Javascript Interview Question and Answer 1

Question	Answer
What is JavaScript?	- High-level, interpreted scripting language.
	- Makes web pages interactive.
	- One of the web's three main pillars: HTML, CSS, and JavaScript.
Differentiate between Java and JavaScript.	- Java: compiled, object-oriented programming language.
	- JavaScript: interpreted scripting language for web development.
	- Two distinct languages.
What are 'undefined' and 'null' in JavaScript?	- Represent absence of value.
	- 'undefined': variable declared but not yet assigned a value.
	- 'null': assignment value representing no value or object.
What is the use of 'this' keyword in JavaScript?	- Refers to the object it belongs to.
	- Can refer to different objects depending on context.
What's a closure in JavaScript?	- Function accessing its own scope, outer function's scope, and global scope.
What is the difference between '==' and '==='?	- '==': checks value equality with type coercion.
	- '===': checks both value and type equality without type coercion.
What are 'truthy' and 'falsy' values?	- 'truthy': evaluates to true in a boolean context.
	- 'falsy': evaluates to false. Includes 0, "", null, undefined, NaN, and false.
What's an Immediately Invoked Function Expression (IIFE)?	- Function that runs as soon as it's defined: (function(){ /* code */ })().
What are JavaScript data types?	- Undefined, Null, Boolean, String, Symbol, Number, and Object.
What is a callback function?	- Function passed into another function as an argument.
	- Executed after the outer function completes.
What's 'hoisting'?	- JavaScript mechanism moving variables and function declarations to the top of their scope.
What is the difference between let, const, and var?	- var: function-scoped.
	- let and const: block-scoped.
	- const: cannot be reassigned.
Explain event delegation.	- Single event listener on a common ancestor of elements to catch events as they bubble up.
What is the Document Object Model (DOM)?	- Tree-like structure representing content of a webpage.
	- Can be manipulated by JavaScript.
What's the difference between null and undefined?	- null: intentional absence of value.
	- undefined: variable declared but not yet assigned a value.
What's the difference between a method and a function?	- Function: block of reusable code.
	- Method: function associated with an object.
What is a Promise?	- Represents a value: available now, in the future, or never.
	- Has states: pending, fulfilled, rejected.
What's the difference between window, screen, and document in JavaScript?	- window: global object for browser's window.
	- screen: information about user's screen.
	- document: represents the DOM.
What is AJAX?	- Asynchronous JavaScript And XML.
	- Technique for dynamic web pages; makes server requests without reloading the page.
What's a cross-origin request?	- Request for a resource from a different domain, protocol, or port due to same-origin policy.
How does bind() work?	- Creates a new function with a specific context for the this value.
What's the difference between apply(), call(), and bind()?	- All set the this value.
	- call() and apply(): invoke function immediately.
	- apply(): accepts array of arguments.
	- bind(): returns new function with specified this value.
What are arrow functions?	- Introduced in ES6: concise way to write functions.
	- Don't bind their own this value.
What's a prototype in JavaScript?	- Every object has a prototype for inheritance.
What's the difference between a static method and an instance method?	- Static: belongs to the class.
	- Instance: belongs to the object instance, accesses instance data.
What's the event loop?	- Handles asynchronous operations.
	- Checks if call stack is empty; if so, adds functions from the event queue.
What are JavaScript timers?	- setTimeout() and setInterval(): execute a function after a set period.
How can you convert a string to a number?	- parseInt(), parseFloat(), unary + operator, or Number() function.
How do you clone an object in JavaScript?	- Object.assign(), spread operator (...).
	- Deep clone: libraries like lodash or JSON.parse(JSON.stringify(object)).
What's the difference between a shallow copy and a deep copy?	- Shallow: clones outermost object.
	- Deep: clones entire hierarchy of objects.
What is a JavaScript class?	- Blueprint for creating objects.
	- Introduced in ES6; supports inheritance, constructors, methods.
What's the difference between forEach and map?

