Q. What is JavaScript? 
Ans: JavaScript is a programming language used for creating interactive websites and web applications. It is a client-side scripting language that can be used to create dynamic web content and interact with user interfaces.

Q. What are the differences between JavaScript and Java? 
Ans: JavaScript is a scripting language used primarily for creating web content, while Java is a general-purpose programming language used for a wide range of applications. Java code needs to be compiled before it can be executed, while JavaScript code can be executed directly in a web browser.


Q. What are the data types supported by JavaScript? 
Ans: JavaScript supports several data types, including strings, numbers, booleans, null, undefined, and objects. Additionally, JavaScript supports the concept of dynamic typing, which means that the data type of a variable can change during the execution of a program.

Q. What is an object in JavaScript? 
Ans: In JavaScript, an object is a collection of key-value pairs, where the key is a string and the value can be any data type, including other objects. Objects in JavaScript are used to represent real-world entities and can be used to store and manipulate complex data structures.

Q. What is a closure in JavaScript? 
Ans: A closure in JavaScript is a function that has access to variables in its outer scope, even after that scope has closed. This allows the function to maintain access to variables that it needs to reference, even after the function that created those variables has finished executing.

Q. What is the difference between null and undefined in JavaScript? 
Ans: In JavaScript, null represents the intentional absence of any object value, while undefined represents the absence of a value of any type. Null is typically used to indicate that a variable has no value, while undefined is often used to indicate that a variable has not been assigned a value.

Q. What is the difference between == and === in JavaScript? 
Ans: In JavaScript, the == operator is used to test for equality between two values, but it allows for type coercion, which means that it will attempt to convert values to a common type before making the comparison. The === operator, on the other hand, tests for strict equality, which means that it will only return true if both values have the same type and value.

Q. What is the difference between let, const, and var in JavaScript? 
Ans: In JavaScript, let and const are used to declare variables with block scope, while var is used to declare variables with function scope. Variables declared with let can be reassigned, while variables declared with const cannot. Additionally, const variables must be initialized when they are declared.

Q. What is a callback function in JavaScript? 
Ans: In JavaScript, a callback function is a function that is passed as an argument to another function and is intended to be called when the first function has completed its operation. Callback functions are commonly used in event-driven programming and asynchronous operations.

Q. What is event bubbling in JavaScript? 
Ans: In JavaScript, event bubbling is a mechanism by which events that occur on a nested element will also propagate up to its parent elements, triggering any event listeners attached to those elements as well. Event bubbling can be stopped using the stopPropagation() method.

Q. What is the difference between let and var in JavaScript? 
Ans: In JavaScript, let and var are used to declare variables, but they have different scoping rules. Variables declared with let have block scope, meaning they are only accessible within the block they are declared in. Variables declared with var have function scope, meaning they are accessible throughout the entire function they are declared in.

Q. What are the different types of error in JavaScript? 
Ans: There are three main types of error in JavaScript: syntax errors, runtime errors, and logic errors. Syntax errors occur when there is a mistake in the code that prevents it from being executed. Runtime errors occur when the code is executed and encounters a problem, such as trying to access a variable that doesn't exist. Logic errors occur when the code executes without error, but produces the wrong result.

Q. What is the difference between synchronous and asynchronous programming in JavaScript? 
Ans: Synchronous programming in JavaScript is when code is executed in a sequential and blocking manner, meaning that the code waits for each line to execute before moving on to the next. Asynchronous programming in JavaScript allows code to execute in a non-blocking manner, meaning that it can move on to other tasks while waiting for a task to complete. Asynchronous programming is commonly used in event-driven programming and with APIs that use callbacks or promises.

Q. What is a promise in JavaScript? 
Ans: A promise in JavaScript is an object that represents the eventual completion or failure of an asynchronous operation and its resulting value. Promises allow for asynchronous code to be written in a more synchronous style and can be used to handle errors and control flow.

Q. What is event delegation in JavaScript? 
Ans: Event delegation in JavaScript is a technique where event listeners are attached to a parent element rather than to individual child elements. This allows the parent element to listen for events that occur on its child elements and handle those events appropriately.

