1. What is JavaScript?
JavaScript is a programming language used to make web pages interactive and dynamic. It runs on the client side and is supported by all modern web browsers.

2. What are the data types supported by JavaScript?
JavaScript supports six primitive data types: string, number, boolean, null, undefined, and symbol. It also supports one complex data type called object.

3. What is a closure?
A closure is an inner function that has access to variables from its outer (enclosing) function. It allows for data encapsulation and provides a way to create private variables and methods.

4. What is hoisting in JavaScript?
Hoisting is a behavior in JavaScript where variable and function declarations are moved to the top of their containing scope during the compilation phase. However, only declarations are hoisted, not initializations.

5. What is the difference between == and === operators?
The == operator checks for equality after performing necessary type conversions, while the === operator checks for equality without performing any type conversion. So, === is stricter than ==.

6. What is the difference between null and undefined?
Null represents the intentional absence of any object value, while undefined is the absence of a value that has not been assigned. null is an assigned value, whereas undefined is an uninitialized value.

7. What is the use of the let and const keywords introduced in ES6?
The let keyword allows the declaration of block-scoped variables. Once declared, they can be reassigned but not redeclared within the same scope. The const keyword is used to declare block-scoped variables that cannot be reassigned.

8. What is event delegation in JavaScript?
Event delegation is a technique in which a parent element listens for an event that occurs on its child elements. Instead of attaching event handlers to each individual child element, the parent handles the events on behalf of its children.

9. What is the difference between call() and apply() methods?
The call() and apply() methods are used to invoke a function with a specified this value and arguments provided individually or as an array, respectively. The only difference is in the way arguments are passed.

10. How can you prevent default behavior in an event handler?
To prevent the default behavior in an event handler, you can use the event object’s preventDefault() method. This method stops the default action of an element from occurring, such as clicking a link or submitting a form.

11. What is the difference between an arrow function and a regular function?
Arrow functions are anonymous functions defined using a shorter syntax (=>). Unlike regular functions, arrow functions do not have their own this value and cannot be used as constructors.

12. What is the purpose of the ‘use strict’ directive in JavaScript?
The ‘use strict’ directive enables strict mode in JavaScript, which helps in writing more reliable and maintainable code. It catches common mistakes and prevents the use of certain error-prone features.

13. How can you convert a string to a number in JavaScript?
You can convert a string to a number in JavaScript using the parseInt() or parseFloat() functions. parseInt() converts a string to an integer, while parseFloat() converts it to a floating-point number.

14. What is the difference between let and var in JavaScript?
The let keyword introduces block scope for variables, meaning they are limited to the block (enclosed by curly braces) in which they are defined. The var keyword, on the other hand, introduces function scope.

15. What is the purpose of the XMLHttpRequest object in JavaScript?
The XMLHttpRequest object is used to interact with servers asynchronously. It can send HTTP requests and receive responses, allowing web pages to update content dynamically without reloading the entire page.

16. How do you check if a variable is an array in JavaScript?
You can check if a variable is an array in JavaScript using the Array.isArray() method introduced in ES5. It returns true if the variable is an array and false otherwise.

17. How can you clone an object in JavaScript?
To clone an object in JavaScript, you can use the Object.assign() method or the spread operator (…). These techniques create a shallow copy of the original object, meaning any nested objects or arrays are still referenced.

18. What are the different ways to loop through an array in JavaScript?
You can loop through an array in JavaScript using a for loop, while loop, forEach() method, map() method, or for…of loop introduced in ES6.

19. What is the purpose of the this keyword in JavaScript?
The this keyword refers to the current context or object in which a function is being executed. It allows you to access and manipulate the properties and methods of the object it belongs to.

20. What is event bubbling in JavaScript?
Event bubbling is a phenomenon in which an event triggered on a particular element propagates up the DOM tree, triggering the same event on each parent element, with the document object as the final destination.

1. What is JavaScript Hoisting?
JavaScript hoisting refers to the process in which the JavaScript interpreter moves the variable and function declarations to the top of their respective scopes during the compilation phase. This allows you to use the variables or functions before they are declared.

2. What are Immediately Invoked Function Expressions (IIFE) in JavaScript?
IIFE is a JavaScript design pattern that executes a function immediately after it is created. It encapsulates the function within its own scope, preventing any variables declared within it from polluting the global scope.

