Understanding undefined Values Returned by API Calls with Promises in JavaScript

Описание: Discover why your JavaScript API calls return `undefined` values and learn how to fix it with `async` and `await` for effective promise handling.
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This video is based on the question https://stackoverflow.com/q/75108667/ asked by the user 'CodeoDE' ( https://stackoverflow.com/u/18664246/ ) and on the answer https://stackoverflow.com/a/75108787/ provided by the user 'zanderwar' ( https://stackoverflow.com/u/2266583/ ) at 'Stack Overflow' website. Thanks to these great users and Stackexchange community for their contributions.

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Understanding undefined Values Returned by API Calls with Promises in JavaScript

Working with APIs is a crucial skill for any JavaScript developer, especially when dealing with asynchronous calls. One common issue that newcomers often face is getting undefined values when attempting to retrieve data from a promise. In this post, we will break down this problem and provide a comprehensive solution that will help you understand how to appropriately handle asynchronous functions in JavaScript.

The Problem: undefined Values in Promises

Imagine you're trying to call an API that retrieves geographical data based on certain parameters, such as longitude and latitude. After setting up the function to execute this call, you find that the result is undefined. Here’s a representative snippet from a user struggling with this issue:

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

When this promise is resolved, it returns undefined, leading to confusion and frustration. But why is this happening?

Analyzing the Code

To resolve the confusion, let’s look deeper into the potential reasons behind this issue:

Async Function Handling: In JavaScript, functions that return a promise must be handled correctly using the async and await keywords. If you do not use await, the function does not pause and allow the promise to resolve before attempting to access its result.

Improper Promise Usage: In the provided code snippet, the getClosestSchachtInSeg() function is an asynchronous function, but calling it inside a promise without await leads to undefined being returned.

The Solution: Utilize async and await

To fix this issue, you should change the way you call your asynchronous function. Here’s how to properly restructure your code:

Make the Parent Function Async: First, ensure that your function where you call the API is marked as async.

Use Await for the API Call: Replace the promise structure with await to properly handle the asynchronous call. Here’s the corrected code:

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

By implementing these changes, you allow the execution of your code to pause until the promise resolved, thus ensuring that you capture the actual value returned by your API call.

Conclusion

Understanding how to effectively manage promises in JavaScript, especially with APIs, is essential for any developer. By utilizing async and await, you can avoid returning undefined values and ensure smoother handling of asynchronous operations. This not only enhances your coding skills but also aids in building robust applications that handle data effectively.

Don’t forget: Promise management makes a significant difference in your JavaScript programming journey!