800+ Front end Developer Interview Questions and Answers

typeof() operator in JavaScript is used to determine the data type of a variable or expression.

• typeof() returns a string indicating the type of the operand.

• Example: typeof 42 will return 'number'.

• Example: typeof 'hello' will return 'string'.

Futuristic skills for a frontend developer include AI, VR/AR, PWAs, and accessibility.

• Proficiency in AI and machine learning for creating intelligent interfaces

• Knowledge of VR/AR for developing immersive experiences

• Expertise in Progressive Web Apps (PWAs) for building fast and reliable web applications

• Understanding of accessibility guidelines and techniques for creating inclusive designs

Components in React go through various stages like mounting, updating, and unmounting.

• Components are created and inserted into the DOM during the mounting phase.

• During the updating phase, components can re-render due to changes in props or state.

• Components are removed from the DOM during the unmounting phase.

• Lifecycle methods like componentDidMount, componentDidUpdate, and componentWillUnmount are used to perform actions at different stages.

Prototype chaining in JavaScript is the mechanism by which objects inherit properties and methods from other objects.

• In JavaScript, each object has a prototype property that points to another object. When a property or method is accessed on an object, JavaScript will look for it in the object itself first, and then in its prototype chain.

• If the property or method is not found in the object, JavaScript will continue to look up the prototype chain until it finds the property or...read more

The 'this' operator in JavaScript refers to the current context or object.

• Refers to the current object or context in which a function is being executed

• Can be used to access properties and methods of the current object

• The value of 'this' is determined by how a function is called

React middlewares are functions that intercept and modify requests and responses in a React application.

• Middlewares are used to add additional functionality to an application without modifying the core code.

• They can be used for authentication, logging, error handling, and more.

• Examples of React middlewares include Redux Thunk, Redux Saga, and React Router.

• They are typically implemented using higher-order functions.

• Middlewares can be chained together to create a pipeline of fu...read more

The filter method is a built-in JavaScript method used to create a new array with all elements that pass a certain condition.

• Filter method is more concise and readable compared to for loop

• Filter method is more declarative and functional programming oriented

• Example: const numbers = [1, 2, 3, 4, 5]; const evenNumbers = numbers.filter(num => num % 2 === 0);

• Example: const words = ['hello', 'world', 'example']; const shortWords = words.filter(word => word.length < 6);

Hoisting in JavaScript is a behavior where variable and function declarations are moved to the top of their containing scope during the compilation phase.

• Variable declarations are hoisted to the top of their scope, but not their assignments.

• Function declarations are fully hoisted, meaning they can be called before they are declared.

• Hoisting can lead to unexpected behavior if not understood properly.

Types of data binding in Angular include interpolation, property binding, event binding, two-way binding.

• Interpolation: {{ data }}

• Property binding: [property]="data"

• Event binding: (event)="function()"

• Two-way binding: [(ngModel)]="data"

Promises are objects representing the eventual completion or failure of an asynchronous operation.

• Promises have three states: pending, fulfilled, and rejected.

• Pending: initial state, neither fulfilled nor rejected.

• Fulfilled: operation completed successfully.

• Rejected: operation failed.

• Promises can be chained using .then() to handle success and failure.

• Example: const promise = new Promise((resolve, reject) => { ... });

JavaScript is single-threaded.

• JavaScript is single-threaded, meaning it can only execute one piece of code at a time.

• This is because JavaScript runs in the browser's main thread, which is responsible for handling user interactions, rendering, and running JavaScript code.

• Asynchronous operations in JavaScript, such as AJAX requests and setTimeout, allow for non-blocking code execution without the need for multi-threading.

• Web Workers can be used to run JavaScript code in the bac...read more

The 'this' operator refers to the object that is currently executing the code.

• The value of 'this' depends on how a function is called.

• In a method, 'this' refers to the object that owns the method.

• In a regular function, 'this' refers to the global object (window in a browser).

• In an event handler, 'this' refers to the element that received the event.

• The value of 'this' can be explicitly set using call(), apply(), or bind() methods.

Promises are objects representing the eventual completion or failure of an asynchronous operation.

• Promises are used to handle asynchronous operations in JavaScript.

• Promise.all() takes an array of promises and returns a single promise that resolves when all of the input promises have resolved.

• A polyfill for Promise.all() can be implemented using a combination of Promise and Array.prototype.reduce().

The CSS Box Model is a fundamental concept in CSS that defines the layout and spacing of elements on a webpage.

• The Box Model consists of content, padding, border, and margin.

• Content: The actual content or text of the element.

• Padding: Space between the content and the border.

• Border: The border surrounding the padding and content.

• Margin: Space outside the border, separating the element from other elements.

• Example: div { width: 200px; padding: 20px; border: 1px solid black; marg...read more

Functional components are simpler and easier to test, while class components offer more features and lifecycle methods.

• Use functional components for simple UI components

• Use class components for more complex UI components

• Functional components are easier to read and maintain

• Class components offer more lifecycle methods and state management options

Scss/Sass is a CSS preprocessor that adds features like variables, nesting, and mixins to make writing CSS easier and more efficient.

