20 Common JavaScript Interview Questions and Answers

JavaScript is one of the most popular programming languages in the world. As a result, it is also one of the most frequently asked topics in technical interviews. Whether you are a beginner or an experienced developer, it’s important to be well-prepared for JavaScript interview questions.

In this article, we’ll discuss 20 common JavaScript interview questions and provide detailed answers to help you prepare.

1. What is JavaScript, and what are its features?

   JavaScript is a scripting language that is used to create interactive web pages. It was created by Brendan Eich in 1995 and is now one of the most widely used programming languages. JavaScript is known for its ability to create dynamic and interactive web pages, its flexibility, and its simplicity.

2. What is the difference between let, var, and const in JavaScript?

   let, var, and const are all used for declaring variables in JavaScript, but they have different scoping rules. var is function-scoped, and let and const are block-scoped. const is used for declaring constants that cannot be reassigned, whereas let and var can be reassigned.

3. What is hoisting in JavaScript?

   Hoisting is a JavaScript mechanism that allows variables and function declarations to be moved to the top of their scope. This means that variables and functions can be used before they are declared. However, only the declarations are hoisted, not the assignments.

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

   “null” represents an intentional absence of any object value, while undefined represents an uninitialized, undeclared or out-of-scope variable. In other words, null is an explicit value that means “no value”, while undefined means that a variable has not been assigned a value.

5. What is a closure in JavaScript?

   A closure is a function that has access to variables in its outer lexical scope, even after the outer function has returned. This allows for persistent variables that can be accessed and modified by inner functions.

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

   Synchronous code is executed in sequence, one statement at a time, while asynchronous code is executed out of order, with callbacks or promises used to handle the execution order. Asynchronous code is used for time-consuming operations, such as network requests or file system operations.

7. What are callback functions in JavaScript?

   A callback function is a function that is passed as an argument to another function and is executed inside that function. Callback functions are often used in asynchronous code to handle the results of an operation.

8. What is event bubbling in JavaScript?

   Event bubbling is a mechanism in JavaScript where an event triggered on an element will propagate up through its parent elements until it is handled or reaches the document object. This allows for event handling on a parent element that affects all its child elements.

9. What is the purpose of the “this” keyword in JavaScript?

   The “this” keyword refers to the object that is currently executing the code. In JavaScript, “this” is dynamically determined based on how the function is called. It is often used in object-oriented programming to refer to the object to which the method belongs.

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

    “==” is a loose comparison operator that checks for equality after type coercion, while === is a strict comparison operator that checks for equality without type coercion. For example, 1 == "1" would return true, but 1 === "1" would return false.

11. What is the difference between let and const in JavaScript?

    The main difference between let and const is that let variables can be reassigned to new values, while const variables cannot be reassigned. However, the scope of both let and const variables is limited to the block in which they are declared.

12. What are the different data types available in JavaScript?

    There are six primitive data types in JavaScript: string, number, boolean, null, undefined, and symbol. Additionally, there is one non-primitive data type, which is an object.

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

    Undefined is a value that a variable can have if it has not been assigned a value yet, while null is a value that a variable can have if it has been explicitly assigned the value null.

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

    A function declaration is a statement that creates a function with a given name, while a function expression creates a function that is assigned to a variable or passed as an argument to another function.

15. What is an IIFE in JavaScript?

    An IIFE (Immediately Invoked Function Expression) is a function that is executed as soon as it is defined. It is typically used to create a private scope for variables and functions that are not exposed to the global scope.

16. What is a promise in JavaScript?

    A promise is an object that represents a value that may not be available yet but will be available at some point in the future. Promises are often used to handle asynchronous operations, such as network requests.

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

    Call and apply are both methods that can be used to invoke a function with a given object as its context. The difference is in how arguments are passed: call takes arguments directly, while apply takes arguments as an array.

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

    A for loop is a traditional loop that is used to iterate over an array or other iterable object, while a forEach loop is a method of the array object that is used to execute a callback function for each element of the array.

19. What is the purpose of the “use strict” directive in JavaScript?

    The “use strict” directive is a feature that was introduced in ECMAScript 5 and is used to enable strict mode in JavaScript. Strict mode enforces stricter parsing and error handling and can help prevent common programming mistakes.

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

    In JavaScript, a class is a blueprint for creating objects, while an object is an instance of a class. Classes in JavaScript are actually just syntactic sugar for constructor functions and prototypal inheritance.

In conclusion, being well-prepared for JavaScript interview questions can help you land your dream job as a web developer. By understanding the concepts and answering these 20 common JavaScript interview questions, you’ll be ready to demonstrate your expertise and impress your interviewer.