3. Explain the concept of Prototypal Inheritance in JavaScript.
Prototypal inheritance is a feature of JavaScript that allows an object to inherit properties and methods from another object. Each object in JavaScript has a prototype object, and if a property or method is not found in the current object, JavaScript looks for it in the prototype chain.

4. What is the difference between ‘undefined’ and ‘null’ in JavaScript?
In JavaScript, `undefined` is a primitive value that is assigned automatically to variables that have been declared but have not been assigned a value. On the other hand, `null` is an object that represents the intentional absence of any object value.

5. What are JavaScript Promises?
Promises are a way to handle asynchronous operations in JavaScript. They represent the result of an asynchronous operation that may eventually produce a value. Promises help in chaining multiple asynchronous operations together and provide a cleaner way to write asynchronous code.

6. Explain the concept of ‘currying’ in JavaScript.
Currying is a technique in functional programming where a function that takes multiple arguments is transformed into a series of functions, each taking a single argument. This allows for partial application of the function, making it more flexible and reusable.

7. What is the purpose of the ‘bind()’ method in JavaScript?
The `bind()` method in JavaScript creates a new function that, when called, has its `this` value set to a specific value. It is often used in event handling or when passing a method as a callback function, while preserving the proper context of `this`.

8. Explain the concept of the event loop in JavaScript.
The event loop is a mechanism in JavaScript that allows for asynchronous execution of code. It continuously checks if there are any events in the event queue, and if so, it executes the corresponding event handlers. This allows for non-blocking I/O operations and better responsiveness.

9. What is a generator function in JavaScript?
A generator function is a special type of function in JavaScript that can be paused and resumed during its execution. It uses the `yield` keyword to pause the function and return a value, and the `next()` method to resume the function execution from where it was paused.

10. What is the purpose of the ‘this’ keyword in JavaScript?
The `this` keyword in JavaScript refers to the object that is currently executing the code. It is determined by how a function is called, and can dynamically change based on the context in which the function is executed.

11. How does ‘async/await’ work in JavaScript?
The `async/await` syntax in JavaScript allows for more intuitive handling of asynchronous code. The `async` keyword is used to define an asynchronous function, and the `await` keyword is used to wait for a promise to resolve or reject before proceeding further in the code.

12. What is the purpose of the ‘reduce()’ method in JavaScript?
The `reduce()` method in JavaScript is used to apply a function to an accumulator and each element in an array, reducing it to a single value. It is commonly used for summing up numbers, finding the maximum or minimum value, or calculating averages.

13. Explain the concept of closures in JavaScript.
Closures are functions in JavaScript that have access to variables from their outer scope, even after the outer function has finished executing. They allow for encapsulation and data privacy, as well as for creating higher-order functions.

14. What are the different ways to create objects in JavaScript?
In JavaScript, there are multiple ways to create objects: using object literals, using the `new` keyword and a constructor function, using the `Object.create()` method, or using ES6 classes.

15. What is the purpose of the ‘Map’ and ‘Set’ objects in JavaScript?
The `Map` object in JavaScript is used for storing key-value pairs, while the `Set` object is used for storing unique values. Both objects provide efficient methods for adding, removing, and searching for elements.

16. Explain the concept of ‘call’, ‘apply’, and ‘bind’ in JavaScript.
`call`, `apply`, and `bind` are methods in JavaScript that are used to change the value of `this` inside a function. `call` and `apply` immediately invoke the function, while `bind` returns a new function with the specified `this` value.

17. What is the purpose of the ‘toString()’ method in JavaScript?
The `toString()` method in JavaScript is used to convert an object to its string representation. It is automatically called when an object is concatenated with a string or used in a context where a string is expected.

18. What is memoization in JavaScript?
Memoization is a technique in JavaScript where the return value of a function is cached based on its input parameters. It helps in optimizing performance by storing the results of expensive function calls and returning the cached result when the same inputs are provided.

19. Explain the concept of event delegation in JavaScript.
Event delegation is a technique in JavaScript where instead of attaching an event listener to each individual element, you attach a single event listener to a parent element. The event listener then checks for the target element that triggered the event and executes the corresponding code.

20. What are the differences between ‘let’, ‘const’, and ‘var’ in JavaScript?
`let` and `const` are block-scoped variables introduced in ES6, while `var` is function-scoped. `let` allows for reassignment of values, `const` is used for constants, and `var` is hoisted to the top of its scope.