• Scss/Sass stands for Syntactically Awesome Style Sheets

• It is a CSS preprocessor that compiles into regular CSS

• It adds features like variables, nesting, and mixins to make writing CSS easier and more efficient

• It also allows for more advanced features like functions and loops

• Example: $primary-color: #007bff; .button { background-color: $primary-co...read more

React.js is a JavaScript library used for building user interfaces by creating reusable UI components.

• React.js allows developers to create interactive user interfaces efficiently.

• It uses a virtual DOM for optimal performance by updating only the necessary components.

• React components can be reused across different parts of an application.

• It follows a unidirectional data flow, making it easier to manage state and data.

• React.js is maintained by Facebook and a community of develo...read more

Controlled components are React components whose value is controlled by React, while uncontrolled components are not.

• Controlled components are typically used for forms and inputs

• Uncontrolled components are typically used for simple inputs like text fields

• Controlled components use the 'value' prop to set the value of the component

• Uncontrolled components use the 'defaultValue' prop to set the initial value of the component

• Controlled components allow for more control and validat...read more

Life cycle methods are methods that are automatically called at different stages of a component's life cycle in React.

• ComponentDidMount is called after the component has been rendered to the DOM.

• ComponentDidUpdate is called after the component's state or props have been updated.

• ComponentWillUnmount is called before the component is removed from the DOM.

Position absolute is positioned relative to its closest positioned ancestor, while position relative is positioned relative to its normal position in the document flow.

• Position absolute elements are taken out of the normal document flow.

• Position relative elements are still in the normal document flow but can be moved from their original position using top, bottom, left, and right properties.

• Position fixed elements are positioned relative to the viewport and do not move when t...read more

To center 3 divs vertically and horizontally, use flexbox and align-items/justify-content properties.

• Wrap the 3 divs in a parent container

• Set the parent container's display property to flex

• Set the parent container's align-items and justify-content properties to center

Server-Side Rendering is rendering the web page on the server and sending the fully rendered page to the client, while Client-Side Rendering is rendering the web page on the client's browser using JavaScript.

• Server-Side Rendering generates the HTML on the server and sends it to the client, resulting in faster initial page load.

• Client-Side Rendering loads a basic HTML page first and then uses JavaScript to render the content on the client's browser, which can result in slower ...read more

CSS Flexbox is a layout module that helps in creating flexible and responsive layouts.

• Flexbox is used to align and distribute space among items in a container.

• It has properties like display, flex-direction, justify-content, align-items, and flex-wrap.

• Flexbox is widely used in modern web development for creating responsive designs.

• Example: display: flex; flex-direction: row; justify-content: center; align-items: center;

Dependency injection in Angular is a design pattern where components are given their dependencies rather than creating them themselves.

• In Angular, dependency injection is achieved by declaring the dependencies in the constructor of a component or service.

• It helps in making components more modular, reusable, and easier to test.

• For example, if a component needs a service to fetch data from an API, instead of creating an instance of the service within the component, the service ...read more

Modernizing legacy systems and integrating new technologies in a global infrastructure.

• Identify key pain points in the legacy system that need modernization

• Develop a clear roadmap for integration of new technologies while minimizing disruption

• Collaborate with stakeholders to ensure alignment with business goals

• Implement gradual changes and conduct thorough testing to mitigate risks

• Provide training and support for users to adapt to the new technologies

A chat app db should have tables for users, conversations, messages, and attachments.

• Create a table for users with their unique ID, username, and password.

• Create a table for conversations with their unique ID, name, and participants.

• Create a table for messages with their unique ID, conversation ID, sender ID, and message content.

• Create a table for attachments with their unique ID, message ID, and attachment URL.

Event bubbling can be implemented by attaching event listeners to the child, parent, and grandparent divs.

• Add event listeners to the child, parent, and grandparent divs

• Use the event.stopPropagation() method to stop the event from bubbling up to the parent and grandparent divs

• Handle the event in each div's event listener function

Media elements in HTML are used to embed audio and video content on a webpage.

• Media elements include <audio> and <video> tags in HTML.

• They allow for the playback of audio and video files directly on a webpage.

• Attributes like src, controls, autoplay, and loop can be used to customize the behavior of media elements.

• Example: <video src='video.mp4' controls></video>

display none removes element from layout, visibility hidden hides element but still takes up space

• display none removes the element from the layout completely

• visibility hidden hides the element but still takes up space

• display none will not render the element, while visibility hidden will render the element but make it invisible

Session storage is temporary and data is cleared when the browser is closed, while local storage persists even after the browser is closed.

• Session storage is temporary and data is cleared when the browser is closed

• Local storage persists even after the browser is closed

• Session storage is limited to the current tab or window, while local storage is shared across tabs and windows of the same origin

React lifecycle methods are functions that are called at different stages of a component's life.

• Mounting: constructor(), render(), componentDidMount()

• Updating: render(), componentDidUpdate()

• Unmounting: componentWillUnmount()

• Error Handling: componentDidCatch()

Observables are streams that can emit multiple values over time, while promises can only emit a single value.

