Top 10 Interview Questions and Answers for JavaScript Frontend Engineers

Mastering JavaScript is crucial for becoming a good web developer because it is one of the most commonly used programming languages in web development. In this article, we’ll cover some common JavaScript interview questions that cover a wide range from the basics to advanced concepts. Also, this can help you prepare for JavaScript interviews.

1. What is JavaScript, and how does it differ from other programming languages?

JavaScript is a programming language that is primarily used to create interactive web applications. It is a high-level, interpreted language that supports object-oriented, imperative, and functional programming paradigms.

2. What is the difference between “undefined” and “null” in JavaScript?

In JavaScript, “undefined” and “null” are both values that represent the absence of something. However, there are some important differences between them. We indicate that a variable or property lacks a value by using Undefined, whereas we intentionally give a value of nothing to a variable or property by using null.

3. What are some ways to improve the performance of JavaScript code?

Some techniques to improve the performance of JavaScript code include minimizing DOM manipulation, using caching and memoization, using event delegation, and minimizing the use of global variables.

4. What is a closure in JavaScript, and why is it important?

A closure is a function that has access to its parent function’s variables and parameters. Closures are important in JavaScript because they allow functions to maintain state and to retain access to variables that would otherwise be out of scope.

5. What is the “this” keyword in JavaScript, and how does it work?

The “this” keyword in JavaScript refers to the current execution context, which can be the global context, a function context, or an object context. How a function is called determines the value of “this”.

6. What is the difference between “==” and “===” in JavaScript?

“==” is a loose equality operator that compares two values for equality after converting them to a common type. “===” is a strict equality operator that compares two values for equality without converting them to a common type.

7. What is event bubbling, and how does it work?

Event bubbling is a mechanism in JavaScript where events triggered on a nested or child element “bubble up” to its parent elements in the DOM tree, triggering their corresponding event handlers in the process.

8. What is the difference between “var”, “let”, and “const” in JavaScript?

“var” is a keyword that declares a variable with function scope. “let” and “const” are keywords that declare variables with block scope. “let” allows a variable to be reassigned, while “const” does not.

9. What is the difference between “map” and “forEach” in JavaScript?

In JavaScript, you can use both “map” and “forEach” methods to iterate over arrays. However, “map” returns a new array with the results of a callback function applied to each element, while “forEach” simply iterates over the array and performs an action for each element.

10. What is the difference between “async” and “defer” in script tags?

We can use attributes “async” and “defer” in script tags to control the loading and execution of scripts. “async” loads the script asynchronously, which means that it does not block the rendering of the page. “defer” loads the script asynchronously as well, but it waits until the page has finished parsing before executing the script.

Conclusion

In summary, preparing for an interview is an important step in becoming a JavaScript front-end engineer. During the interview process, make sure you have a good understanding and grasp of core JavaScript concepts, DOM manipulation, AJAX, event handling, front-end frameworks, and that you can clearly communicate your ideas and problem-solving approach.