Top 100 JavaScript Interview Questions and Answers

JavaScript is a high-level, interpreted scripting language primarily used for web development to create interactive and dynamic web pages. JavaScript runs in browsers and can be used server-side with environments like Node.js.

The difference between JavaScript and ECMAScript is that JavaScript is a general-purpose scripting language, while ECMAScript (often abbreviated as ES) is the standardized specification that defines its core features. Think of ECMAScript as the blueprint and JavaScript as one of the implementations of that blueprint.

The difference between JavaScript and Java is that Java is an OOP programming language while JavaScript is an OOP scripting language. JavaScript is interpreted and follows dynamic typing. Java is compiled and strongly typed.

The primitive data types in JavaScript are string, number, boolean, undefined, null, symbol, Object, and BigInt.

Undefined in JavaScript is a primitive value automatically assigned to variables that have been declared but not yet initialized. It represents the absence of a defined value.

Null in JavaScript is a special value that signifies no value or no object. It needs to be explicitly set and denotes the intentional absence of any value.

You can differentiate null from undefined as undefined is the default state of a declared variable that hasn't been assigned a value, whereas null is an explicit assignment indicating the deliberate absence of a value.

NaN stands for "Not-a-Number" and indicates a value that cannot be represented as a valid number. To check if a value is NaN, use the global function NaN() or the more reliable Number is NaN().

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

The difference between let, const, and var is that var declarations are globally scoped or function scoped while let and const are block scoped. var and let allows for reassignment, const creates a read-only reference, meaning the value it holds cannot be changed after declaration.

An Immediately Invoked Function Expression (or IIFE) is a JavaScript function that is defined and executed immediately after its creation.

Closures in JavaScript are functions which retains access to variables from its enclosing scope, even after that outer function has finished executing

The this keyword in JavaScript refers to the current execution context or the object that the function is bound to. The value of this keyword changes if its call type is a method, constructor or a standalone function.

bind(), call(), and apply() are methods used to change the context of this keyword within a function. bind() creates a new function with a specified `this` value, while call() and apply() immediately invoke the function with the provided `this` value and arguments.

The prototype chain is a mechanism that defines how objects inherit properties and methods from their prototype objects. JavaScript looks up the prototype chain to find it in higher-level prototypes, when a property or method is not found on an object.

Event delegation is a method where a single event handler is placed on a common parent element of multiple child elements. The parent element captures and handles events triggered by the child elements.

JavaScript Promises are objects representing the eventual completion or failure of an asynchronous operation. Promises in JavaScript provide a cleaner and more structured way to handle asynchronous code compared to callback functions, making it easier to work with asynchronous tasks like HTTP requests.

Use try-catch block to handle errors in JavaScript. Errors are thrown explicitly using the throw statement. The try statement is used to define a code block to be executed. The catch statement defines a code block that handles any errors that occur during execution. The finally block defines a code block that should always be executed, regardless of the outcome of the try statement. The throw statement can be used to define a custom error caught and handled by the catch statement.

Event Loop in JavaScript continuously checks the call stack for executed functions and the message queue for events or tasks to process ensuring that JavaScript remains single-threaded while handling asynchronous operations efficiently.

The difference between == and === in JavaScript is that == is a loose equality operator that compares values after type coercion, while === is a strict equality operator that compares both values and types. === requires both values to be of the same type and have the same value, making it a safer choice for most comparisons to avoid unexpected type conversions.

The importance of use strict directive is that it enforces a stricter set of rules and helps catch common coding mistakes. It prevents the use of undeclared variables, eliminates ambiguous behavior, and encourages a cleaner, more reliable codebase.

Callback Hell, or Pyramid of Doom, occurs when multiple nested callback functions are used in asynchronous JavaScript code, leading to deeply indented and hard-to-read code structures making code maintenance and debugging challenging.

To avoid callback Hell use Promises, async/await, or modularize the code into smaller functions.

A callback function is a function passed as an argument to another function, which is then invoked or executed at a later point in the program's execution.

map(), reduce(), and filter() are array methods in JavaScript. map() transforms each element in an array into a new array based on a provided function. reduce() reduces an array to a single value by applying a function cumulatively to elements. filter() creates a new array with elements that pass a given test.

Closure in JavaScript is when a function retains access to variables from its enclosing scope, even after that outer function has finished executing. Practical examples include creating private variables and functions in JavaScript, maintaining state in event handlers, and implementing data hiding patterns.

To clone an object in JavaScript use methods like Object.assign(), the spread operator (...), or by create a custom cloning functions.

To create a Promise, use the Promise constructor with two parameters: resolve and reject. Then use .then() and .catch() to handle successful and failed outcomes, respectively.

JavaScript handles asynchronous operations using mechanisms like callbacks, Promises, async/await, and the Event Loop.

A function expression is distinguished from a function declaration as function declaration defines a named function with a specific name and is hoisted to the top of its containing scope. A function expression assigns an anonymous or named function to a variable, and it is not hoisted.

The Document Object Model (DOM) is a programming interface and representation of structured documents. It represents a web page's structure as a tree of objects, where each object corresponds to a part of the page, such as an element or an attribute.

