Get Ready for Your Next JavaScript Interview: 40 Common Questions You Might Encounter

Mastering the Basics: Commonly Asked JavaScript Interview Questions

If you're looking for a job as a JavaScript developer, you probably already know that the interview process can be tough. But don't worry - we're here to help you prepare! In this blog post, we've compiled a list of 40 common questions you might encounter during your next JavaScript interview. By studying and practising these questions, you'll be ready to ace your interview and land your dream job.

Photo by Nangialai Stoman on Unsplash

Javascript basics:

1. What are the basic data types in JavaScript?

   There are seven basic data types in JavaScript: null, undefined, boolean, number, string, symbol, and object.

2. What are the different ways to define a variable in JavaScript?
   There are three ways to define a variable in JavaScript: using var, let, or const.

3. What is the difference between == and === in JavaScript?
   The double equals (==) compares two values for equality, but it can perform type coercion, which means it will try to convert the values to a common type before comparing. The triple equals (===) compares two values for equality without performing type coercion, which means it will only return true if both values have the same type and the same value.

4. How do you declare a function in JavaScript?
   Functions can be declared using the function keyword followed by the function name, followed by parentheses that contain the function parameters, and then the function body enclosed in curly braces.

5. What are the different types of loops available in JavaScript?
   There are three types of loops in JavaScript: the for loop, the while loop, and the do-while loop.

6. What is a closure in JavaScript?
   A closure is a function that has access to variables in its outer lexical scope, even after that scope has been destroyed.

7. How does the hoisting mechanism work in JavaScript?
   Hoisting is a mechanism in JavaScript where variable and function declarations are moved to the top of their respective scopes, allowing them to be used before they are declared.

8. What are the differences between let, var, and const in JavaScript?
   The main differences between let, var, and const in JavaScript are their scoping rules and mutability. var has function scope, let and const have block scope. var can be reassigned and redeclared within its scope, while let can be reassigned but not redeclared, and const cannot be reassigned or redeclared.

9. What is the difference between null and undefined in JavaScript?
   null and undefined are both used to represent the absence of a value in JavaScript. null is explicitly assigned to a variable to indicate that it has no value, while undefined means that a variable has been declared but has not been assigned a value.

10. What is an IIFE in JavaScript?
    IIFE stands for Immediately Invoked Function Expression. It is a function that is executed as soon as it is defined, without being explicitly called.

    
    DOM manipulation:

11. What is the Document Object Model (DOM)?

    The Document Object Model (DOM) is a programming interface for web documents. It represents the page so that programs can change the document structure, style, and content. The DOM represents the document as nodes and objects. That way, programming languages can interact with the page.

12. How do you select elements in the DOM using JavaScript?

    JavaScript provides several methods to select elements in the DOM, including getElementById, getElementsByClassName, getElementsByTagName, and querySelector. These methods allow you to select elements based on their ID, class name, tag name, or any CSS selector.

13. What is the difference between innerHTML and textContent?

    innerHTML and textContent are both properties of DOM elements. innerHTML returns the HTML content inside an element, while textContent returns only the text content of an element. This means that innerHTML includes all HTML tags and text inside an element, while textContent only includes the raw text content.

14. How do you add and remove elements to the DOM using JavaScript?

    JavaScript provides several methods to add and remove elements to the DOM, including createElement, appendChild, removeChild, and replaceChild. These methods allow you to create new elements, add them to the DOM tree, remove existing elements, or replace them with new elements.

15. What is event bubbling in JavaScript?

    Event bubbling is a feature in JavaScript where events triggered on an element will also trigger on all its parent elements. This means that if you click on a child element inside a parent element, the click event will also trigger on the parent element. Event bubbling can be useful for handling events on multiple elements at once.

16. How do you prevent default behaviour in JavaScript events?

    JavaScript events have default behaviours that can be prevented using the preventDefault() method. For example, you can prevent a form from submitting or a link from navigating to its destination by calling event.preventDefault() inside an event handler.

17. How do you add and remove event listeners in JavaScript?

    JavaScript provides several methods to add and remove event listeners, including addEventListener, removeEventListener, and onclick. These methods allow you to attach event listeners to elements, remove existing event listeners, or specify a function to run when an event occurs.

18. What are the different types of events in JavaScript?

    JavaScript provides a variety of events that can be triggered by user interactions, such as mouse clicks, keyboard inputs, form submissions, and page loading. These events include click, submit, keydown, load, and many others.

19. How do you create and use custom events in JavaScript?

    JavaScript allows you to create custom events using the CustomEvent constructor and dispatch them using the dispatchEvent method. Custom events can be useful for creating more complex event-driven applications or for integrating with other libraries.

20. How do you animate elements in the DOM using JavaScript?

    JavaScript provides several methods for animating elements in the DOM, including setInterval, setTimeout, and the Web Animations API. These methods allow you to create animations by changing the style, position, or other properties of elements over time.
    

    Asynchronous programming:

21. What is asynchronous programming in JavaScript?

    Asynchronous programming in JavaScript is a way of executing non-blocking code that allows other code to run while waiting for an asynchronous operation to complete. Asynchronous programming is commonly used for tasks that may take a long time to complete, such as making network requests or accessing a database.

22. What are callbacks in JavaScript?

    Callbacks in JavaScript are functions that are passed as arguments to other functions and are called when a particular event occurs. They are commonly used in asynchronous programming to handle the results of an asynchronous operation once it completes.

