Understanding undefined in Fulfilled Promises: A JavaScript Deep Dive

Описание: Explore why JavaScript yields `undefined` in fulfilled promises, learn how to fix it, and grasp important promise concepts with easy examples.
---
This video is based on the question https://stackoverflow.com/q/64931070/ asked by the user 'Mateusz W' ( https://stackoverflow.com/u/14153983/ ) and on the answer https://stackoverflow.com/a/64932052/ provided by the user 'Mechanic' ( https://stackoverflow.com/u/5953610/ ) at 'Stack Overflow' website. Thanks to these great users and Stackexchange community for their contributions.

Visit these links for original content and any more details, such as alternate solutions, latest updates/developments on topic, comments, revision history etc. For example, the original title of the Question was: Why there is undefined on fulfilled promise?

Also, Content (except music) licensed under CC BY-SA https://meta.stackexchange.com/help/l...
The original Question post is licensed under the 'CC BY-SA 4.0' ( https://creativecommons.org/licenses/... ) license, and the original Answer post is licensed under the 'CC BY-SA 4.0' ( https://creativecommons.org/licenses/... ) license.

If anything seems off to you, please feel free to write me at vlogize [AT] gmail [DOT] com.
---
Understanding undefined in Fulfilled Promises: A JavaScript Deep Dive

In the world of JavaScript, asynchronous programming can often lead to confusion, especially when working with promises. A common question that arises is: Why do I see undefined in the console when a promise is fulfilled?

In this guide, we’ll break down this question, explore what’s happening behind the scenes, and provide a clear solution.

The Scenario

Consider a simple promise structure in JavaScript, as illustrated in the following code:

[[See Video to Reveal this Text or Code Snippet]]

The Output Explained

You may find that the console logs 'success' and you also see something like this:

[[See Video to Reveal this Text or Code Snippet]]

This leads to the question: Why is there undefined despite the promise returning a valid string?

The Core of the Issue

Implicit Returns in JavaScript

When a function does not explicitly return a value, JavaScript automatically appends an undefined return value to it. This happens with the callback function in the then() method of a promise if it does not return anything explicitly.

Here’s the breakdown:

Your promise is resolved perfectly with 'success'.

The then() method runs the provided callback function, which logs the result but does not return anything.

Hence, the function's implicit return value is undefined, leading to the displayed promise output: Promise {<fulfilled>: undefined}.

How to Fix It

To have a more useful output from your promise, you should modify your promise chain to include explicit returns in the then() method. Here’s how you can do that:

[[See Video to Reveal this Text or Code Snippet]]

What Changed?

Explicit Return: By adding return result; and return error;, we ensure that the then() method returns meaningful values instead of undefined.

Improved Logging: Now, in the console, you can also see both the log messages and the results of the promise returns.

Conclusion

Understanding why undefined appears in fulfilled promises helps us write better, cleaner asynchronous code in JavaScript. By ensuring we provide explicit return values in promise callbacks, we can avoid confusion and communicate clearly through our code.

As you continue to work with promises, remember the importance of the return statement. It can make a significant difference in how your code behaves and is understood.

Happy coding!