Q. What is the difference between the document object and the window object in JavaScript? 
Ans: In JavaScript, the document object represents the web page that is being displayed, while the window object represents the browser window or tab that is displaying the page. The window object provides methods for interacting with the browser, such as opening new windows or navigating to a different URL.

Q. What is the difference between the innerHTML and textContent properties in JavaScript? 
Ans: The innerHTML property in JavaScript returns the HTML content of an element, including any nested elements and tags. The textContent property, on the other hand, returns only the text content of an element, without any HTML tags.



Q. What is hoisting in JavaScript? 
Ans: Hoisting in JavaScript is a mechanism by which variables and functions are moved to the top of their respective scopes during compilation, regardless of where they are declared in the code. This allows variables and functions to be used before they are declared, but can also lead to unexpected behavior if not understood properly.

Q. What is a constructor function in JavaScript? 
Ans: A constructor function in JavaScript is a special function used to create objects with a specific set of properties and methods. Constructor functions are called using the "new" keyword and are used to create new instances of an object with the same properties and methods.

Q. What is the difference between call and apply methods in JavaScript? 
Ans: Both call and apply are methods used to invoke a function with a specific context or value of "this". The difference between the two is that call takes a comma-separated list of arguments, while apply takes an array of arguments.

Q. What is closure in JavaScript? 
Ans: A closure in JavaScript is a function that has access to variables in its outer (enclosing) function scope, even after the outer function has returned. This allows for the creation of private variables and functions that are not accessible outside of the closure.

Q. What is the difference between null and undefined in JavaScript? 
Ans: In JavaScript, null and undefined are both used to represent the absence of a value. The difference between the two is that undefined is a variable that has been declared but has not been assigned a value, while null is an explicitly assigned value representing no value.

Q. What is JSON in JavaScript? 
Ans: JSON (JavaScript Object Notation) is a lightweight data interchange format that is easy for humans to read and write and easy for machines to parse and generate. It is often used to transmit data between a server and a web application, as well as for storing and exchanging data within a web application.

Q. What is the difference between let and const in JavaScript? 
Ans: In JavaScript, let and const are used to declare variables, but they have different reassignment rules. Variables declared with let can be reassigned, while variables declared with const cannot be reassigned. However, the values of variables declared with const can be mutated, meaning that their properties can be changed.

Q. What is the purpose of the keyword "this" in JavaScript? 
Ans: In JavaScript, the keyword "this" refers to the current object or context in which the code is executing. The value of "this" is determined by the way in which a function is called and can vary depending on the context in which the function is called.

Q. What is the difference between a for loop and a forEach loop in JavaScript? 
Ans: A for loop in JavaScript is a traditional loop that executes a block of code for a specific number of iterations, while a forEach loop is a method used to iterate over the elements of an array or other iterable object. A forEach loop executes a specified function once for each element in the array and can be used to perform an operation on each element.

Q. What is a generator function in JavaScript? 
Ans: A generator function in JavaScript is a special type of function that can be paused and resumed during execution, allowing for more flexible and efficient control flow. Generator functions are declared using the function* syntax and are called using the yield keyword.

Q. What is the difference between event bubbling and event capturing in JavaScript? 
Ans: Event bubbling and event capturing are two different ways in which events are propagated in the DOM (Document Object Model) in JavaScript. Event bubbling is the default behavior, in which an event first fires on the target element and then bubbles up to its parent elements. Event capturing, on the other hand, involves capturing the event at the highest level of the DOM and then working down to the target element.

Q. What is a closure in JavaScript and how is it used? 
Ans: A closure in JavaScript is a function that has access to variables in its outer (enclosing) function scope, even after the outer function has returned. Closures are used to create private variables and functions that are not accessible outside of the closure. They can also be used to create a "factory function" that generates a set of related functions with shared variables and context.

Q. What is hoisting in JavaScript? 
Ans: Hoisting is a JavaScript mechanism where variable and function declarations are moved to the top of their respective scopes during the compilation phase, before the code is executed. This means that variables and functions can be used before they are declared in the code.

Q. What is the difference between synchronous and asynchronous programming in JavaScript? 
Ans: Synchronous programming in JavaScript is when code is executed in a sequential, blocking manner, meaning that each line of code is executed in order and the program waits for each line to finish before moving on to the next. Asynchronous programming, on the other hand, allows code to be executed in a non-blocking manner, meaning that the program can move on to the next line of code before the previous line has finished executing.