• Observables can emit multiple values over time, while promises can only emit a single value.

• Observables are cancellable, while promises are not.

• Observables support operators like map, filter, and reduce for transforming data streams, while promises do not.

• Observables are lazy, meaning they do not run until they are subscribed to, while promises start executing as soon ...read more

Lifecycle hooks in Angular are methods that allow you to tap into specific points in a component's lifecycle.

• Lifecycle hooks include ngOnInit, ngOnChanges, ngDoCheck, ngOnDestroy, etc.

• ngOnInit is used for initialization logic, ngOnChanges for reacting to input changes, ngDoCheck for custom change detection, ngOnDestroy for cleanup tasks, etc.

Find a pair of numbers in a list that subtract to a target value

• Iterate through the list of numbers and store each number in a hashmap along with its index

• For each number, check if the difference between the target and the number exists in the hashmap

• Return the pair of numbers if found, otherwise return an empty array

Count the number of words in given strings of array using higher order function.

• Use the map function to transform each string into an array of words

• Use the reduce function to count the total number of words in all strings

• Handle edge cases such as empty strings or strings with leading/trailing spaces

Ensuring scalability and performance optimization involves utilizing efficient data management techniques and leveraging appropriate technologies.

• Utilize database indexing and caching to improve data retrieval speed

• Implement data sharding to distribute data across multiple servers for better performance

• Use content delivery networks (CDNs) to reduce latency for global users

• Optimize front end code by minimizing HTTP requests and using lazy loading for large datasets

• Regularly mo...read more

Answers to questions related to programming languages and concepts.

• Data types in programming languages define the type of data that can be stored and manipulated.

• Keywords are reserved words in programming languages that have a specific meaning and cannot be used as variable names.

• C++ is a high-level programming language that is used for developing software applications.

• Python is a popular programming language that is known for its simplicity and ease of use.

• Operators in C++ a...read more

Flutter is a UI toolkit for building natively compiled applications for mobile, web, and desktop from a single codebase.

• Flutter is developed by Google and uses the Dart programming language.

• It allows for fast and efficient development of cross-platform applications.

• Flutter provides a rich set of pre-designed widgets and allows for customization.

• Hot reload feature in Flutter enables quick code changes and updates in real-time.

• Flutter has a strong community support and a growin...read more

Closures in JavaScript are functions that have access to variables from their outer scope even after the outer function has finished executing.

• Closures allow for maintaining state in JavaScript functions.

• They are created whenever a function is defined within another function.

• Closures can access variables from their outer scope, even after the outer function has finished executing.

React virtual DOM is a lightweight copy of the actual DOM, used for efficient rendering of UI components.

• Virtual DOM is a concept where a lightweight copy of the actual DOM is created and updated by React.

• When changes are made to the UI, React compares the virtual DOM with the actual DOM to determine the minimal number of updates needed.

• This process helps in optimizing performance by reducing the number of DOM manipulations required.

• Example: When a user interacts with a React...read more

My salary expectation is up to 600,000 per annum based on my experience and skills.

• Consider factors such as experience, skills, industry standards, and location when determining salary expectations.

• Research average salaries for front end developers in your area to ensure your expectations are realistic.

• Be prepared to negotiate and discuss other benefits in addition to salary.

I built a responsive web application for managing tasks and deadlines.

• Used HTML, CSS, and JavaScript to create the frontend

• Implemented drag and drop functionality for task organization

• Integrated a calendar API for deadline tracking

No, it is not optimal to use redux for single state

• Redux is typically used for managing complex state across multiple components

• Using redux for a single state can add unnecessary complexity to the codebase

• Consider using React's built-in state management for simpler cases

Websocket i.o is a JavaScript library that enables real-time, bidirectional and event-based communication between the browser and the server.

• Allows for real-time communication between client and server

• Enables bidirectional data transfer

• Supports event-based communication

• Used for applications requiring real-time updates like chat applications, online gaming, live tracking, etc.

Firebase is a Backend-as-a-Service platform that provides tools and infrastructure for building web and mobile applications.

• Firebase is owned by Google and offers a suite of cloud-based services.

• It provides features like real-time database, authentication, hosting, storage, and more.

• Firebase can be used to develop and deploy web and mobile applications quickly and easily.

• It offers SDKs for various platforms and integrates well with other Google services.

• Firebase allows develo...read more

Use CSS flexbox to center three divs inside each other

• Use display: flex; justify-content: center; align-items: center; on the grandparent div

• Set width and height on each div to see the effect clearly

• Example: .grandparent { display: flex; justify-content: center; align-items: center; }

JavaScript closures, TDZ, hoisting, and the JavaScript engine are key concepts in understanding how JavaScript works.

• JavaScript closures are functions that have access to variables from their outer scope even after the outer function has finished executing.

• Temporal Dead Zone (TDZ) is the period between entering a scope and the variable being declared where accessing the variable results in a ReferenceError.

• Hoisting is the JavaScript behavior where variable and function declar...read more