JavaScript Questions

 Question: What is JavaScript?

Answer: JavaScript is a high-level, interpreted programming language primarily used for creating interactive web pages.

Question: What are the key features of JavaScript?

Answer: Key features include dynamic typing, prototype-based object orientation, and first-class functions.

Question: Differentiate between null and undefined.

Answer: null represents the intentional absence of any object value, while undefined represents a variable that has been declared but has not been assigned a value.

Question: Explain the concept of closures in JavaScript.

Answer: Closures are functions that have access to the outer function's scope even after the outer function has finished executing.

Question: What is the difference between == and === in JavaScript?

Answer: == is the equality operator that performs type coercion, while === is the strict equality operator that checks both value and type.

Question: Explain the event bubbling and event capturing in JavaScript.

Answer: Event bubbling refers to the propagation of events from inner elements to outer elements, while event capturing is the opposite, propagating events from outer elements to inner elements.

Question: How does hoisting work in JavaScript?

Answer: Hoisting is a JavaScript mechanism where variables and function declarations are moved to the top of their containing scope during the compilation phase.

Question: What is the purpose of the this keyword in JavaScript?

Answer: The this keyword refers to the context in which a function is called.

Question: Explain the concept of prototypes in JavaScript.

Answer: Prototypes are used for inheritance in JavaScript, allowing objects to inherit properties and methods from other objects.

Question: What are the differences between let, const, and var?

Answer: let and const are block-scoped variables, while var is function-scoped. let allows reassignment, const does not.

Question: Explain the concept of callback functions in JavaScript.

Answer: Callback functions are functions passed as arguments to other functions to be executed later.

Question: What are Promises in JavaScript? How do they differ from callbacks?

Answer: Promises are objects representing the eventual completion or failure of an asynchronous operation. They provide better control flow compared to callback functions.

Question: How does asynchronous programming work in JavaScript?

Answer: Asynchronous programming allows multiple operations to be performed concurrently, avoiding blocking the main execution thread.

Question: Explain the difference between synchronous and asynchronous code execution in JavaScript.

Answer: Synchronous code executes one statement at a time in order, while asynchronous code allows multiple statements to execute simultaneously without waiting for each other to complete.

Question: What is the Event Loop in JavaScript?

Answer: The Event Loop is a mechanism that handles asynchronous callbacks and ensures non-blocking I/O operations in JavaScript.

Question: How would you handle errors in JavaScript?

Answer: Errors in JavaScript can be handled using try...catch blocks or by attaching error event listeners.

Question: Explain the differences between ES5, ES6 (ES2015), ES7, and ES8.

Answer: ES5 introduced significant improvements over ES3, ES6 (ES2015) introduced classes, arrow functions, and let/const declarations, ES7 and ES8 introduced features like async/await and shared memory.

Question: What are Arrow functions? How do they differ from regular functions?

Answer: Arrow functions are concise functions introduced in ES6, with implicit returns and lexical scoping of this.

Question: What are Generators in JavaScript?

Answer: Generators are functions that can pause and resume their execution, allowing for asynchronous-like behavior.

Question: Explain the concept of closures and how they affect memory management in JavaScript.

Answer: Closures retain references to their outer scope, potentially causing memory leaks if not managed properly.

Question: What are modules in JavaScript? How do you use them?

Answer: Modules in JavaScript allow for better organization and encapsulation of code. They can be imported and exported using import and export statements.

Question: Explain the concept of currying in JavaScript.

Answer: Currying is the process of converting a function that takes multiple arguments into a sequence of functions that each take a single argument.

Question: How does async/await work in JavaScript? Provide an example.

Answer: async/await is a syntactic sugar for asynchronous programming, making asynchronous code look synchronous.

Question: What are the differences between the map(), filter(), and reduce() methods in JavaScript?

Answer: map() creates a new array by applying a function to each element, filter() creates a new array with elements that pass a test, reduce() applies a function to each element and reduces the array to a single value

Question: What is the DOM (Document Object Model) and how does it relate to HTML?

Answer: The DOM represents the structure of an HTML document as a tree of objects, allowing JavaScript to interact with and manipulate the document's content and structure.

Question: Explain how you would add an event listener to an HTML element using JavaScript.

Answer: Event listeners can be added using the addEventListener() method, specifying the event type and the function to execute when the event occurs.

Question: What are Web APIs? Provide examples.

Answer: Web APIs provide additional functionalities to JavaScript, such as the Fetch API for making HTTP requests and the Geolocation API for obtaining geographical information.

Question: How do you make an AJAX call in JavaScript?

Answer: AJAX calls can be made using the XMLHttpRequest object or the fetch() API.

Question: Explain the concept of local storage and session storage in HTML5

Answer: Local storage and session storage are mechanisms for storing key-value pairs locally in the user's browser. Session storage data is cleared when the session ends, while local storage data persists until explicitly cleared.

Question: What are some best practices for writing clean and efficient JavaScript code?

Answer: Best practices include using descriptive variable names, avoiding global variables, minimizing DOM manipulation, and using efficient algorithms and data structures.

Question: How would you optimize the performance of a JavaScript application?

Answer: Performance optimization techniques include minimizing network requests, optimizing images and other media, lazy loading resources, and using caching strategies.

Question: Explain the concept of cross-origin resource sharing (CORS) in JavaScript.

Answer: CORS is a security feature implemented by browsers to restrict cross-origin HTTP requests initiated from within scripts.