Q. What is a callback function in JavaScript? 
Ans: A callback function in JavaScript is a function that is passed as an argument to another function and is executed after some operation has been completed. Callback functions are often used in asynchronous programming to handle the result of an asynchronous operation.

Q. What is a promise in JavaScript? 
Ans: A promise in JavaScript is an object that represents the eventual completion (or failure) of an asynchronous operation and allows for more readable and maintainable asynchronous code. Promises have three states: pending, fulfilled, and rejected, and can be chained together to handle multiple asynchronous operations.

Q. What is the difference between a shallow copy and a deep copy in JavaScript? 
Ans: In JavaScript, a shallow copy of an object or array creates a new object or array with the same references to the original object or array's properties, meaning that changes to the original object or array will also be reflected in the copied object or array. A deep copy, on the other hand, creates a new object or array with copies of all of the original object or array's properties, so changes to the original object or array will not be reflected in the copied object or array.

Q. What is the difference between a stack and a queue in JavaScript? 
Ans: In JavaScript, a stack is a data structure that follows the Last In, First Out (LIFO) principle, meaning that the last element added to the stack is the first one to be removed. A queue, on the other hand, follows the First In, First Out (FIFO) principle, meaning that the first element added to the queue is the first one to be removed.

Q. What is the difference between let and var in JavaScript? 
Ans: In JavaScript, let and var are both used to declare variables, but they have different scoping rules. Variables declared with var are function-scoped, meaning that they are accessible within the function in which they are declared, as well as any nested functions. Variables declared with let, on the other hand, are block-scoped, meaning that they are only accessible within the block in which they are declared.

Q. What is a rest parameter in JavaScript? 
Ans: A rest parameter in JavaScript is a parameter that allows a function to accept an indefinite number of arguments as an array. Rest parameters are declared using the ... syntax before the parameter name in a function definition and can be used to capture any remaining arguments that are not explicitly named.

Q. What is destructuring in JavaScript? 
Ans: Destructuring in JavaScript is a way of extracting values from objects and arrays and assigning them to variables using a shorthand syntax. Object destructuring allows for the extraction of values from an object using the property names as variable names, while array destructuring allows for the extraction of values from an array using their index positions.



Q. What is the difference between null and undefined in JavaScript? 
Ans: In JavaScript, null and undefined are both used to represent the absence of a value, but they are not the same. Null is a value that represents a deliberate non-value, meaning that a variable with a null value has been explicitly set to have no value. Undefined, on the other hand, is a value that represents the absence of a value, meaning that a variable with an undefined value has not been assigned a value at all.

Q. What is a closure in JavaScript? 
Ans: A closure in JavaScript is a function that has access to the variables and functions defined in its outer (enclosing) scope, even after the outer function has returned. Closures are created by nesting functions within each other, and can be used to create private variables and functions, as well as to create functions with persistent state.

Q. What is the difference between a for loop and a forEach loop in JavaScript? 
Ans: In JavaScript, a for loop is a traditional loop that iterates over an array or other iterable using a counter variable and a condition, while a forEach loop is a higher-order function that accepts a callback function and iterates over an array or other iterable, executing the callback function once for each element in the array. The main difference between the two is that a for loop can be used to break or continue the loop, while a forEach loop cannot.

Q. What is the difference between a regular function and an arrow function in JavaScript? 
Ans: In JavaScript, a regular function is created using the function keyword, while an arrow function is created using the => syntax. The main difference between the two is that arrow functions have a shorter syntax and do not have their own this value, meaning that they inherit the this value of their enclosing scope.

Q. What is a generator function in JavaScript? 
Ans: A generator function in JavaScript is a special type of function that can be paused and resumed during execution, allowing for the generation of a sequence of values over time. Generator functions are declared using the function* syntax and use the yield keyword to pause execution and return a value to the caller.

Q. What is a template literal in JavaScript? 
Ans: A template literal in JavaScript is a string literal that allows for the interpolation of variables and expressions using placeholders indicated by ${}. Template literals are created using the ` (backtick) character instead of the traditional ' or " quotation marks.

