /* initial state */ }
}
})
Note: React.createClass() is deprecated and removed in React v16. Use plain JavaScript classes instead.
Can you force a component to re-render without calling setState?
By default, when your component’s state or props change, your component will re-render. If your render() method depends on some other data, you can tell React that the component needs re-rendering by calling forceUpdate().
component.forceUpdate(callback)
It is recommended to avoid all uses of forceUpdate() and only read from this.props and this.state in render().
What is the difference between super() and super(props) in React using ES6 classes?
When you want to access this.props in constructor() then you should pass props to super() method.
Using super(props):
class MyComponent extends React.Component {
constructor(props) {
super(props)
console.log(this.props) // { name: 'John', ... }
}
}
Using super():
class MyComponent extends React.Component {
constructor(props) {
super()
console.log(this.props) // undefined
}
}
Outside constructor() both will display same value for this.props.
How to loop inside JSX?
You can simply use Array.prototype.map with ES6 arrow function syntax. For example, the items array of objects is mapped into an array of components:
{items.map(item => )}
You can’t iterate using for loop:
for (let i = 0; i < items.length; i++) {
}
This is because JSX tags are transpiled into function calls, and you can’t use statements inside expressions. This may change thanks to do expressions which are stage 1 proposal.
How do you access props in attribute quotes?
React (or JSX) doesn’t support variable interpolation inside an attribute value. The below representation won’t work:
<img></img>
But you can put any JS expression inside curly braces as the entire attribute value. So the below expression works:
<img></img>
Using template strings will also work:
<img></img>
What is React proptype array with shape?
If you want to pass an array of objects to a component with a particular shape then use React.PropTypes.shape() as an argument to React.PropTypes.arrayOf().
ReactComponent.propTypes = {
arrayWithShape: React.PropTypes.arrayOf(React.PropTypes.shape({
color: React.PropTypes.string.isRequired,
fontSize: React.PropTypes.number.isRequired
})).isRequired
}
How to conditionally apply class attributes?
You shouldn’t use curly braces inside quotes because it is going to be evaluated as a string.
<div> Instead you need to move curly braces outside (don't forget to include spaces between class names):
<div>
Template strings will also work:
<div></div></div>
</div>
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().
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.
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.
{‘User’}
Since for is a reserved keyword in JavaScript, use htmlFor instead.
{‘User’}
How to combine multiple inline style objects?
You can use spread operator in regular React:
{‘Submit’}
If you’re using React Native then you can use the array notation:
{‘Submit’}
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.
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 <span>{this.state.width} x {this.state.height}</span>
}
}
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().
How to listen to state changes?
The following lifecycle methods 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.
componentWillUpdate(object nextProps, object nextState)
componentDidUpdate(object prevProps, object prevState)
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.
removeItem(index) {
this.setState({
data: this.state.data.filter((item, i) => i !== index)
})
}
Is it possible to use React without rendering HTML?
render() {
return false
}
render() {
return null
}
render() {
return []
}
render() {
return
}
render() {
return <>>
}
Returning undefined won't work.How to pretty print JSON with React?
We can use <pre> tag so that the formatting of the JSON.stringify() is retained:
const data = { name: ‘John’, age: 42 }
class User extends React.Component {
render() {
return (
<pre>
{JSON.stringify(data, null, 2)}
</pre>
)
}
}
React.render(, document.getElementById(‘container’))</pre>
Why you can’t update props in React?
The React philosophy is that props should be immutable and top-down. This means that a parent can send any prop values to a child, but the child can’t modify received props.
How to focus an input element on page load?
class App extends React.Component{
componentDidMount() {
this.nameInput.focus()
}
render() {
return (
<div>
this.nameInput = input}
defaultValue={'Will focus'}
/>
</div>
)
}
}
ReactDOM.render(, document.getElementById(‘app’))
What are the possible ways of updating objects in state?
Calling setState() with an object to merge with state:
Using Object.assign() to create a copy of the object:
const user = Object.assign({}, this.state.user, { age: 42 })
this.setState({ user })
Using spread operator:
const user = { ...this.state.user, age: 42 }
this.setState({ user })
Calling setState() with a function:
this.setState(prevState => ({
user: {
…prevState.user,
age: 42
}
}))
Why function is preferred over object for setState()?
React may batch multiple setState() calls into a single update for performance. Because this.props and this.state may be updated asynchronously, you should not rely on their values for calculating the next state.
This counter example will fail to update as expected:
// Wrong
this.setState({
counter: this.state.counter + this.props.increment,
})
The preferred approach is to call setState() with function rather than object. 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.
// Correct
this.setState((prevState, props) => ({
counter: prevState.counter + props.increment
}))
How can we find the version of React at runtime in the browser?
You can use React.version to get the version.
const REACT_VERSION = React.version
ReactDOM.render(
<div>{`React version: ${REACT_VERSION}`}</div>
,
document.getElementById(‘app’)
)
What are the approaches to include polyfills in your create-react-app?
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.
import ‘core-js/fn/array/find’
import ‘core-js/fn/array/includes’
import ‘core-js/fn/number/is-nan’
Using Polyfill service:
Use the polyfill.io CDN to retrieve custom, browser-specific polyfills by adding this line to index.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.
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:
“scripts”: {
“start”: “set HTTPS=true && react-scripts start”
}
or just run set HTTPS=true && npm start
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.
How to add Google Analytics for React Router?
history.listen(function (location) {
window.ga(‘set’, ‘page’, location.pathname + location.search)
window.ga(‘send’, ‘pageview’, location.pathname + location.search)
})
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.
componentDidMount() {
this.interval = setInterval(() => this.setState({ time: Date.now() }), 1000)
}
componentWillUnmount() {
clearInterval(this.interval)
}
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.
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