23. What are promises in JavaScript?

    Promises in JavaScript are objects that represent the eventual completion or failure of an asynchronous operation and its resulting value. Promises allow for cleaner, more organized asynchronous code by allowing developers to chain together multiple asynchronous operations and handle errors in a more structured way.

24. What is the async/await syntax in JavaScript?

    The async/await syntax in JavaScript is a way to write asynchronous code that looks and behaves more like synchronous code. The async keyword is used to define a function as asynchronous, and the await keyword is used to wait for the completion of an asynchronous operation.

25. How do you handle errors in asynchronous JavaScript code?

    Error handling in asynchronous JavaScript code can be handled through the use of try/catch blocks, or by attaching a .catch() method to a Promise object. Proper error handling is important in order to prevent your application from crashing or entering an invalid state.

26. What are the benefits of using asynchronous programming in JavaScript?

    Asynchronous programming in JavaScript allows for more efficient use of resources by not blocking the main thread while waiting for long-running operations to complete. It also allows for more responsive user interfaces and can lead to better performance in network-intensive applications.

27. How do you handle race conditions in JavaScript?

    Race conditions in asynchronous JavaScript code can be handled through the use of mutexes or other synchronization mechanisms to ensure that only one operation is modifying shared data at a time. Proper synchronization is important to prevent data corruption and ensure the correct behaviour of your application.

28. How do you work with multiple asynchronous operations in JavaScript?

    Multiple asynchronous operations in JavaScript can be handled through the use of Promises, callbacks, or the async/await syntax to coordinate the execution of multiple asynchronous operations. The Promise.all() method can also be used to wait for multiple Promises to complete before proceeding.

29. What are some common pitfalls to avoid when working with asynchronous JavaScript code?

    Common pitfalls when working with asynchronous JavaScript code include failing to properly handle errors, failing to properly synchronize access to shared data, and failing to properly cancel or clean up resources used by asynchronous operations.

30. How do you debug asynchronous JavaScript code?

    Debugging asynchronous JavaScript code can be challenging due to the non-linear nature of asynchronous execution. Techniques such as logging, stepping through code with a debugger, and using browser developer tools can all be helpful in identifying and fixing bugs in asynchronous code.

    
    Functional programming:

31. What is functional programming in JavaScript?

    Functional programming is a programming paradigm that emphasizes the use of functions as the primary building blocks of software. In JavaScript, functional programming involves creating functions that operate on data and return new data, rather than changing data in place. It also emphasizes immutability, avoiding side effects, and using higher-order functions to compose complex behaviours.

32. What is a pure function in JavaScript?

    A pure function is a function that always returns the same output for a given input and has no side effects. In JavaScript, a pure function does not modify any external state or mutate any input. It relies solely on the arguments passed to it and any internal variables created within the function.

33. What is immutability in JavaScript?

    Immutability is the concept of creating data structures that cannot be changed after they are created. In JavaScript, this is often accomplished by creating new copies of data structures rather than modifying existing ones. Immutable data structures are useful in functional programming because they make it easier to reason about the behaviour of functions and avoid unexpected side effects.

34. What is a higher-order function in JavaScript?

    A higher-order function is a function that takes another function as an argument or returns a function as its result. In JavaScript, higher-order functions are often used to create reusable code and to implement common patterns like map, filter, and reduce.

35. How do you use map, filter, and reduce in JavaScript?

    Map, filter, and reduce are higher-order functions that are used to transform arrays in JavaScript. Map applies a function to each element of an array and returns a new array with the transformed values. Filter creates a new array containing only the elements that pass a certain condition. Reduce aggregates all elements of an array into a single value using a given function.

36. What are some common functional programming patterns in JavaScript?

    Some common functional programming patterns in JavaScript include currying, partial application, and function composition. These patterns are used to create reusable functions that can be combined to create more complex behaviours.

37. How do you compose functions in JavaScript?

    Function composition in JavaScript involves combining multiple functions to create a new function that performs a sequence of operations. This can be accomplished using higher-order functions like compose or pipe, or by manually chaining functions together using the return value of one function as the input to the next.

38. What are the benefits of using functional programming in JavaScript?

    Using functional programming in JavaScript can lead to code that is easier to understand, test, and maintain. It can also lead to more modular code and fewer bugs, as well as improved performance in certain situations.

39. How do you test functional JavaScript code?

    Functional JavaScript code can be tested using a variety of testing frameworks and libraries, including Jest, Mocha, and Jasmine. Tests can be written to verify that functions return the expected output for a given input and that they do not have any unexpected side effects.

40. How do you debug functional JavaScript code?

    Debugging functional JavaScript code can be challenging because it often involves working with complex data structures and higher-order functions. However, tools like the JavaScript debugger and console.log statements can be used to step through code and identify errors. Additionally, functional programming techniques like immutability and pure functions can make it easier to reason about the behaviour of code and avoid errors in the first place.
    

Preparing for a JavaScript interview can be overwhelming, but we hope this list of questions has given you a good starting point for your preparation. Remember to not only memorize the answers but to also understand the concepts behind them. Practice coding, read documentation, and stay up-to-date with the latest developments in the JavaScript world. With these tips and your hard work, you're sure to impress your interviewer and land your next JavaScript job. Good luck!