Q. What is event bubbling in JavaScript? 
Ans: Event bubbling in JavaScript is a mechanism where an event triggered on a child element is propagated up through its parent elements in the DOM hierarchy, triggering any event listeners attached to those parent elements along the way. This can be useful for creating complex event handling logic, but can also lead to unexpected behavior if not handled properly.

Q. What is a web worker in JavaScript? 
Ans: A web worker in JavaScript is a separate thread of execution that can be used to run scripts in the background, separate from the main UI thread. Web workers are useful for running long-running tasks or tasks that require significant computation, without blocking the main UI thread and causing the page to become unresponsive.

Q. What is a proxy object in JavaScript? 
Ans: A proxy object in JavaScript is an object that wraps another object (the target object) and intercepts calls to its properties and methods.

Q. What is the difference between let, const, and var in JavaScript? 
Ans: In JavaScript, let and const are block-scoped variables that were introduced in ECMAScript 6, while var is a function-scoped variable that has been around since the early days of JavaScript. The main difference between let and var is that let variables can only be accessed within the block they are declared in, while var variables are accessible throughout the entire function they are declared in. Const variables, on the other hand, are similar to let variables, but they cannot be reassigned once they are assigned a value.

Q. What is an immediately-invoked function expression (IIFE) in JavaScript?
Ans: An immediately-invoked function expression (IIFE) in JavaScript is a function that is defined and immediately called, without being assigned to a variable or otherwise named. IIFEs are often used to create a private scope for variables and functions, to prevent them from polluting the global namespace.

Q. What is the difference between event.preventDefault() and event.stopPropagation() in JavaScript? 
Ans: In JavaScript, event.preventDefault() is a method that is used to prevent the default behavior of an event, such as submitting a form or following a link. event.stopPropagation() is a method that is used to stop the propagation of an event from bubbling up the DOM hierarchy to its parent elements. The main difference between the two is that preventDefault() stops the default behavior of the event, while stopPropagation() only stops the event from being propagated further up the DOM hierarchy.

Q. What is the difference between an object and an array in JavaScript? 
Ans: In JavaScript, an object is a collection of key-value pairs, where each key is a string and each value can be of any type, while an array is an ordered list of values, where each value can be of any type. Objects are typically used to represent more complex data structures, while arrays are typically used for storing collections of similar items.

Q. What is a promise in JavaScript? 
Ans: A promise in JavaScript is an object that represents the eventual completion (or failure) of an asynchronous operation and allows for the execution of callbacks once the operation has completed. Promises are typically used to handle asynchronous operations such as network requests or file I/O, and can help to simplify complex asynchronous code.

Q. What is the difference between synchronous and asynchronous code in JavaScript? 
Ans: In JavaScript, synchronous code is executed in order, one line at a time, blocking further execution until the current line has completed, while asynchronous code is executed out of order, allowing other code to continue running while the asynchronous operation is being performed. Asynchronous code is typically used for tasks that take a long time to complete, such as network requests or file I/O.

Q. What is a higher-order function in JavaScript? 
Ans: A higher-order function in JavaScript is a function that accepts another function as an argument, or returns a function as a result. Higher-order functions are often used for tasks such as filtering or mapping arrays, or for creating new functions from existing ones.

Q. What is a callback function in JavaScript? 
Ans: A callback function in JavaScript is a function that is passed as an argument to another function, and is executed at a later time by that function. Callback functions are often used in asynchronous code to handle the result of an operation once it has completed.

Q. What is the difference between null and undefined in JavaScript? 
Ans: In JavaScript, null is a value that represents the intentional absence of any object value, while undefined is a value that represents the absence of any value. Null is typically used to indicate that a variable has no value, while undefined is typically used to indicate that a variable has not been assigned a value.

Q. What is the difference between == and === in JavaScript? 
Ans: In JavaScript, the == operator is used to compare values for equality, while the === operator is used to compare values for strict equality. The == operator performs type coercion, meaning that it will attempt to convert values of different types to the same type before comparing them, while the === operator does not perform type coercion and will only return true if the values being compared are of the same type.



