JavaScript interview questions with answers

 Basic Concepts:

• What is JavaScript?
  • JavaScript is a high-level, interpreted programming language used to create interactive and dynamic web pages.
• What are the differences between JavaScript and Java?
  • JavaScript is a scripting language primarily used for client-side web development, while Java is a general-purpose programming language used for various applications.
• What are the data types in JavaScript?
  • Primitive data types include string, number, boolean, null, and undefined. The object data type is object.
• Explain the difference between null and undefined.
  • null represents the intentional absence of any value, while undefined represents the absence of a value in general, often due to the variable being declared but not assigned.
• What is the use of the typeof operator?
  • The typeof operator is used to determine the data type of a variable or expression in JavaScript.
• Explain the concept of hoisting in JavaScript.
  • Hoisting is a JavaScript mechanism where variables and function declarations are moved to the top of their containing scope during the compilation phase.

Variables and Scope:

• What is the difference between var, let, and const?
  • var is function-scoped and can be reassigned and redeclared. let is block-scoped and can be reassigned but not redeclared. const is block-scoped and cannot be reassigned or redeclared after initialization.
• Explain variable scope in JavaScript.
  • Scope refers to the accessibility of variables in different parts of the code. JavaScript has global scope, function scope, and block scope.
• What is closure in JavaScript?
  • A closure is a function bundled together with references to its surrounding state (the lexical environment). It allows the function to access and manipulate outer variables even after the outer function has finished executing.

Functions:

• What are JavaScript functions?
  • JavaScript functions are blocks of reusable code that perform a specific task. They can be called multiple times and accept parameters.
• What is the difference between function declarations and function expressions?
  • Function declarations are defined using the function keyword followed by a name and parameters. Function expressions are defined by assigning a function to a variable.
• What is an arrow function? How does it differ from regular functions?
  • Arrow functions are a concise way to write functions in JavaScript. They have a shorter syntax compared to regular functions and do not have their own this keyword.

Arrays and Objects:

• How do you create an array in JavaScript?
  • Arrays can be created using square brackets [], and elements can be separated by commas.
• Explain the difference between arrays and objects.
  • Arrays are a special type of object used to store multiple values in a single variable. Objects are collections of key-value pairs.
• How do you iterate over objects in JavaScript?
  • You can iterate over objects using for...in loop, Object.keys(), Object.values(), or Object.entries() methods.

DOM Manipulation:

• What is the Document Object Model (DOM)?
  • The DOM is a programming interface for web documents. It represents the structure of a document as a tree of nodes, allowing JavaScript to interact with and manipulate the content and structure of a web page.
• How do you select elements in the DOM?
  • You can select elements using methods like getElementById(), getElementsByClassName(), getElementsByTagName(), querySelector(), and querySelectorAll().
• How do you create new elements dynamically using JavaScript?
  • You can create new elements using the document.createElement() method and append them to the DOM using methods like appendChild() or insertBefore().

Events:

• What are events in JavaScript?
  • Events are actions or occurrences that happen in the system you are programming, which the system tells you about so you can respond to them.
• How do you attach event handlers to HTML elements?
  • Event handlers can be attached to HTML elements using attributes like onclick, or through JavaScript using methods like addEventListener().
• Explain event bubbling and event delegation.
  • Event bubbling is the propagation of events from the innermost target element to the outermost ancestor. Event delegation is a technique where you attach an event listener to a parent element instead of individual child elements.

Asynchronous Programming:

• What is asynchronous programming in JavaScript?
  • Asynchronous programming allows code execution to continue while waiting for asynchronous operations to complete, preventing blocking of the main thread.
• Explain the difference between synchronous and asynchronous code.
  • Synchronous code executes line by line and waits for each operation to complete before moving to the next. Asynchronous code allows multiple operations to be executed concurrently, and callbacks or promises are used to handle the results.
• How do you handle asynchronous operations in JavaScript?
  • Asynchronous operations can be handled using callbacks, promises, or async/await syntax.

ES6 and Beyond:

• What are the new features introduced in ES6 (ECMAScript 2015)?
  • ES6 introduced several new features including let and const for variable declarations, arrow functions, template literals, destructuring, spread/rest operators, classes, and modules.
• Explain the concepts of destructuring, template literals, and spread/rest operators.
  • Destructuring allows you to extract values from arrays or objects into variables. Template literals are string literals allowing embedded expressions. Spread/rest operators allow an iterable to be expanded or collected into multiple elements.

Error Handling:

• How do you handle errors in JavaScript?
  • Errors in JavaScript can be handled using try...catch blocks. The try block contains the code where an exception might occur, and the catch block handles the exception if it occurs.
• What is the purpose of try, catch, and finally blocks?
  • The try block contains the code to be executed and may throw an exception. The catch block handles the exception thrown by the try block. The finally block contains code that will be executed regardless of whether an exception is thrown or not.

Module System:

• What is the module system in JavaScript?
  • The module system in JavaScript allows you to split your code into multiple files or modules, making it easier to manage and maintain. Modules can export and import functionalities.
• How do you export and import modules in JavaScript?
  • You can export functionalities using the export keyword and import them using the import keyword.

Prototype and Inheritance:

• Explain the concept of prototype in JavaScript.
  • In JavaScript, each object has a prototype, which serves as a blueprint for the object. Objects inherit properties and methods from their prototype.
• How does prototypal inheritance work in JavaScript?
  • Prototypal inheritance allows objects to inherit properties and methods from their prototype. If a property or method is not found in the object itself, JavaScript looks for it in the object's prototype chain.

Functional Programming:

• What is functional programming?
  • Functional programming is a programming paradigm focused on building software by composing pure functions, avoiding shared state, mutable data, and side effects.
• Explain higher-order functions and their use cases.
  • Higher-order functions are functions that can take other functions as arguments or return functions as results. They are commonly used for abstraction and code reuse.

Testing and Debugging:

• What are some common techniques for debugging JavaScript code?
  • Common techniques for debugging JavaScript code include using console.log(), breakpoints in browser developer tools, and debugging tools like Chrome DevTools.
• What are unit tests and how do you write them in JavaScript?
  • Unit tests are tests that validate the behavior of individual units or components of a software system. In JavaScript, you can write unit tests using testing frameworks like Jasmine, Mocha, or Jest.

Performance Optimization:

• How do you optimize JavaScript code for better performance?
  • JavaScript code can be optimized for better performance by minimizing DOM manipulation, reducing network requests, using efficient algorithms, and optimizing rendering performance.
• Explain the concepts of lazy loading and code splitting.
  • Lazy loading is a technique where resources are loaded asynchronously only when they are needed, reducing initial page load time. Code splitting involves breaking up code into smaller chunks and loading them on-demand, improving load times for large applications.