1. What is JavaScript?

JavaScript is a high-level, interpreted programming language that is widely used for creating interactive web pages and web applications. It is a scripting language that is mainly used for client-side web development, but can also be used for server-side development with Node.js.

2. What are the different data types in JavaScript?

JavaScript supports several data types, including strings, numbers, booleans, null, undefined, objects, and symbols. Strings are used to represent text, numbers are used to represent numerical values, booleans are used to represent true/false values, null represents the intentional absence of any object value, undefined represents the absence of a defined value, objects are used to represent complex data structures, and symbols are used to create unique identifiers.

3. What is the difference between null and undefined in JavaScript?

Null represents the intentional absence of any object value, while undefined represents the absence of a defined value. In other words, null is a value that is explicitly set by a programmer to indicate the absence of an object, while undefined is a value that is automatically assigned by JavaScript when a variable or property has not been assigned a value.

4. What is the difference between == and === in JavaScript?

The == operator is used to compare values for equality, but it performs type coercion if the types of the operands are different. The === operator is used to compare values for equality, but it does not perform type coercion, so the types of the operands must be the same.

5. What is the difference between let and var in JavaScript?

The let keyword was introduced in ES6 and is used to declare variables with block scope. Variables declared with let can only be accessed within the block they are defined in. The var keyword is used to declare variables with function scope. Variables declared with var can be accessed within the entire function they are defined in.

6. What is hoisting in JavaScript?

Hoisting is a JavaScript mechanism where variable and function declarations are moved to the top of their respective scopes before any code is executed. This means that variables and functions can be used before they are declared, although it is not recommended to do so.

7. What is a closure in JavaScript?

A closure is a feature in JavaScript that allows a function to access variables from its parent scope, even after the parent function has returned. Closures are created when a function is defined inside another function and the inner function has access to variables from the outer function.

8. What is the difference between synchronous and asynchronous code in JavaScript?

Synchronous code is executed sequentially, meaning that each statement is executed one after the other, and the next statement does not begin until the previous statement has finished. Asynchronous code, on the other hand, allows multiple tasks to be executed concurrently, without blocking the execution of other tasks.

9. What is the event loop in JavaScript?

The event loop is a mechanism in JavaScript that enables asynchronous processing. It continuously checks the message queue for new tasks to execute, and when a task is found, it is executed on the call stack.

10. What is the difference between call and apply in JavaScript?

Both call and apply are used to call a function with a given this value, but they differ in how additional arguments are passed to the function. The call expects the arguments to be passed individually, while apply expects an array of arguments to be passed as the second argument.

11. What is the difference between slice and splice in JavaScript?

The slice method is used to extract a portion of an array and return a new array, without modifying the original array. The splice method, on the other hand, is used to add or remove elements from an array, and it modifies the original array.

12. What is the difference between forEach and map in JavaScript?

Both forEach and map are used to iterate over an array, but they differ in how they handle the return value. forEach executes a function for each element in the array, but it does not create a new array. map, on the other hand, creates a new array by applying a function to each element in the array and returning the result.

13. What is the difference between async and defer in JavaScript?

Both async and defer are attributes that can be used to load JavaScript files asynchronously, but they differ in how they affect the execution of the script. async allows the script to be executed as soon as it is downloaded, without waiting for the HTML document to finish parsing. defer, on the other hand, allows the script to be downloaded asynchronously, but it defers the execution of the script until the HTML document has finished parsing.

14. What is the difference between innerHTML and textContent in JavaScript?

innerHTML is a property that sets or returns the HTML content within an element, including any HTML tags. textContent, on the other hand, sets or returns the text content within an element, without any HTML tags.

15. What is the difference between querySelector and getElementById in JavaScript?

querySelector is a method that returns the first element within a document that matches a specified CSS selector. getElementById, on the other hand, is a method that returns the element with the specified ID.

16. What is JSON in JavaScript?

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 a text format that is used to transmit data between a server and a client.

