Javascript Promise

Michael Zippo 04.11.2021

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asynchronous code was a problem writing in JavaScript. To write asynchronous operations in your code, you must be dealing with multiple levels of callbacks. The more features you introduce into your code, the harder it is to read.

In SS6, promises have come to the rescue. Promises are one way to efficiently write asynchronous code in JavaScript.

In this guide we are going to talk about what promises are and how they work. We’ll see an example that promises to help you learn how to use them in your code. Let’s get started!

What is a promise?

A promise is an object that returns a response that you want to receive in the future.

A A good way to think of JavaScript promises is to compare them to the way people make promises. When you make a promise, it is a guarantee that you will do something at a later date. You won’t do this thing now; you will do this at a later time

A promise can exist in any of the three states.

Pending: A promise has not been. still completed
rejected. the promise does not have to return a value
Fulfilled. a promise has been fulfilled

This is similar to promises in real life. You can have an exceptional promise which you say will keep in the future. You can keep a promise. You can refuse or "break‚" a promise and not keep what you agreed to do.

When you make a promise, it will be pending. The promise will exist in this state until a response is received and the promise is fulfilled or rejected

If our promise is rejected, the contents of the catch () declaration will be executed

Let’s see what will happen when we run our code:

cinnamon

cinnamon value returned because our promise was successfully resolved. If the value of "name" was not "Cinnamon" in our promise, an error would be returned:

Not Detected (in promise) This promise does not have

<. p> We specified the error message: "This promise failed. " In our require () function in our promise.

finally () with promises

What if you want to execute a block of code after a promise has been executed, regardless of whether the promise succeeded or failed

> < / div>

is where the last () statement comes in handy>. The last () statement is executed regardless of whether the promise is honored or rejected . Let last day call our promise finally use a statement:

Our code returns:.

Cinnamon

The promise returnName has been executed

our promise statements cinnamon because it was successful. The contents of the finally () statement are then executed, which returns a message to our console.

Conclusion

promises JavaScript allow you to write own code asynchronously .

In this guide, we’ve covered the basics of promises. There are a lot more promises that we’ve covered in this guide. The next step in your learning journey is to write some promises in your code.

Here’s a challenge:. Write a promise that makes a web request and returns its response. To do this, you need to use the fetch () API to make a request. This request must be enclosed in a promise. You need to write code that handles the promise both if it succeeds and if it fails.

If you want to dig deeper, check out the JavaScript async works. These can be used to write asynchronous code and are commonly used with promises.

You are now ready to start writing JavaScript promises like an expert!

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We hope this article has helped you to resolve the problem. Apart from Javascript Promise, check other __future__ Python module-related topics.

Want to excel in Python? See our review of the best Python online courses 2023. If you are interested in Data Science, check also how to learn programming in R.

By the way, this material is also available in other languages:

Cornwall Jackson

Paris | 2023-02-01

Simply put and clear. Thank you for sharing. Javascript Promise and other issues with time Python module was always my weak point 😁. Will use it in my bachelor thesis

Chen Wu

Moscow | 2023-02-01

Simply put and clear. Thank you for sharing. Javascript Promise and other issues with code Python module was always my weak point 😁. I am just not quite sure it is the best method

Manuel Schteiner

Massachussetts | 2023-02-01

Thanks for explaining! I was stuck with Javascript Promise for some hours, finally got it done 🤗. I just hope that will not emerge anymore