Are you preparing for a MERN Stack interview but don’t know where to start? Don’t worry, you’re not alone. MERN stands for MongoDB, Express.js, React.js, and Node.js, and it’s one of the most popular full-stack development frameworks today. With more companies building modern, scalable applications using MERN, the demand for skilled developers is growing fast.
To help you get ready, we’ve put together a list of the most common MERN Stack interview questions and answers. These questions cover both technical concepts and real-world coding problems you may face during an interview. Mern stack is covered exhaustively in Synergisticit’s Java Devops JOPP.
Whether you’re just starting out or brushing up on your skills,or want to start a career in Fullstack development this guide will boost your confidence and give you a clear idea of what to expect. So let’s dive into the MERN stack and help you land that job in web development!
Answer: As Node.js is a single-thread process, it does not expose child threads & thread management methods to developers. Node.js does spawn child threads for some tasks like asynchronous I/O, but that runs behind the scenes and does not execute any application JavaScript code nor block the main event loop. If threading support is required in a Node.js application, several tools are available to enable it, e.g., the ChildProcess module.
Answer: Node.js offers a single thread to programmers to let them write code easily without a bottleneck. Node uses multiple POSIX threads internally for several I/O operations. When Node receives an I/O request, it uses or creates a thread to perform that I/O operation. Once the operation is complete, it pushes the result to an event queue. On each of such events, the event loop runs & checks the queue & if the execution stack of Node is empty, it adds the queue result to the execution stack.
Answer: PureComponent is similar to a Component; the only difference is it handles the shouldComponentUpdate method. When the state or props change, the PureComponent performs a shallow comparison on both the state and the props. On the other hand, the Component does not compare the current state & props. Therefore, the component re-renders by default when shouldComponentUpdate is called.
Answer: React Hooks are a new addition to the latest version of React 16.8. React Hooks lets you use state & other React features without writing a class. You can extract stateful logic from a component to be reused & tested independently by using Hooks. It allows you to reuse stateful logic without the need to change your component hierarchy. Thus making it easier to share Hooks with the community or among many components.
Answer: The key advantage of React Hooks is that it facilitates the re-usability of stateful logic. It can be done through custom hooks without changing the component hierarchy. Thus, one can reuse stateful logic and easily share custom hooks with other components. React Hooks are easier to work & test. It has less no. of code & makes the code look easier to read & cleaner.
Answer: Aggregation processes data records & returns computed results. It groups values from several documents together and performs a variety of operations on that grouped data to return a single outcome. MongoDB offers three ways to perform Aggregation, namely:
- Aggregation pipeline;
- Map-reduce function;
- Single-purpose aggregation commands & methods.
Answer: JSX is like an XML/HTML syntax that extends ECMAScript so that XML or HTML text can co-exist with React/JavaScript code. JSX enables developers to write concise HTML/XML-like structures in the same file as they write JavaScript code; Babel then transforms those expressions into actual JS code. It helps us to put HTML into JavaScript.
Answer: ReactDOM refers to a package that provides DOM-specific methods used at the top level of a web application to manage DOM elements efficiently. ReactDOM renders the developers with an API containing the below methods:
- render() Function
- findDOMNode() Function
- render() Function- It is one of the most important methods of ReactDOM. render()Function helps to render a single React Component or several Components wrapped together in a div element or Component. It uses React’s efficient methods for updating the DOM by changing only a subtree, efficient diff methods, etc.
- findDOMNode() Function- It is used to receive the DOM node where a particular React component was rendered.
Answer: Sharding implies a method through which you can store data across multiple machines. MongoDB uses sharding to support deployments with massive data sets & high throughput operations.
Answer: A Stream is a collection of data that doesn’t have to fit in the memory & may not be available. It continuously provides a chunk of data and helps to read & process it. There are four types of streams available in Node.js:
- Writable: This stream can write data.
- Readable: Through this stream, the data can be read.
- Duplex: It facilitates both reading and writing the data.
- Transform: This stream can transform or modify the data as it is written and read.
Answer: When an application needs to wait for some I/O operation to complete its execution, the code responsible for waiting is called a blocking code.
Answer: When building a ReactJS application, there is always a need for a deeply nested component to utilize data from another component with a much higher hierarchy component. The simplest approach to do so is by passing a prop from each component to the next in a hierarchy from the source component to the deeply nested component. It is called prop drilling. To avoid prop drilling, use React context. It allows a Provider component that supplies data to be defined and allows nested components to consume context data by either a Consumer component or a use Context hook.
Answer: The Shadow DOM refers to a browser technology mainly designed for scoping variables and CSS in the web components. Libraries implement virtual DOM in JavaScript on top of the browser APIs. The only thing common between a ShadowDOM & Virtual DOM is that they help with performance issues. Otherwise, a Virtual DOM creates a copy of the entire DOM object, & the Shadow DOM creates small pieces of the DOM object that has their own isolated scope for the element they represent.
Answer:
- createRef always creates a new ref on each render.
- useRef only takes care of returning the same ref each time during the initial rendering.
Answer:
- A smart component manages its state or is connected to the Redux store in a Redux environment.
- A dumb component is driven by the props passed in from the parent & maintains no state of its own.
Answer: React operates by creating a Virtual DOM. Initially, it employs a “diffing” algorithm whenever there is a change in the state of a component. Following this, it goes through a process called reconciliation, where it updates the actual DOM based on the differences identified during the “diffing” process.
Answer: Mongoose, also known as an Object Document Mapper (ODM), is utilized to offer a schema-based approach for modeling application data. It encompasses functionalities such as typecasting, validation, query construction, business logic hooks, and more.
Answer: An Asynchronous API, or non-blocking API, is a component within the Node.js library. It enables Node.js servers to continue processing without waiting for API responses. Node.js leverages a notification process to manage responses from previous API calls efficiently.
Answer: `process.nextTick()` executes a function immediately after the current operation completes but before yielding to the event loop. On the other hand, `setImmediate()` schedules a callback to run in the next iteration of the event loop.
Answer: A Higher Order Component (HOC) is essentially a function that takes a component as input and returns a new component. It follows a pattern derived from React’s compositional nature and is sometimes referred to as a pure component. HOCs accept child components provided to them but do not alter the behavior of these input components.