50 interview questions for javascript developer

questions:

1. What is JavaScript and what are its features? Answer: JavaScript is a programming language that is primarily used to create dynamic web content. Its key features include dynamic typing, functional programming, and event-driven programming.

2. What is the difference between null and undefined in JavaScript? Answer: Null represents the intentional absence of any object value, while undefined represents the unintentional absence of an object value.

3. What are the primitive data types in JavaScript? Answer: The primitive data types in JavaScript include strings, numbers, booleans, null, and undefined.

4. What is a closure in JavaScript? Answer: A closure is a function that has access to variables from its outer scope, even after the outer function has returned.

5. What is the event loop in JavaScript and how does it work? Answer: The event loop is a mechanism in JavaScript that allows asynchronous code to be executed in a non-blocking way. It continuously checks for new events or tasks that are queued up and processes them one at a time.

6. What is the difference between synchronous and asynchronous programming in JavaScript? Answer: Synchronous programming means that code is executed in a single thread, and the program waits for one operation to complete before moving on to the next. Asynchronous programming allows multiple operations to be executed concurrently, with the program continuing to run while waiting for operations to complete.

7. What is a callback function and how is it used in JavaScript? Answer: A callback function is a function that is passed as an argument to another function and is invoked when a specific event occurs or when a task is completed.

8. What is a Promise in JavaScript and how does it work? Answer: A Promise is an object that represents the eventual completion of an asynchronous operation and provides a way to handle the result or error that is returned.

9. What is a generator function in JavaScript and how is it used? Answer: A generator function is a special type of function that can be paused and resumed at any point during its execution. It is used to create iterators that can produce a sequence of values.

10. What is the difference between let, var, and const in JavaScript? Answer: let and const are block-scoped, while var is function-scoped. const is a constant that cannot be reassigned, while let can be reassigned.

11. What is the difference between == and === in JavaScript? Answer: == is a loose equality comparison operator that performs type coercion, while === is a strict equality comparison operator that does not perform type coercion.

12. What is a module in JavaScript and how is it used? Answer: A module in JavaScript is a file that contains a set of related functions and variables. It is used to organize code into reusable and testable units.

13. What is hoisting in JavaScript? Answer: Hoisting is a JavaScript behavior where variable and function declarations are moved to the top of their respective scopes, regardless of where they are defined in the code.

14. What is the scope in JavaScript? Answer: Scope in JavaScript refers to the visibility of variables and functions in different parts of the code. There are two types of scope: global and local.

15. What is the this keyword in JavaScript? Answer: The this keyword in JavaScript refers to the object that the current code is being executed in.

16. What are the differences between let and const in JavaScript? Answer: let allows reassignment, while const does not. Also, let is block-scoped, while const is also block-scoped, but its value cannot be changed.

17. What is destructuring in JavaScript? Answer: Destructuring in JavaScript is a way to extract data from arrays or objects into separate variables.

18. What are arrow functions in JavaScript? Answer: Arrow functions in JavaScript are a more concise syntax for creating functions, and they also have some differences in how they handle this keyword.

19. What is the difference between call, apply, and bind in JavaScript? Answer: call and apply are used to invoke a function with a specific context and a set of arguments, while bind is used to create a new function with a specific context.

20. What are the different ways to create an object in JavaScript? Answer: Objects in JavaScript can be created using object literals, constructor functions, and the Object.create() method.

21. What is the difference between a regular function and an arrow function in JavaScript? Answer: An arrow function does not have its own this keyword and is more concise, while a regular function has its own the this keyword and can be used as a constructor.

22. What is event bubbling in JavaScript? Answer: Event bubbling is a process in which events are first handled by the innermost element, and then propagated outwards to the outer elements.

23. What is event capturing in JavaScript? Answer: Event capturing is a process in which events are first handled by the outermost element, and then propagated inwards to the inner elements.

24. What are the different types of error handling in JavaScript? Answer: Error handling in JavaScript can be done using try-catch blocks, throw statements, and error objects.

25. What is the difference between a synchronous and asynchronous function in JavaScript? Answer: A synchronous function blocks the main thread until it completes, while an asynchronous function does not block the main thread and allows other code to execute while it is running.

26. What is a prototype in JavaScript? Answer: A prototype in JavaScript is an object that is used as a template for creating other objects.