To select an element in the DOM use methods like document.getElementById(), document.querySelector(), or document.getElementsByClassName(), depending on the selection criteria.

The difference between innerHTML and textContent is that innerHTML retrieves or sets the HTML content within an element, while textContent retrieves or sets only the text content, excluding any HTML tags.

To add a class using the element.classList.add('classname') method and remove it using the element.classList.remove('classname') method.

Event bubbling and event capturing are two phases of event propagation in the DOM. In event bubbling, the event starts from the target element that triggered the event and bubbles up to the root of the DOM. In event capturing, the process is reversed and the event descends from the root to the target element.

The data-* attribute allows to storage of custom data on an element, providing a way to store extra information that doesn't have any visual representation.

To create a new element and add it to the DOM, use the document.createElement('elementName') method. Use the parentNode.appendChild(newElement) method after setting its properties.

To attach an event handler to a DOM element use the element.addEventListener('eventname', handlerFunction) method.

To prevent the default behavior in an event handler use the event.preventDefault() method within the handler function.

document.ready in jQuery ensures that the DOM is fully loaded and ready for manipulation. It guarantees that scripts run only after the entire page's elements are available, preventing potential errors or unexpected behaviors.

The shadow DOM enables web developers to encapsulate their code by attaching separate DOM trees to specific elements. This mechanism isolates the internal structure, composition, and styling of these elements from the main document, ensuring functional and styling independence. It becomes easier to manage and maintain the code, by keeping them hidden from the rest of the document.

a JavaScript generator is a special type of function that can pause its execution and later resume from where it left off. Regular functions run to completion when invoked and generators produce a sequence of values using the yield keyword and are controlled by the next() method.

A Proxy in JavaScript is an object that wraps another object (target) and intercepts operations, like reading or writing properties, offering a way to customize default behaviors.

An asynchronous function in JavaScript returns a promise. The await keyword can be used inside the function to pause its execution until the promise settles, simplifying the flow of asynchronous code.

JavaScript’s Module pattern provides a way to create private scopes and expose only chosen parts to the external world. It's a design pattern that utilizes closures to encapsulate functionality, allowing for public and private access levels.

A Service Worker is a script running in the background of a browser, separate from the web page. A Service Worker is used for caching resources, enabling offline capabilities, and intercepting network requests to enhance performance and user experience.

A Web Worker is a script executed in the background, on a separate thread from the main execution thread. A Web Worker ensures long-running tasks don't block the main thread, keeping web applications responsive.

Memoization in JavaScript is an optimization technique that stores the results of expensive function calls and returns the cached result when the same inputs reoccur thus reducing the computational time.

Large applications handle state management using centralized state management solutions such as Redux, Vuex, or MobX. These libraries offer a structured and predictable method for managing and updating state across components.

Using JavaScript frameworks/libraries like React or Angular provides advantages such as rapid development, reusable components, and robust ecosystems. The main disadvantages include steep learning curves, potential overengineering, and increased initial load times due to library overheads.

Template literals in ES6 are string literals allowing embedded expressions. They utilize backticks ( ) instead of quotes and can span multiple lines, making string interpolation more intuitive.

The spread operator (...) expands an array or object into its elements or properties. The rest operator, also represented by ..., collects the remaining elements or properties into an array or object.

Arrow functions are a concise way to write functions in ES6. Arrow functions differ from regular functions in syntax and behavior, particularly in their handling of the this keyword, which arrow functions do not bind.

Destructuring assignment in ES6 allows unpacking values from arrays or properties from objects into distinct variables, simplifying the extraction of multiple properties or values.

Default parameters in ES6 allow functions to have predefined values for arguments that are not passed, ensuring the function behaves correctly even if some parameters are missing.

The import and export statements in ES6 modules facilitate modular programming by allowing developers to split code into multiple files and then import or export functionalities as required.

JavaScript Symbols are unique and immutable data types introduced in ES6. They often serve as object property keys to ensure uniqueness and avoid overwriting existing properties.

Iteration in ES6 is a new mechanism for traversing over data. New constructs like the for...of loop, are used to seamlessly iterable objects, such as arrays, strings, maps, and sets, providing a more intuitive way to loop through items.

JavaScript Sets are collections of values where each value must be unique. JavaScript Maps are collections of key-value pairs where keys can be any type. Both of these collections are introduced in ES6 to supplement traditional objects and arrays.

Async functions are an enhancement to promises, allowing to write asynchronous code in a more synchronous-like fashion using the async and await keywords.

The Virtual DOM in React is a lightweight representation of the actual DOM. Virtual DOM enables efficient updates and rendering by minimizing direct interactions with the actual DOM.

The difference between state and props is that the state in React represents the internal data of a component that can change over time. Props are immutable data passed from parent to child components.

Angular's two-way data binding automatically synchronizes the model and the view. When the model changes, the view reflects the change, and vice versa, without additional code to detect and respond to these changes.

The difference between Angular and React is that Angular is a full-fledged framework offering a wide array of built-in tools and features, while React is a library primarily focused on building user interfaces. They have different philosophies, such as Angular using two-way data binding and React opting for one-way data flow.

