1. What is JavaScript and what is its purpose?
Answer: JavaScript is a high-level, interpreted programming language used to make dynamic and interactive web pages. Its purpose is to add interactivity and other dynamic elements to websites.
2. What is the difference between let, var, and const in JavaScript?
Answer: Var is function scoped, while let and const are block scoped. Var can be re-declared and re-assigned, while let can only be re-assigned and const cannot be re-declared or re-assigned.
3. What is closure in JavaScript?
Answer: A closure is a function that has access to variables in its outer scope even after the outer function has returned.
4. What is hoisting in JavaScript?
Answer: Hoisting is a mechanism in JavaScript where variable and function declarations are moved to the top of their scope, regardless of where they are defined in the code.
5. What is an anonymous function in JavaScript and how is it used?
Answer: An anonymous function is a function without a name. It can be used as an argument in another function or assigned to a variable.
6. What is the difference between == and === in JavaScript?
Answer: The double equal sign (==) performs type coercion, meaning it converts the type of the operands if they are different before comparing them. The triple equal sign (===) performs strict equality comparison, meaning it returns false if the operands have different types.
7. What is a Promise in JavaScript?
Answer: A Promise is an object representing the eventual completion or failure of an asynchronous operation. It provides a way to register callbacks to be called when the operation is complete or when an error occurs.
8. What is the event loop in JavaScript?
Answer: The event loop is a mechanism in JavaScript that continuously checks the message queue for new messages and executes the code associated with each message. This allows for the non-blocking execution of code.
9. What is the difference between null and undefined in JavaScript?
Answer: Undefined means a variable has been declared but has not been assigned a value, while null is an assignment value, meaning the variable has been declared and assigned the value of null.
10. What is a higher-order function in JavaScript?
Answer: A higher-order function is a function that takes one or more functions as arguments and/or returns a function.
11. What is the difference between synchronous and asynchronous code in JavaScript?
Answer: Synchronous code runs in order, blocking further execution until it has been completed. Asynchronous code runs in the background and allows other code to run while it is being executed.
12. What is a module in JavaScript?
Answer: A module is a reusable piece of code in JavaScript that can be imported into other code to make use of its functionality.
13. What is the difference between null and NaN in JavaScript?
Answer: Null is a value that represents the intentional absence of any object value, while NaN stands for "Not a Number" and represents the result of an invalid mathematical operation.
14. What is the difference between classical inheritance and prototypal inheritance in JavaScript?
Answer: Classical inheritance is based on classes and the concept of inheritance is achieved through the use of prototypes. Prototypal inheritance is based on prototypes and objects inherit directly from other objects.
15. What is a ternary operator in JavaScript?
Answer: A ternary operator is a shorthand for an if-else statement in JavaScript and takes the form of "condition ? expression1: expression2". If the condition is true, expression1 is executed, otherwise, expression2 is executed.
16. What is a switch statement in JavaScript and how is it used?
Answer: A switch statement is a control flow statement that tests a single value against multiple cases and executes the code associated with the matching case. It is often used as an alternative to a long chain of if-else statements.
17. What is the difference between .call() and .apply() in JavaScript?
Answer: Both .call() and .apply() are used to invoke a function and set the value of "this" within the function. The difference is in the arguments they take: .call() takes individual arguments, while .apply() takes an array of arguments.
18. What is the difference between an object and an array in JavaScript?
Answer: An object is a collection of key-value pairs, while an array is a collection of values that can be accessed by an index.
19. What is the difference between null and undefined in JavaScript?
Answer: Undefined means a variable has been declared but has not been assigned a value, while null is an assignment value, meaning the variable has been declared and assigned the value of null.
20. What is the difference between deep copy and shallow copy in JavaScript?
Answer: A shallow copy only copies the first level of an object's properties and does not copy any nested objects. A deep copy, on the other hand, creates a new copy of the entire object and its nested objects, so that the original object and its copy are completely independent.