20 Event Loop Interview Questions and Answers

An Event Loop is a programming construct that allows for asynchronous programming and can be used to improve the performance of an application. If you’re applying for a position that involves Event Loop programming, it’s important to be prepared to answer questions about your experience and knowledge. In this article, we review some common Event Loop interview questions and provide guidance on how to answer them.

Event Loop Interview Questions and Answers

Here are 20 commonly asked Event Loop interview questions and answers to prepare you for your interview:

1. What is an event loop?

An event loop is a programming construct that allows a program to handle multiple events or tasks simultaneously. It works by having the program keep track of all the events or tasks that need to be processed, and then processing them one by one as they come in. This can be a very efficient way to handle a lot of events or tasks, as it prevents the program from getting bogged down by having to process everything at once.

2. Can you explain how a JavaScript event loop works?

The event loop is a mechanism that allows JavaScript to perform non-blocking operations. When an asynchronous task is started, the event loop will start running. Once the task is completed, the event loop will again check for any other tasks that need to be performed. This process will continue until all tasks have been completed.

3. How do you handle multiple events in the context of an event loop?

When you are handling multiple events in the context of an event loop, you need to be careful to avoid what is known as a “race condition.” This is when two events happen so close together that the order in which they are handled can affect the outcome of the program. To avoid this, you need to use a mechanism known as “locking.” This ensures that only one event can be handled at a time, and that the events are handled in the order in which they occurred.

4. Can you give me some examples of situations where it’s a good idea to use an event loop?

Event loops are often used in situations where there is a need to constantly check for new events or data. For example, a chat program may use an event loop to check for new messages from other users, or a game may use an event loop to check for user input or new game data.

5. Where are event loops used most often?

Event loops are most often used in programming languages that are based on the event-driven programming paradigm. This programming paradigm is based on the idea of responding to events as they occur, rather than executing a set of instructions in a predetermined order. Many graphical user interface (GUI) programs are written using the event-driven programming paradigm, as are many video games.

6. Are there any drawbacks to using event loops? If yes, then what are they?

While event loops are generally considered to be a good thing, there are some potential drawbacks to using them. One is that if an event loop is not used properly, it can lead to what is known as an “event storm” which can cause a program to become unresponsive. Another potential drawback is that if an event loop is not coded correctly, it can cause a program to enter into an infinite loop.

7. What is a callback function?

A callback function is a function that is passed as an argument to another function. The callback function is then invoked inside of the other function. Callback functions are often used in event-driven programming, such as in the browser when handling user events.

8. Can you explain what first-class functions are and why they’re important?

In JavaScript, functions are considered first-class citizens, which means that they can be treated like any other value in the language. This means that they can be assigned to variables, passed as arguments to other functions, and returned from functions. First-class functions are important because they allow for a great deal of flexibility and creativity in how we write our code.

9. How many phases does an event loop have?

There are four main phases in an event loop:

1. The event loop starts and waits for an event to occur.
2. When an event occurs, the event loop calls the appropriate event handler.
3. The event handler processes the event and may generate new events.
4. The event loop goes back to waiting for another event.

10. Can you explain the difference between callbacks and promises? Which one would you choose for asynchronously executing code in a NodeJS application?

Callbacks are functions that are executed after a certain event has occurred. Promises are similar to callbacks, but they provide a more convenient way to handle asynchronous code. With promises, you can chain together multiple asynchronous operations, and you can be sure that the code will execute in the order that you expect. For these reasons, promises are generally the better choice for asynchronously executing code in a NodeJS application.

11. Do all programming languages support event loops?

No, not all programming languages support event loops. Some languages are designed for more traditional, linear execution. However, more and more languages are starting to support event loops as they become more popular.

12. What are the various types of events that can be generated by the browser or user actions?

The various types of events that can be generated by the browser or user actions include:

-Mouse events: These events are generated when the user interacts with the mouse, such as when they click on an element or move the mouse over an element.

-Keyboard events: These events are generated when the user interacts with the keyboard, such as when they press a key or type a character.

-Touch events: These events are generated when the user interacts with a touch screen, such as when they tap on an element or swipe their finger across an element.

-Resize events: These events are generated when the browser window is resized.

-Scroll events: These events are generated when the user scrolls up or down the page.

13. What is the purpose of the Event Queue?

The Event Queue is a data structure that stores information about events that have not yet been processed. When an event occurs, it is added to the end of the queue. The Event Loop then processes events in the order that they were added to the queue. This ensures that all events are processed in a timely manner.

14. What is the main advantage of using an event queue over multithreading?

The main advantage of using an event queue is that it allows for a more efficient use of resources. With an event queue, you can avoid having multiple threads running at the same time, which can lead to issues with race conditions and other problems.

15. What is the difference between a task and a microtask?

A task is a unit of work that is typically executed by the JavaScript engine in a single turn of the event loop. A microtask is a smaller unit of work that is typically executed by the JavaScript engine in between turns of the event loop.

16. Why is it a bad idea to block the main thread when running JavaScript code?

Blocking the main thread can cause performance issues because it prevents the event loop from being able to process other tasks. This can lead to delays and can make your code less responsive.

17. Is it possible to manipulate the order in which tasks are executed? If yes, then how?

Yes, it is possible to manipulate the order in which tasks are executed. This can be done by using the setTimeout() or setInterval() methods. These methods allow you to specify a time delay before a task is executed. This can be used to manipulate the order in which tasks are executed.

18. What is the best way to execute asynchronous tasks in JavaScript?

The best way to execute asynchronous tasks in JavaScript is to use the event loop. The event loop is a mechanism that allows JavaScript to execute asynchronous tasks in a sequential order. This means that tasks that are dependent on each other can be executed in the correct order, and tasks that are not dependent on each other can be executed in parallel.

19. Can you explain how setTimeout() works?

The setTimeout() function takes two arguments: a callback function and a delay in milliseconds. The callback function is executed once the delay has elapsed. The delay is not guaranteed to be accurate, but is usually within a few milliseconds.

20. What is the maximum number of concurrent HTTP requests allowed by browsers?

The maximum number of concurrent HTTP requests allowed by browsers is typically six. This number can vary depending on the browser, but it is generally six. This limit is in place because too many concurrent requests can overload the browser and cause it to crash.