A hook in React is a function that lets you tap into React features, like state and lifecycle methods, from function components, making them more versatile without converting them to class components.

React handles state management using component local state, context, or third-party libraries like Redux or MobX, depending on the application's complexity and requirements.

Vue.js is different from Angular and React as Vue.js is a progressive JavaScript framework for building user interfaces. Unlike React, which is a library, and Angular, a comprehensive framework, Vue offers a middle ground with an easier learning curve, combined with a flexible and modular approach.

A service in Angular is a class that provides reusable data or functionalities across the application. It is used for tasks like HTTP requests, logging, or data sharing between components.

Redux is a predictable state management library for JavaScript applications. Redux maintains application state in a single immutable object, with changes made through pure functions called reducers, ensuring a consistent and centralized data flow.

A reducer in Redux is a pure function that takes the current state and an action, then returns a new state. It defines how the application's state changes in response to actions dispatched to the store.

The Critical Rendering Path is the sequence of steps browsers undergo to convert HTML, CSS, and JavaScript into pixels rendered on the screen. Optimizing this path ensures web pages render quickly and responsively.

JavaScript is included in HTML using the <script> tag either inline, by placing code directly between the tags, or externally, by linking to an external .js file using the src attribute.

Progressive Web Apps (PWAs) are web applications that offer native-app-like experiences with features like offline access, push notifications, and installation on the home screen, while still being accessible via browsers.

Performance of a JavaScript application is improved by optimizing code, minimizing DOM interactions, leveraging browser caching, deferring non-critical JavaScript, and using Webpack tool for bundling and minification.

Lazy Loading is a performance optimization technique where specific assets, like images or scripts, are loaded only when needed or when they appear in the viewport, reducing initial load times.

Ensuring cross-browser compatibility involves using feature detection, leveraging Babel for transpilation, and testing the code across various browsers and browser versions.

JavaScript code is debugged using browser developer tools, setting breakpoints, inspecting variables, and utilizing console.log() statements or more advanced methods like profiling and monitoring.

Common performance bottlenecks include extensive DOM manipulations, memory leaks, unoptimized images, synchronous blocking calls, and redundant or unused code.

Webpack is vital in modern web development as it's a module bundler and task runner. Webpack bundles JavaScript files, stylesheets, images, and other assets into a single package, optimizing and transforming them for optimal performance.

Tree shaking is an optimization technique used in module bundling, where dead code, or unused modules, are eliminated from the final bundled file, resulting in smaller and more efficient output.

Unit testing in JavaScript involves testing individual units or components of code in isolation to ensure that each part functions as intended.

Testing in JavaScript is performed using testing libraries or frameworks, writing test cases, and then running these tests to verify code correctness.

Popular JavaScript testing libraries/frameworks are Jasmine, Jest, Mocha, Chai, and Karma.

Test-Driven Development (TDD) is a software development methodology where tests are written before the actual code, ensuring that code is developed with testing in mind and meets defined requirements.

Mocking in testing involves creating mock objects or functions to replicate and control the behavior of real objects, enabling isolated testing of specific parts without actual dependencies.

The difference between end-to-end testing and unit testing is that end-to-end testing evaluates the entire application's flow in a real-world scenario, while unit testing focuses on individual components in isolation.

Code linting is crucial for maintaining code quality, as it detects and warns about stylistic errors, potential bugs, and deviations from coding standards.

Ensuring JavaScript code adheres to coding standards involves using tools like ESLint or TSLint, setting up custom rules, and integrating them into the development process.

Continuous Integration (CI) and Continuous Deployment (CD) are vital for ensuring code consistency, automating testing, and speeding up the delivery of software updates or features to end users.

Key principles of writing clean code are writing readable and understandable code, keeping functions and modules short, using meaningful names, avoiding code duplication, and adhering to the Single Responsibility Principle.

Use the built-in JavaScript methods, to reverse a string.

Iterate through the string and use a frequency counter.

A string is a palindrome if it reads the same backward as forward.

Use a two-pointer technique to merge sorted arrays.

Debounce ensures a function doesn't fire too frequently.

A basic Promise implementation involves resolve and reject functions.

You can use recursion to deep clone objects.

Binary search halves the search interval with each step.

A pub-sub system involves subscribing to events and publishing them.

A recursive approach to compute Fibonacci.

Yes, a JavaScript developer can work remotely. With the ubiquity of online collaboration tools, cloud platforms, and version control systems like Git, JavaScript developers can efficiently collaborate, code, and deploy applications from anywhere in the world. The nature of web development, combined with the digital tools available, makes remote work a viable and often preferred option for many JavaScript developers.

Yes, JavaScript developers need continuous learning. The world of web development, and particularly JavaScript, is dynamic and rapidly evolving. New frameworks, tools, and best practices emerge regularly. JavaScript developers must stay updated with the latest advancements, adopt new methodologies, and continuously hone their skills, to remain relevant and efficient. This ongoing learning ensures they deliver optimal solutions and keep pace with the ever-changing landscape of web technology.

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