Q. What is hoisting in JavaScript? 
Ans: Hoisting in JavaScript is a mechanism by which variable and function declarations are moved to the top of their respective scopes, allowing them to be used before they are declared. This means that even if a variable or function is declared at the bottom of a file or function, it can be used at the top without causing an error.

Q. What is a closure in JavaScript? 
Ans: A closure in JavaScript is a function that has access to variables in its outer (enclosing) scope, even after the outer function has returned. Closures are created when a function returns another function that uses variables from the outer function's scope, and can be used to create private variables and functions that are not accessible from outside the closure.

Q. What is the prototype in JavaScript? 
Ans: In JavaScript, the prototype is an object that is attached to every function and object, which contains properties and methods that can be inherited by instances of the function or object. When an object or function is created, it automatically inherits properties and methods from its prototype, which can be used to share behavior and functionality between multiple instances.

Q. What is the difference between arrow functions and regular functions in JavaScript? 
Ans: In JavaScript, arrow functions are a more concise syntax for defining functions, with a different behavior for the this keyword. Arrow functions do not have their own this context, but instead inherit the this context from the enclosing scope. This makes arrow functions particularly useful for working with asynchronous code and for creating more readable code.

Q. What is the event loop in JavaScript? 
Ans: The event loop in JavaScript is a mechanism that allows for non-blocking I/O operations by constantly checking for new events in a queue and executing them as they arrive. This allows for asynchronous code to be executed without blocking the main thread, and is a key feature of modern web development.

Q. What is the difference between let and var in JavaScript? 
Ans: In JavaScript, let is a block-scoped variable that was introduced in ECMAScript 6, while var is a function-scoped variable that has been around since the early days of JavaScript. The main difference between let and var is that let variables can only be accessed within the block they are declared in, while var variables are accessible throughout the entire function they are declared in.

Q. What is a generator function in JavaScript? 
Ans: A generator function in JavaScript is a function that can be paused and resumed during execution, allowing for the creation of iterators that can be used to generate a series of values. Generator functions are defined using the function* syntax and use the yield keyword to pause execution and return a value.

Q. What is a template literal in JavaScript? 
Ans: A template literal in JavaScript is a string literal that allows for the interpolation of variables and expressions using ${...} syntax. Template literals can also span multiple lines and can be used to create more readable and expressive code.

Q. What is destructuring in JavaScript? 
Ans: Destructuring in JavaScript is a feature that allows for the extraction of values from arrays and objects using a concise syntax. This can be useful for assigning multiple variables at once or for extracting specific properties from an object.

Q. What is the difference between synchronous and asynchronous code in JavaScript? 
Ans: In JavaScript, synchronous code is executed sequentially, with each line of code being executed in order and blocking the main thread until the operation is complete. Asynchronous code, on the other hand, is executed concurrently, with the main thread continuing to execute while waiting for the asynchronous operation to complete.

Q. What is a promise in JavaScript? 
Ans: A promise in JavaScript is an object that represents a value that may not be available yet, but will be at some point in the future. Promises are used to handle asynchronous operations in a more elegant way than traditional callback functions, and can be used to chain together multiple asynchronous operations.

Q. What is async/await in JavaScript? 
Ans: Async/await is a newer feature in JavaScript that provides a more concise syntax for working with promises and asynchronous code. Async functions are defined using the async keyword and allow for the use of the await keyword to pause execution and wait for a promise to resolve.

Q. What is a callback function in JavaScript? 
Ans: A callback function in JavaScript is a function that is passed as an argument to another function and is called when that function has finished executing. Callback functions are commonly used in asynchronous programming to handle the result of an asynchronous operation.

Q. What is the difference between call and apply in JavaScript? 
Ans: In JavaScript, both call and apply are used to invoke a function with a specific context and arguments. The main difference between the two is the way in which arguments are passed to the function. With call, arguments are passed as a comma-separated list, while with apply, arguments are passed as an array.

Q. What is the difference between slice and splice in JavaScript? 
Ans: In JavaScript, both slice and splice are used to manipulate arrays. Slice returns a new array containing a subset of the elements in the original array, while splice is used to add or remove elements from an array in place.