Question	Answer
What's a prototype in JavaScript?	- Every JavaScript object has a prototype. <br> - It inherits properties from this prototype. <br> - Mechanism for achieving inheritance.
What is AJAX?	- Stands for Asynchronous JavaScript And XML. <br> - Technique for creating fast, dynamic web pages. <br> - Allows requests to server without reloading entire page.
What is the Document Object Model (DOM)?	- Tree-like structure representing content of a webpage. <br> - Can be manipulated using JavaScript.
How does bind() work?	- bind() creates a new function. <br> - Sets a certain context for the this value.
What are arrow functions?	- Introduced in ES6. <br> - Concise way to write functions. <br> - Doesn't bind its own this value.
What's the difference between splice and slice?	- splice: modifies original array (add/remove elements). <br> - slice: returns portion of array without modifying original.
What is a JavaScript class?	- Introduced in ES6. <br> - Blueprint for creating objects. <br> - Supports inheritance, constructors, and methods.
What are template literals in JavaScript?	- Introduced in ES6. <br> - Allows for embedded expressions and string interpolation. <br> - Uses backticks ( ).
What is destructuring in JavaScript?	- Allows binding of parts of data structures (arrays or objects) to variables.
What is the difference between target and currentTarget in event context?	- target: element where event occurred. <br> - currentTarget: element with the event listener attached.
How do you add an event listener to a DOM element?	- Using the addEventListener method.
What is a JavaScript generator?	- Function that can pause/resume its execution. <br> - Introduced in ES6. <br> - Uses function* syntax and yield keyword.
Explain the concept of a "single page application" (SPA).	- Web application that rewrites current page for interaction. <br> - Doesn't load new pages from the server.
How can you share code between files in JavaScript?	- Using modules with import and export statements.
What is the async/await syntax useful for?	- Cleaner way to handle asynchronous operations. <br> - Makes them appear synchronous.
What's the difference between a "function declaration" and a "function expression"?	- Function declaration: defines named function, no variable assignment. <br> - Function expression: assigns unnamed function to a variable.
What is Babel?	- JavaScript compiler/transpiler. <br> - Converts next-gen JavaScript to older versions for browser compatibility.
How do you handle errors in Promises?	- Use .catch() method. <br> - Or use try/catch with async/await.
How do you prevent a function from being called multiple times in a row?	- Use debouncing or throttling techniques.
What is transpiling?	- Converting source code from one language/version to another. <br> - E.g., from ES6 to ES5 for browser compatibility.
What are the benefits of using TypeScript?	- Typed superset of JavaScript. <br> - Provides type safety, better refactoring, early bug detection.
How do closures work in JavaScript?	- Function with access to the parent scope even after parent function has closed. <br> - Allows data privacy and emulating private methods.
How can you make a JavaScript function sleep or wait?	- Use setTimeout function. <br> - Or use async/await with a promise-based delay.
What is a Set in JavaScript?	- ES6 collection of values. <br> - Each value must be unique.
What is the purpose of the data- attribute in HTML?	- Stores custom data private to the page or application. <br> - In JavaScript, can access using the dataset property.
What is CORS?	- Stands for "Cross-Origin Resource Sharing". <br> - Browser security feature controlling requests for resources from different origins.
What is JSON?	- Stands for JavaScript Object Notation. <br> - Lightweight data-interchange format. <br> - Easy for humans to read/write.
What are JavaScript cookies?	- Small pieces of data stored on user's browser. <br> - Used to track actions or save state.
What is local storage?	- Web storage for storing key-value pairs in a web browser. <br> - Data persists with no expiration time.
What's the difference between local storage and session storage?	- Local storage: persists data after browser closes. <br> - Session storage: retains data for one session, deleted after browser closes.

Question	Answer
What's the difference between forEach and map?	- forEach: iterates over elements; doesn't return new array. <br> - map: iterates and transforms each element; returns new array.
How does the filter method work in JavaScript?	- Iterates over array. <br> - Returns new array with elements passing a test (predicate function).
Describe the reduce method in JavaScript.	- Iterates over array. <br> - Accumulates array values into a single value. <br> - Takes a reducer and initial value as arguments.
What is a Promise in JavaScript?	- Object representing eventual completion or failure of async operation. <br> - Has three states: pending, resolved, and rejected.
How does the finally method work with Promises?	- Executes after promise is either fulfilled or rejected. <br> - Doesn't receive promise results.
What are callback functions?	- Functions passed as arguments to another function. <br> - Executed after the outer function completes.
What is event delegation?	- Technique of adding event listener to common parent. <br> - Relies on event propagation to detect events on child elements.
Describe the this keyword in JavaScript.	- Refers to object executing current function. <br> - Context depends on invocation (e.g., global, object, event).
How do you clone an object in JavaScript?	- Using Object.assign(). <br> - Or using spread operator (...).
What's the difference between == and ===?	- ==: loose equality, type coercion can occur. <br> - ===: strict equality, no type coercion.
What are the different types of JavaScript loops?	- for, while, do...while, for...in, for...of.
Describe variable hoisting.	- Variables declared with var are "moved" to top of their scope. <br> - Initialization remains where it is.

Question	Answer
What's the difference between forEach and map?	- forEach: Iterates over array elements, doesn't return a new array. <br> - map: Iterates and creates a new array based on the function provided.

Question	Answer
What's the difference between forEach and map?	- forEach: iterates over elements; doesn't return new array. <br> - map: iterates and transforms each element; returns new array.
How does the filter method work in JavaScript?	- Iterates over array. <br> - Returns new array with elements passing a test (predicate function).
Describe the reduce method in JavaScript.	- Iterates over array. <br> - Accumulates array values into a single value. <br> - Takes a reducer and initial value as arguments.
What is a Promise in JavaScript?	- Object representing eventual completion or failure of async operation. <br> - Has three states: pending, resolved, and rejected.
How does the finally method work with Promises?	- Executes after promise is either fulfilled or rejected. <br> - Doesn't receive promise results.
What are callback functions?	- Functions passed as arguments to another function. <br> - Executed after the outer function completes.
What is event delegation?	- Technique of adding event listener to common parent. <br> - Relies on event propagation to detect events on child elements.
Describe the this keyword in JavaScript.	- Refers to object executing current function. <br> - Context depends on invocation (e.g., global, object, event).
How do you clone an object in JavaScript?	- Using Object.assign(). <br> - Or using spread operator (...).
What's the difference between == and ===?	- ==: loose equality, type coercion can occur. <br> - ===: strict equality, no type coercion.
What are the different types of JavaScript loops?	- for, while, do...while, for...in, for...of.
Describe variable hoisting.	- Variables declared with var are "moved" to top of their scope. <br> - Initialization remains where it is.