17. What is a promise in JavaScript?

A promise is an object that represents the eventual completion (or failure) of an asynchronous operation and its resulting value. It allows asynchronous code to be written in a synchronous style, making it easier to reason about.

18. What is the difference between a callback function and a promise in JavaScript?

A callback function is a function that is passed as an argument to another function, and is executed when the function has finished its task. A promise, on the other hand, is an object that represents the eventual completion of an asynchronous operation, and can be used to handle success or failure in a more structured way.

19. What is the difference between a factory function and a constructor function in JavaScript?

A factory function is a function that returns a new object, while a constructor function is a function that is used with the new keyword to create a new object. Factory functions are more flexible and can be used to create different types of objects, while constructor functions are used to create objects with a specific type or class.

20. What is the difference between the this keyword and the bind method in JavaScript?

The this keyword is used to refer to the current object, while the bind method is used to create a new function with a specified this value. Bind can be used to create a new function with a fixed this value, which is useful when passing a method as a callback function.

21. What is the purpose of the filter() method in JavaScript?

The filter() method is used to create a new array with all elements that pass a test specified by a function. It is often used to extract a subset of an array based on some condition.

22. What is event bubbling in JavaScript?

Event bubbling is a phenomenon in which an event that is triggered on a child element "bubbles up" through its parent elements, and can be handled by multiple elements along the way. It can be useful for handling events on a parent element that affect its children.

23. What is the difference between null and undefined in JavaScript?

Null represents a deliberate non-value, while undefined represents an uninitialized value. In other words, null is explicitly set by the programmer, while undefined is the default value of uninitialized variables and function parameters.

24. What is the difference between a let and a var in JavaScript?

Both let and var are used to declare variables, but they differ in their scoping rules. let is block-scoped, which means that it is only visible within the block in which it is declared, while var is function-scoped, which means that it is visible throughout the entire function in which it is declared.

25. What is a callback function in JavaScript?

A callback function is a function that is passed as an argument to another function, and is executed when the function has finished its task. It is often used with asynchronous functions, to handle the result of the asynchronous operation.

26. What is the difference between a for loop and a forEach loop in JavaScript?

Both for loops and forEach loops are used to iterate over arrays, but they differ in their syntax and functionality. A for loop allows more flexibility in the way the array is iterated, while a forEach loop is simpler and easier to read.

27. What is the purpose of the reduce() method in JavaScript?

The reduce() method is used to apply a function to each element in an array and reduce the array to a single value. The result of each function call is passed as an argument to the next function call, until a single value is left.

28. What is the difference between a class and an object in JavaScript?

A class is a blueprint for creating objects, while an object is an instance of a class. A class defines the properties and methods that an object will have, while an object has its own unique state and behavior.

29. What is the difference between a static and a non-static method in JavaScript?

In JavaScript, all methods are non-static by default. A static method is a method that belongs to the class itself, rather than to a specific instance of the class. It can be called without creating an instance of the class.

30. What is the difference between a function declaration and a function expression in JavaScript?

A function declaration is a statement that defines a function and makes it available in the current scope, while a function expression is a function that is assigned to a variable or passed as an argument to another function. Function declarations are hoisted to the top of the current scope, while function expressions are not.

31. What is the use of the try...catch statement in JavaScript?

The try...catch statement is used for error handling in JavaScript. It allows you to execute a block of code and catch any errors that may occur, allowing you to gracefully handle the error and prevent your code from crashing.

32. What is a Promise in JavaScript?

A Promise is an object that represents the eventual completion (or failure) of an asynchronous operation and its resulting value. It allows you to write asynchronous code that is easier to read and reason about, by using the then() and catch() methods to handle the result of the operation.

33. What is the purpose of the async/await keywords in JavaScript?

The async/await keywords are used to write asynchronous code that looks and behaves more like synchronous code. The async keyword is used to mark a function as asynchronous, while the await keyword is used to wait for the completion of a promise.