Q. What is the difference between map and forEach in JavaScript? 
Ans: In JavaScript, both map and forEach are used to iterate over arrays. The main difference between the two is that map returns a new array containing the results of calling a function on each element of the original array, while forEach simply executes a function for each element of the array without returning a value.

Q. What is the difference between let and const in JavaScript? 
Ans: In JavaScript, let and const are both used to declare variables, but const variables cannot be reassigned after they have been declared, while let variables can be reassigned. This makes const useful for declaring constants or values that should not be changed, while let is useful for variables that may need to be reassigned.

Q. What is the difference between filter and find in JavaScript? 
Ans: In JavaScript, both filter and find are used to search through arrays. Filter returns a new array containing all elements that match a given condition, while find returns the first element in the array that matches the given condition.

Q. What is the difference between == and === in JavaScript? 
Ans: In JavaScript, both == and === are used for equality comparisons. The main difference between the two is that == performs type coercion before comparing values, while === does not. This means that == will attempt to convert values of different types to a common type before comparing them, while === will only return true if the values are of the same type.

Q. What is hoisting in JavaScript? 
Ans: Hoisting is a feature of JavaScript that allows variables and function declarations to be moved to the top of their respective scopes, regardless of where they are actually declared in the code. This means that variables and functions can be used before they are declared, as long as they are declared within the same scope.

Q. What is closure in JavaScript? 
Ans: A closure in JavaScript is a function that has access to variables in its outer scope, even after that scope has been exited. Closures are created when a function is defined inside another function and has access to the outer function's variables.

Q. What is the difference between null and undefined in JavaScript? 
Ans: In JavaScript, null and undefined are both used to represent absence of value, but in slightly different ways. Undefined is used to indicate that a variable has not been assigned a value, while null is used to indicate that a variable has been explicitly set to a value of null.

Q. What is an immediately-invoked function expression (IIFE) in JavaScript? 
Ans: An immediately-invoked function expression (IIFE) in JavaScript is a function that is immediately executed after it is defined. This is achieved by wrapping the function definition in parentheses and then immediately invoking it with another set of parentheses.

Q. What is a higher-order function in JavaScript? 
Ans: A higher-order function in JavaScript is a function that either takes another function as an argument or returns a function as a result. Higher-order functions are used to create abstractions and enable more generic and reusable code.



Q. What is the difference between var, let, and const in JavaScript? 
Ans: In JavaScript, var is used to declare variables in the global or function scope, while let and const are used to declare variables in block scope. Var variables can be reassigned and redeclared, while let variables can be reassigned but not redeclared, and const variables cannot be reassigned or redeclared.

Q. What is the event loop in JavaScript? 
Ans: The event loop in JavaScript is a mechanism that allows for asynchronous code to be executed in a non-blocking way. It continuously checks the call stack and the message queue to see if there are any functions that need to be executed, and schedules them for execution when appropriate.

Q. What is the this keyword in JavaScript? 
Ans: The this keyword in JavaScript is a reference to the object that the current function is a method of. It is used to access properties and methods of the object that the function is a part of.

Q. What is the difference between a shallow and deep copy of an object in JavaScript? 
Ans: In JavaScript, a shallow copy of an object only copies the object's properties and values, while a deep copy creates a new object with copies of all of the original object's properties and values, including nested objects and arrays.

Q. What is a callback function in JavaScript? 
Ans: A callback function in JavaScript is a function that is passed as an argument to another function and is executed when the first function has completed its task. Callbacks are commonly used in asynchronous code, such as event handlers and AJAX requests.

Q. What is the difference between a for loop and a forEach loop in JavaScript? 
Ans: A for loop in JavaScript is a general-purpose loop that can iterate over any kind of iterable object. A forEach loop is a specialized loop that is only used to iterate over arrays. The main advantage of a forEach loop is that it is more concise and easier to read than a for loop.

Q. What is a promise in JavaScript? 
Ans: A promise in JavaScript is a way to handle asynchronous operations. A promise represents the eventual completion or failure of an asynchronous operation and can be in one of three states: pending, fulfilled, or rejected. Promises are commonly used in AJAX requests and other asynchronous code.

