JavaScript and React Interview questions (Junior/Mid-level) 2023

Hi, I am working as Frontend/JavaScript developer for the past 5+ years. I've taken multiple interviews. I want to share some questions I used to ask from candidates. Hope it'll be helpful for you guys. Let's dive into it.

JavaScript Questions:

1. What is an Event loop?
2. What are the various data types that exist in JavaScript?
3. Difference between Primitive and non-primitive?
4. What are the scopes of a variable in JavaScript?
5. What is Hoisting in JS?
6. What is the ‘this’ keyword in JavaScript?
7. What is ES6/ES7?
8. What is Callback in JavaScript?
9. What is the difference between var, let, and const?
10. What is the difference between splice and slice?
11. What is the difference between a map and foreach?
12. What is the difference between an arrow function and a normal function?
13. What is a generator function?
14. What is the difference between cookies, sessions, and local storage?
15. What are Closures in JavaScript?
16. What is the difference between Document and Window in JavaScript?
17. What is the difference between Undefined and Undeclared in JavaScript?
18. What is the difference between Call and Apply?
19. Difference between “ == “ and “ === “ operators?
20. What is NaN property in JavaScript?
21. What is the difference between Passed by value and passed by reference?
22. What is an Immediately Invoked Function in JavaScript?
23. What are Higher Order Functions?
24. What is currying in JavaScript?
25. How many Types of errors are in JavaScript?
26. What is BOM?
27. Difference between client-side and server-side?
28. What is a spread operator?
29. What are promises in JS?
30. What is Object Destructuring?
31. Difference between Async and Promise?
32. What is the role of deferred scripts?
33. What is the difference between Event Capturing and Event Bubbling?
34. What is the Strict mode in JavaScript?
35. What is the difference between filter and find?
36. Is JavaScript Case-Sensitive?
37. How can you convert an object to an array?
38. What Is Garbage Collection in JavaScript?

React Js:

1. Differentiate between Real DOM and Virtual DOM.
2. What is the significance of keys in React?
3. What is the purpose of render() in React?
4. What are Props?
5. What is a state in React?
6. Differentiate between stateful and stateless components.
7. What are lifecycle methods in React?
8. What is the difference between class and functional components?
9. What is an event in React?
10. What are Refs in React?
11. Explain useEffect in detail?
12. What are Higher Order Components(HOC)?
13. What are Pure Components?
14. What is the significance of keys in React?
15. What is JSX?
16. What is Redux?
17. What are the principles of Redux?
18. How can you access value from Redux?
19. What is the difference between controlled and uncontrolled components?
20. What is the difference between useMemo and useCallback?
21. What is the difference between useEffect and useLayoutEffect?
22. What is SPA?
23. What are custom hooks?
24. When to use util or custom hook?
25. How do you handle optimization in a large React application?
26. What is Context API?
27. Explain the concept of a Portal in React?
28. How do you handle asynchronous data loading in a React application?
29. How do you handle localization in a React application?
30. What is the difference between static and dynamic components in React?
31. How do you handle server-side rendering in a React application?
32. What is PropTypes?
33. Explain the concept of Memoization in React?
34. Does React have one or two-way binding?
35. How do you send data from the child to the parent component?
36. Does useEffect triggers when we change by reference?
37. How to convert functional components to pure components?
38. What is Axios interceptor?
39. How to validate forms in React?
40. What is unit testing?
41. What is JWT?

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Other links:

• Highlight on the new features of Next JS 13 config 2022
• How to configure React JS app on WSL2 Windows 10
• Custom 'Tooltip' component using Tailwind and React Js