Javascript Interview Questions and Answers 1.0

 Hi everyone! In this video, we'll be diving into some of the most frequent JavaScript interview questions and their answers. Stay tuned!

Let's start with an easy one. What's the difference between `forEach` and `map` in JavaScript?

 Sure, `forEach` is used to execute a function on each element in an array but doesn't return anything, whereas `map` creates a new array with the results of calling a provided function on every element in the calling array.

 Interesting. And how does the `filter` method work in JavaScript?

 `filter` creates a new array with all elements that pass the test implemented by the provided function. It's great for conditional filtering of array elements.

 Can you describe the `reduce` method?

 Certainly! `reduce` executes a reducer function on each element of the array, resulting in a single output value. It's like folding an array into a single value using some operation.

 What about Promises? What is a Promise in JavaScript?

 A Promise represents an operation that hasn't completed yet but is expected in the future. It allows you to write asynchronous code in a more manageable way.

 How does the `finally` method work with Promises?

 `finally` is a method used with Promises which allows you to execute some code regardless of the promise's outcome, be it resolved or rejected. It's great for cleanup operations.

 What are callback functions?

 Callback functions are functions passed into another function as an argument, which are then invoked inside the outer function to complete some kind of routine or action.

 Can you explain event delegation?

 Event delegation is a technique of handling events where you use a single event listener on a parent element to manage all of its children. It's efficient for dynamic elements and reduces memory usage.

 Describe the `this` keyword in JavaScript.

 The `this` keyword in JavaScript refers to the object it belongs to. It has different values depending on where it's used: in a method, alone, in a function, in an event, etc.

 How do you clone an object in JavaScript?

 You can clone an object using techniques like the spread syntax, `Object.assign()`, or deep cloning methods like `JSON.parse(JSON.stringify(object))`.

 What's the difference between `==` and `===`?

 `==` checks for equality after type coercion, while `===` checks for both value and type equality, which is why `===` is generally recommended for more predictable results.

 What are the different types of JavaScript loops?

 In JavaScript, we have `for`, `while`, `do...while` loops, and array methods like `forEach`, `map`, `filter`, `reduce` which also loop through elements.

 Can you describe variable hoisting?

 Hoisting is JavaScript's default behavior of moving declarations to the top of the current scope. So, a variable can be used before it has been declared.

 Moving on, what is a prototype in JavaScript?

 A prototype is a mechanism by which JavaScript objects inherit features from one another. It's a core concept for understanding object-oriented programming in JavaScript.

 What is AJAX?

 AJAX stands for Asynchronous JavaScript and XML. It's a set of web development techniques enabling web applications to send and retrieve data from a server asynchronously, without interfering with the display and behavior of the existing page.

 Can you explain the Document Object Model (DOM)?

 The DOM is a programming interface for web documents. It represents the page so that programs can change the document structure, style, and content. The DOM represents the document as nodes and objects; that way, programming languages can interact with the page.

 How does `bind()` work?

 The `bind()` method creates a new function that, when called, has its `this` keyword set to the provided value. It's useful for making sure that a function has the correct `this` value regardless of how it's used.

 What are arrow functions?

 Arrow functions are a shorter syntax for writing functions in JavaScript. They are anonymous and change the way `this` binds in functions.

 What's the difference between `splice` and `slice`?

 `splice` modifies an array by adding, removing, or replacing elements, whereas `slice` returns a shallow copy of a portion of an array without modifying the original array.

 Can you define what a JavaScript class is?

 A JavaScript class is a type of function designed to create objects and encapsulate data and behavior related to that object. Classes support inheritance, base class access, static methods, and constructors.

 What are template literals in JavaScript?

 Template literals are string literals allowing embedded expressions. They can contain placeholders, which are indicated by the dollar sign and curly braces (`${expression}`). They're used for string interpolation and multiline strings in JavaScript.

 Explain destructuring in JavaScript.

 Destructuring is a JavaScript expression that allows you to unpack values from arrays, or properties from objects, into distinct variables.

 What is the difference between `target` and `currentTarget` in the event context?

 In an event, `target` refers to the element that triggered the event, while `currentTarget` refers to the element that the event listener is attached to.

 How do you add an event listener to a DOM element?

 You can add an event listener to a DOM element using the `addEventListener` method. You specify the type of event to listen for and the callback function to execute when the event is triggered.

 What is a JavaScript generator?

 A JavaScript generator is a function that can stop midway and then continue from where it stopped. This is achieved using `yield` and `function*` syntax.

 Can you explain the concept of a "single page application" (SPA)?

 A single page application (SPA) is a web application or website that interacts with the user by dynamically rewriting the current page rather than loading entire new pages from the server. This results in a more fluid and faster user experience.

 How can you share code between files in JavaScript?

 In JavaScript, code can be shared between files using export and import statements. This modular approach helps in maintaining and organizing large codebases.

 What is the async/await syntax useful for?

 The async/await syntax in JavaScript is used for writing asynchronous code in a more synchronous, cleaner, and more readable manner. It simplifies the process of working with promises.

 What's the difference between a "function declaration" and a "function expression"?

 A function declaration defines a function with the specified parameters and is hoisted, allowing it to be used before it's defined. A function expression, on the other hand, is not hoisted and can be anonymous. It's created at runtime.

 What is Babel?

 Babel is a JavaScript transpiler that converts edge JavaScript into plain old ES5 JavaScript that can run in any browser, even the older ones.

 How do you handle errors in Promises?

 Errors in Promises can be handled using the `.catch()` method, which is used to catch any errors that occur during the execution of the promise.

 How do you prevent a function from being called multiple times in a row?

 This can be achieved by using debouncing or throttling techniques. Debouncing prevents a function from being called again until a certain amount of time has passed, whereas throttling limits the number of times a function can be called over time.

 What is transpiling?

 Transpiling is the process of converting source code written in one language or a newer version of a language into another language or an older version. It's commonly used in JavaScript to convert ES6 code into ES5.

 What are the benefits of using TypeScript?

 TypeScript offers benefits like strong typing, object-oriented features, compile-time error checking, and improved readability and maintainability of the code, making it easier to write robust software.

 How do closures work in JavaScript?

 Closures in JavaScript are functions that have access to the parent scope, even after the parent function has closed. This is useful for data encapsulation and creating private variables.

 How can you make a JavaScript function sleep or wait?

 You can create a delay in JavaScript by using the `setTimeout` function or by using the `await` keyword with a Promise that resolves after a specified time.

 What is a Set in JavaScript?

 A Set in JavaScript is a collection of unique values. It allows you to store distinct values of any type, whether primitive values or object references.

 What is the purpose of the `data-` attribute in HTML?

 The `data-` attribute is used to store custom data private to the page or application, on HTML elements. The data can then be used in JavaScript to create a more interactive experience.

 What is CORS?

 CORS stands for Cross-Origin Resource Sharing. It's a security feature that allows or restricts resources on a web page to be requested from another domain outside the domain from which the resource originated.

 What is JSON?

 JSON, or JavaScript Object Notation, is a lightweight data-interchange format that's easy for humans to read and write and easy for machines to parse and generate. It's commonly used for transmitting data in web applications.

 What are JavaScript cookies?

 Cookies are small pieces of data stored on the user's computer by the web browser while browsing a website. They are used to remember information about the user, like login information or site preferences.

 What is local storage?

 Local storage is a way to store data on the client's computer. It allows the saving of key/value pairs in a web browser and is part of the Web Storage API.

 What's the difference between local storage and session storage?

 The main difference is that local storage persists even when the browser is closed and reopened, while session storage is limited to a session and is cleared when the browser is closed.