Q. What is the difference between a promise and a callback in JavaScript? 
Ans: A promise and a callback are both used to handle asynchronous operations in JavaScript, but they are used in slightly different ways. A callback is a function that is passed as an argument to another function and is executed when the first function has completed its task. A promise represents the eventual completion or failure of an asynchronous operation and can be in one of three states: pending, fulfilled, or rejected.

Q. What is a generator function in JavaScript? 
Ans: A generator function in JavaScript is a special type of function that can be paused and resumed, allowing for more flexible control flow in asynchronous code. When a generator function is called, it returns a generator object, which can be used to iterate over the values that the generator function yields.

Q. What is the difference between a module and a script in JavaScript? 
Ans: In JavaScript, a module is a self-contained unit of code that can be imported and exported from other modules. A script is a standalone file that is executed in the context of a web page. Modules are commonly used in complex web applications to organize code and avoid name collisions.

Q. What is the difference between the window and document objects in JavaScript? 
Ans: In JavaScript, the window object represents the current browser window or tab, while the document object represents the HTML document that is currently loaded in the window or tab. The window object provides access to browser-specific features and methods, while the document object provides access to the content of the HTML document.

Q. What is a web worker in JavaScript? 
Ans: A web worker in JavaScript is a way to run code in a separate background thread, allowing for more efficient use of resources and better performance in web applications. Web workers are commonly used for tasks such as data processing, file I/O, and network communication.

Q. What is the difference between an arrow function and a regular function in JavaScript? 
Ans: An arrow function in JavaScript is a more concise way to define a function, using the => operator instead of the function keyword. Arrow functions have a few key differences from regular functions, including a more concise syntax, implicit return of single expressions, and lexical scoping of the this keyword.

Q. What is the spread syntax in JavaScript? 
Ans: The spread syntax in JavaScript is a way to spread the elements of an iterable object (such as an array) into a new array or function call. The spread syntax is denoted by the ... operator and can be used to concatenate arrays, create copies of objects and arrays, and pass arguments to functions.

Q. What is the difference between null and undefined in JavaScript? 
Ans: In JavaScript, null and undefined are both values that represent the absence of a value. However, they are used in slightly different ways. null is explicitly set by the programmer to represent the absence of a value, while undefined is used to represent variables that have not been assigned a value.

Q. What is the event loop in JavaScript? 
Ans: The event loop in JavaScript is a mechanism that allows for asynchronous code to be executed in a single-threaded environment. The event loop continuously checks for new events or tasks to execute, and executes them in order as they are added to the queue.

Q. What is a closure in JavaScript? 
Ans: A closure in JavaScript is a function that has access to its outer function's scope, even after the outer function has returned. Closures are commonly used in JavaScript to create private variables and functions, and to maintain state between function calls.

Q. What is hoisting in JavaScript? 
Ans: Hoisting in JavaScript is a behavior where variable and function declarations are moved to the top of their respective scopes during the compilation phase. This means that they can be used before they are declared, although their values will be undefined until they are initialized.

Q. What is the prototype chain in JavaScript? 
Ans: The prototype chain in JavaScript is a mechanism that allows objects to inherit properties and methods from their parent objects. Each object in JavaScript has a prototype property, which points to its parent object's prototype. When a property or method is called on an object, JavaScript will first check the object's own properties, and then follow the prototype chain to check its parent objects.

Q. What is a constructor function in JavaScript? 
Ans: A constructor function in JavaScript is a special type of function that is used to create new objects. Constructor functions are typically used with the new keyword to create instances of the object, and can be used to set default values for object properties and methods.

Q. What is the difference between the var, let, and const keywords in JavaScript? 
Ans: In JavaScript, var is used to declare variables in the global scope or function scope, let is used to declare variables in block scope (e.g. inside a loop or if statement), and const is used to declare variables that cannot be reassigned.

Q. What is the difference between synchronous and asynchronous code in JavaScript? 
Ans: Synchronous code in JavaScript is executed in a sequential and blocking manner, meaning that each statement is executed in order and blocks the execution of subsequent statements until it is complete. Asynchronous code in JavaScript is executed in a non-blocking manner, meaning that the program does not wait for the operation to complete before moving on to the next statement. Asynchronous code is commonly used for tasks such as network communication, file I/O, and user input.