27. What is the difference between an object and a class in JavaScript? Answer: JavaScript does not have classes in the traditional sense, but objects can be used to simulate classes using constructor functions or the ES6 class syntax.

28. What is the difference between the null and undefined keywords in JavaScript? Answer: null is used to indicate the absence of a value, while undefined is used to indicate the absence of a value that has not been initialized.

29. What is the difference between the document and window objects in JavaScript? Answer: The document object represents the HTML document that is currently being displayed in the browser, while the window object represents the browser window or tab that the document is displayed in.

30. What is the difference between the innerHTML and textContent properties in JavaScript? Answer: innerHTML returns or sets the HTML content inside an element, while textContent returns or sets the text content inside an element, without any HTML tags.

31. What is a regular expression in JavaScript? Answer: A regular expression is a pattern that is used to match and manipulate text in JavaScript.

32. What is the difference between let and var in JavaScript? Answer: let is block-scoped, while var is function-scoped. Also, let cannot be accessed before it is declared, while var can be.

33. What is the difference between the window.onload and document.ready events in JavaScript? Answer: window.onload is fired when the entire page and all of its resources have finished loading, while document.ready is fired when the HTML document has finished loading, but not necessarily all of its resources.

34. What is the difference between the arguments object and the rest parameter in JavaScript? Answer: The arguments object is an array-like object that contains all of the arguments passed to a function, while the rest parameter is a way to capture a variable number of arguments into an array.

35. What is the difference between the setTimeout and setInterval methods in JavaScript? Answer: The setTimeout method is used to execute a function after a specified amount of time, while the setInterval method is used to execute a function repeatedly with a specified interval between each execution.

36. What is a closure in JavaScript? Answer: A closure in JavaScript is a function that has access to variables in its outer scope, even after the outer function has returned.

37. What is the difference between the == and === operators in JavaScript? Answer: The == operator performs type coercion, while the === operator does not. This means that the == operator can compare values of different types, while the === operator can only compare values of the same type.

38. What is a callback function in JavaScript? Answer: A callback function is a function that is passed as an argument to another function and is called when a certain event or condition occurs.

39. What is the difference between an anonymous function and a named function in JavaScript? Answer: An anonymous function does not have a name and is usually defined as an expression, while a named function has a name and is usually defined as a statement.

40. What is a promise in JavaScript? Answer: A promise in JavaScript is an object that represents a value that may not be available yet, but will be resolved in the future.

41. What is a generator function in JavaScript? Answer: A generator function in JavaScript is a special type of function that can be paused and resumed, allowing for the creation of iterators and lazy evaluation of values.

42. What is the difference between a map and a set in JavaScript? Answer: A map is a collection of key-value pairs, while a set is a collection of unique values.

43. What is a module in JavaScript? Answer: A module in JavaScript is a way to encapsulate code into reusable and maintainable units, and to prevent naming conflicts between different parts of a program.

44. What is the difference between synchronous and asynchronous code in JavaScript? Answer: Synchronous code is executed in a single thread and blocks the main thread until it is completed, while asynchronous code is executed in multiple threads and does not block the main thread, allowing for better performance and responsiveness.

45. What is the difference between a stack and a queue in JavaScript? Answer: A stack is a last-in, first-out data structure, while a queue is a first-in, first-out data structure.

46. What is the difference between the spread operator and the rest parameter in JavaScript? Answer: The spread operator is used to spread an array or object into individual arguments or elements, while the rest parameter is used to capture a variable number of arguments into an array.

47. What is the difference between the push and pop methods and the unshift and shift methods in JavaScript? Answer: The push and pop methods add and remove elements from the end of an array, while the unshift and shift methods add and remove elements from the beginning of an array.

48. What is the difference between the includes and indexOf methods in JavaScript? Answer: The includes method checks if an element is included in an array, while the indexOf method returns the index of the first occurrence of an element in an array, or -1 if it is not found.

49. What is the difference between the find and filter methods in JavaScript? Answer: The find method returns the first element in an array that matches a certain condition, while the filter method returns an array of all elements that match a certain condition.

50. What is the difference between the reduce and forEach methods in JavaScript? Answer: The reduce method is used to reduce an array to a single value, while the forEach method is used to loop through the elements of an array without modifying it.