JavaScript
Top 40 Interview Question and Answer For JavaScript
1. What is JavaScript?
JavaScript is a high-level, interpreted programming language primarily used for client-side web development. It enables interactive web pages and is essential for front-end development.
2. What are the key features of JavaScript?
Key features include:
- Lightweight and interpreted
- Object-oriented
- Cross-platform
- Versatile and dynamic
3. What are the differences between JavaScript and Java?
Despite similar names, they are different languages with different purposes. JavaScript is primarily used for web development, while Java is a general-purpose language used for various applications.
4. What is the difference between == and === operators in JavaScript?
The == operator checks for equality of values, whereas the === operator checks for equality of values and types.
5. What are closures in JavaScript?
Closures are functions that have access to the outer function's scope, even after the outer function has finished executing.
6. What is event delegation in JavaScript?
Event delegation is a technique where a single event listener is attached to a parent element to handle events for all of its descendants.
7. Explain the concept of hoisting in JavaScript.
Hoisting is a JavaScript mechanism where variables and function declarations are moved to the top of their containing scope during compilation.
8. How do you handle errors in JavaScript?
Errors in JavaScript can be handled using try...catch blocks, where code likely to throw an error is placed inside the try block and the catch block is used to handle any resulting errors.
9. Explain the difference between null and undefined in JavaScript.
Null represents the intentional absence of any value, while undefined indicates a variable that has been declared but not assigned a value.
10. What is the event loop in JavaScript?
The event loop is a mechanism in JavaScript that handles asynchronous operations by processing tasks from the event queue in a non-blocking manner.
11. What is the purpose of the 'use strict' directive in JavaScript?
The 'use strict' directive enables strict mode, which imposes stricter parsing and error handling in JavaScript, helping to identify and fix common coding mistakes.
12. How do you create a new object in JavaScript?
Objects can be created using object literals, constructor functions, or by using the Object.create() method.
13. What is the difference between function declaration and function expression in JavaScript?
A function declaration is hoisted to the top of its containing scope and can be invoked before its declaration, whereas a function expression is not hoisted and must be defined before it can be invoked.
14. What are the different ways to define a function in JavaScript?
Functions in JavaScript can be defined using function declarations, function expressions, arrow functions, and method definitions.
15. Explain the concept of prototypal inheritance in JavaScript.
Prototypal inheritance in JavaScript allows objects to inherit properties and methods from other objects through a prototype chain.
16. What is the purpose of the 'this' keyword in JavaScript?
The 'this' keyword refers to the current execution context and is used to access the object that owns the currently executing code.
17. How do you check the type of a variable in JavaScript?
The typeof operator can be used to check the type of a variable in JavaScript.
18. What are the different ways to loop through an array in JavaScript?
Arrays in JavaScript can be iterated over using for loops, forEach(), map(), filter(), reduce(), and other array methods.
19. What is a callback function in JavaScript?
A callback function is a function that is passed as an argument to another function and is executed after a specific event or action occurs.
20. Explain the concept of event bubbling in JavaScript.
Event bubbling is a phenomenon in which events triggered on nested elements are propagated up through their ancestor elements in the DOM hierarchy.
21. How do you convert a string to a number in JavaScript?
Strings can be converted to numbers in JavaScript using the parseInt() or parseFloat() functions, or by using the unary plus operator (+).
22. What is the difference between local and global scope in JavaScript?
Variables declared inside a function have local scope, meaning they are accessible only within that function, while variables declared outside of any function have global scope, making them accessible throughout the entire script.
23. How do you debug JavaScript code?
JavaScript code can be debugged using browser developer tools like the Chrome DevTools or Firefox Developer Tools, which allow you to inspect variables, set breakpoints, step through code, and view console output for debugging purposes.
24. How do you check if an object has a specific property in JavaScript?
The 'in' operator or the hasOwnProperty() method can be used to check if an object has a specific property in JavaScript.
25. What is the difference between event.stopPropagation() and event.preventDefault() in JavaScript?
event.stopPropagation() stops the propagation of an event through the DOM tree, preventing parent elements from receiving the event, while event.preventDefault() prevents the default action associated with the event from occurring, such as following a link or submitting a form.
26. What is the purpose of the 'async' and 'await' keywords in JavaScript?
The 'async' keyword is used to define asynchronous functions, while the 'await' keyword is used to pause the execution of an asynchronous function until a Promise is resolved or rejected.
27. What is the difference between 'let', 'var', and 'const' in JavaScript?
'let' and 'const' are block-scoped, meaning they are limited to the block in which they are defined, while 'var' is function-scoped. Additionally, variables declared with 'const' cannot be reassigned, whereas variables declared with 'let' and 'var' can.
28. What is a promise in JavaScript?
A promise is an object representing the eventual completion or failure of an asynchronous operation. It allows for more readable and manageable asynchronous code compared to traditional callback-based approaches.
29. How do you remove an element from an array in JavaScript?
Elements can be removed from an array in JavaScript using methods like splice(), pop(), shift(), or by using the filter() method to create a new array without the specified element.
30. What are the different ways to handle asynchronous code in JavaScript?
Asynchronous code in JavaScript can be handled using callbacks, promises, async/await, and event listeners.
31. What is the difference between null and undefined in JavaScript?
Null represents the intentional absence of any value, while undefined indicates a variable that has been declared but not assigned a value.
32. How do you check if an element exists in the DOM using JavaScript?
You can check if an element exists in the DOM using methods like getElementById(), querySelector(), or by checking the length property of a NodeList returned by methods like getElementsByClassName() or getElementsByTagName().
33. How do you dynamically create HTML elements using JavaScript?
HTML elements can be created dynamically using methods like createElement() and appendChild(), allowing for the generation of HTML content based on user input or other dynamic factors.
34. What are the benefits of using strict mode in JavaScript?
Strict mode helps catch common coding errors and unsafe actions by enforcing stricter parsing and error handling rules in JavaScript, leading to more robust and maintainable code.
35. Explain the concept of event delegation in JavaScript.
Event delegation is a technique in JavaScript where a single event listener is attached to a parent element to handle events for all of its descendants. This is useful for dynamically created elements or when there are a large number of elements with similar event handlers.
36. How do you detect the browser's user agent in JavaScript?
You can use the navigator.userAgent property to access the browser's user agent string, which contains information about the browser and operating system.
37. What is the difference between document.readyState and window.onload in JavaScript?
document.readyState indicates the current state of the document's loading process, while window.onload is an event handler that fires when the entire document has finished loading, including all its dependent resources like images and stylesheets.
38. How do you compare two objects in JavaScript?
Objects cannot be compared directly using == or === operators because they are compared by reference, not by value. However, you can compare their properties manually or use external libraries like lodash or Underscore.js for deep comparison.
