13 React.js Developer Interview Questions and Answers

A great way to determine your programmer's depth of knowledge is to explain some features of this coding language. Look for answers with the following included:

1. React uses the virtual DOM instead of the real DOM element
2. React uses server-side rendering
3. React follows data binding or a unidirectional data flow 
4. React uses specific lifecycle methods

A programmer should understand how browsers read JavaScript objects, and they should know how to enable them to read XML JSX. In addition, familiarity with JSX transformers like Babel indicates that they know how to change these java objects to pass them into the browser.

There are many basic triggered reactions within the react application. A qualified contractor should identify these events, such as a mouse hover or onclick press, and describe event functionality through its event handler. Knowing why events react and tracing back their syntax to the event handler is crucial for writing and executing react code.

Your qualified freelancer should understand the functionality of the useState hook and how it allows state variables within a functional component. Understanding useState indicates that the coder knows how to create functional higher-order components without switching to class components. This method should typically be performed using the built-in setState() constructor method.

Since every app has more than one form, knowing how to handle the form state of components is key. Your programmer should clearly distinguish between controlled, which is handled by a react component, and uncontrolled input value, handled by the DOM element itself.

An experienced developer should already be using .PureComponent as a significant way to optimize the react app. This component provides a considerable performance increase because it reduces the number of render operations within the application. Knowing how to use this component shows they understand optimization. They can also use it to identify extra re-rendering to perform debugging and trigger alerts if the front-end or native element components need updating. 

The default initial state of a React.Component always has the value of shouldComponentUpdate set to return "true." Your programmer needs to understand this process of setting default values for props arguments—understanding how default values are set and why they are essential for passing prop properties and component classes. 

Rendered objects are often updated within React, so your experienced programmer should have this skill. Ideally, your developer should mention that since react elements are not mutable, once you create your elements, you cannot change its parent component, child component, current state, or current value attributes. Furthermore, since you cannot reuse elements, you must create new elements, and the only way is to pass it through ReactDOM.render().

A react-router is client-side routing, which allows you to build single-page web applications and navigation without refreshing the page algorithm as a user navigates. These routers can be used for both Android or IOS functionality. While your qualified developer should know what this router does, it's more important that they understand when to and when not to use a react-router. Often, the react-router can be an overkill for specific projects, so ask your programmer why and when they would use the react-router. 

Rendering a button is one of the most basic React coding exercises. Have your programmer go through a few simple codes and see how they work through the specific syntax. 

<START>

import React from "react";

import ReactDOM from "react-dom";

import "./styles.css";

function Button(props) {

return (

<button type="submit">{props.label}</button>

);

}

const rootElement = document.getElementById("root");

ReactDOM.render(<Button label="Save" />, rootElement);

<END>

Redux is an open-source JavaScript library with javascript functions that are used to manage the application state. Ask your developer why they would use Redux for their apps, and their answer should include some of the following:

• The app has large amounts of application states that are needed in many places in the app
• It is a great ecosystem for the application's state management
• The app state is updated frequently.
• The logic to update that state may be complex
• The app is worked by many people

Your programmer should know when and when not to use this componentDidUpdate() feature. For example, it is useful when the DOM is updated, and the queue is empty, and your programmer should know the order in which this update should be executed. In addition, knowing how to use this component indicates your programmer is familiar with complex renders and state or DOM changes.

Flux is the architecture that Facebook uses when working with React. Your programmer should be able to make the distinction that Flux is not a framework or a library. It is just a new kind of architecture that complements React with a concept that enforces a unidirectional data flow, provides stability to the application, and is a run-time error reducer.