20 JavaScript Promise Interview Questions and Answers

A JavaScript Promise is an object that represents the eventual completion (or failure) of an asynchronous operation and its resulting value. When applying for a position that involves JavaScript, it is likely that you will be asked questions about Promises. In this article, we review some of the most common questions about JavaScript Promises and how you should answer them.

JavaScript Promise Interview Questions and Answers

Here are 20 commonly asked JavaScript Promise interview questions and answers to prepare you for your interview:

1. What is a promise?

A promise is an object that represents the result of an asynchronous operation. A promise can be in one of three states: pending, fulfilled, or rejected. A pending promise means that the asynchronous operation has not yet completed. A fulfilled promise means that the asynchronous operation has completed successfully. A rejected promise means that the asynchronous operation has failed.

2. Can you give me an example of how to use promises in JavaScript?

There are a few different ways to use promises in JavaScript. One way is to use the .then() method, which allows you to specify what should happen when the promise is fulfilled. For example:

var promise = new Promise(function(resolve, reject) {
// do something

if (/* everything turned out fine */) {
resolve(“Success!”);
} else {
reject(Error(“Failed!”));
}
});

promise.then(function(result) {
console.log(result); // “Success!”
}, function(error) {
console.log(error); // “Failed!”
});

Another way to use promises is to use the async/await keywords. This syntax makes it easier to read and write code that uses promises. For example:

async function doSomething() {
var result = await promise;
console.log(result); // “Success!”
}

doSomething();

3. How do you create a new Promise object in JavaScript?

You can create a new Promise object by using the Promise() constructor function. This function takes in a callback function as its first argument. The callback function takes in two arguments, resolve and reject. These functions are used to resolve or reject the promise, respectively.

4. Is it possible to chain promises together? If yes, then how?

Yes, it is possible to chain promises together. This can be done by returning a promise from the then() function of a previous promise. The returned promise will then be resolved or rejected based on the resolution of the promise that it is chained to.

5. What are the different states that a promise can be in during its lifetime?

A promise can be in one of three states during its lifetime: pending, fulfilled, or rejected. A promise starts in the pending state, and then either transitions to the fulfilled state if the promise is successfully completed, or the rejected state if the promise is not completed.

6. Can you explain what async/await means in the context of JavaScript and NodeJS?

Async/await is a way of writing asynchronous code that makes it look and feel like synchronous code. With async/await, you can write code as if it were running in a single thread, even though it is actually running in multiple threads. This makes asynchronous code much easier to read and write.

7. Can you explain what the .then() method does?

The .then() method is used to specify what should happen when a promise is resolved. This is usually used to chain together multiple promises, so that each promise is executed in order. The .then() method can also take two arguments: a success callback and a failure callback. The success callback will be executed if the promise is resolved, and the failure callback will be executed if the promise is rejected.

8. Can you explain what the .catch() method does?

The .catch() method is used to handle errors that may occur in a Promise chain. The .catch() method takes a single argument, which is a function that will be executed if an error occurs. The .catch() method is typically used after a .then() method to handle any errors that may have occurred in the .then() method.

9. What does the .all() method do when working with promises?

The .all() method is used to take an array of promises and wait for them all to resolve before running the code inside of the .then() method. This is useful if you have a number of asynchronous tasks that need to be completed before moving on.

10. What does the .race() method do when working with promises?

The .race() method is used to take an array of promises and return a promise that resolves or rejects as soon as one of the promises in the array resolves or rejects. This is useful for situations where you want to cancel a Promise if it takes too long to resolve.

11. What does the .resolve() method do when working with promises?

The .resolve() method is used to resolve a promise and return the corresponding value. This is typically used when a promise has been fulfilled and the corresponding value is known.

12. What does the .reject() method do when working with promises?

The .reject() method is used when you want to indicate that a promise has failed. This is typically used when you want to catch an error that has occurred inside of a promise.

13. What is your understanding of callbacks in the context of JavaScript and NodeJS?

A callback is a function that is passed as an argument to another function and is executed after the outer function has completed. In the context of JavaScript and NodeJS, callbacks are often used to handle asynchronous operations.

14. What is the difference between synchronous and asynchronous programming?

In synchronous programming, the code is executed line by line in the order that it is written. This means that each line of code must be completed before the next line can begin. Asynchronous programming, on the other hand, allows for code to be executed out of order. This means that lines of code can be executed even if the previous line has not yet finished.

15. What is the best way to handle errors in JavaScript?

There is no one-size-fits-all answer to this question, as the best way to handle errors in JavaScript will vary depending on the specific situation. However, one general approach that can be taken is to use JavaScript Promises. Promises provide a way to handle asynchronous operations in JavaScript, and can be used to help handle errors by providing a mechanism for handling rejected Promises.

16. Why should we avoid using callback functions?

There are a few reasons why callback functions should be avoided:

1. They can be hard to read and debug
2. They can lead to what is known as “callback hell”
3. Promises provide a simpler and more elegant way to handle asynchronous code

17. When do you think it’s appropriate to convert a callback-based function into a promise-based function?

I think it’s appropriate to convert a callback-based function into a promise-based function when you want to make sure that a certain task is completed before moving on to the next task. For example, if you have a function that fetches data from a server, you might want to convert it into a promise-based function so that you can be sure that the data has been fetched before you try to use it.

18. What is the advantage of using promises over callbacks?

Promises provide a simpler and cleaner interface for dealing with asynchronous code than callbacks. With promises, you can avoid callback hell, and you don’t have to worry about keeping track of which callback goes with which function. Promises also make it easier to handle errors, because you can use .catch() instead of having to remember to add an error-handling callback.

19. Are there any downsides to using promises?

While promises provide a cleaner and more convenient way to handle asynchronous code, they do have some potential downsides. One is that they can make code more difficult to debug, since the flow of execution is not always easy to follow. Additionally, if not used carefully, promises can lead to what is known as “callback hell,” where code can become excessively nested and difficult to read.

20. What is the main difference between Promises and Futures?

The main difference between Promises and Futures is that Futures are used for synchronous programming while Promises are used for asynchronous programming. With Futures, the result of a computation is available immediately after it is started. With Promises, the result of a computation is not available until it is complete, but the computation can continue while the result is being computed.