34. What is hoisting in JavaScript?

Hoisting is a JavaScript behavior where variable and function declarations are moved to the top of their respective scopes (either the global scope or the scope of a function) during the compilation phase. This means that you can use a variable or function before it has been declared, but it may not have the value you expect.

35. What is the difference between synchronous and asynchronous code in JavaScript?

Synchronous code is executed sequentially, one statement at a time, blocking the execution of subsequent statements until the current one has completed. Asynchronous code, on the other hand, allows multiple operations to be performed concurrently, without blocking the execution of subsequent statements. It uses callbacks, Promises, or async/await to handle the results of the asynchronous operations.

36. What is the purpose of the Object.keys() method in JavaScript?

The Object.keys() method is used to return an array of a given object's own enumerable property names. It is often used to loop over the keys of an object or to check if an object has a certain property.

37. What is the purpose of the "use strict" directive in JavaScript?

The "use strict" directive is used to enable strict mode in JavaScript, which enforces stricter syntax rules and prevents common mistakes. It also disallows the use of undeclared variables and other unsafe behavior.

38. What is the difference between a shallow copy and a deep copy in JavaScript?

A shallow copy creates a new object that has the same properties as the original object, but the values of those properties are still references to the original object's values. A deep copy creates a new object with new values that are not references to the original object's values.

39. What is the purpose of the typeof operator in JavaScript?

The typeof operator is used to determine the data type of a variable or expression in JavaScript. It returns a string indicating the type of the operand, such as "number", "string", "object", "function", "undefined", or "boolean".

40. What is the difference between a map and a set in JavaScript?

A map is a collection of key-value pairs, where each key can only occur once. A set is a collection of unique values, where each value can only occur once. A map is typically used to store data that can be accessed by a unique key, while a set is typically used to store a collection of unique values.

41. What is the purpose of the spread operator in JavaScript?

The spread operator (...) is used to expand an iterable object, such as an array or a string, into individual elements. It is commonly used to concatenate arrays or to create a copy of an existing array.

42. What is event bubbling in JavaScript?

Event bubbling is a behavior in which an event that occurs on a child element is propagated up the DOM tree to its parent and ancestor elements. This means that if you click on a button that is inside a div, both the button and the div will receive the click event.

43. What is the difference between an arrow function and a regular function in JavaScript?

An arrow function is a shorthand way of writing a function expression in JavaScript. It has a concise syntax and lexically binds the this value to the surrounding context. A regular function, on the other hand, has a more verbose syntax and binds the this value to the function itself.

44. What is the purpose of the window object in JavaScript?

A window object is a global object in the browser that represents the current window or tab. It provides a number of properties and methods for manipulating the browser window, such as opening a new window, resizing the current window, or navigating to a new page.

45. What is the purpose of the this keyword in JavaScript?

The this keyword in JavaScript refers to the object that the current function is a method of, or to the global object if the function is not a method of any object. It is commonly used to access properties and methods of the current object.

46. What is the difference between the let and var keywords in JavaScript?

The let keyword is used to declare a block-scoped variable in JavaScript, which means that the variable is only visible within the block it is declared in. The var keyword, on the other hand, declares a variable with function scope or global scope, which means that the variable is visible within the entire function or the entire program.

47. What is a callback function in JavaScript?

A callback function is a function that is passed as an argument to another function, and is executed when the parent function is called back. It is commonly used to handle asynchronous operations, such as reading data from a file or making an HTTP request.

48. What is the purpose of the Array.prototype.map() method in JavaScript?

The Array.prototype.map() method is used to create a new array by applying a function to each element of an existing array. The resulting values are collected into a new array, which is returned by the method.

49. What is the difference between the break and continue statements in JavaScript?

The break statement is used to exit a loop or switch statement, while the continue statement is used to skip over the current iteration of a loop and proceed to the next iteration.

50. What is the purpose of the isNaN() function in JavaScript?

The isNaN() function is used to determine whether a value is NaN (not a number) or not. It returns true if the value is NaN, and false if it is a valid number.