Скачать или смотреть How to Fix undefined Results in Your Fetch POST Request in JavaScript

Discover why your fetch POST request returns `undefined` and learn how to correctly handle JSON responses in JavaScript.
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This video is based on the question https://stackoverflow.com/q/71289194/ asked by the user 'flacdontbetter' ( https://stackoverflow.com/u/18327315/ ) and on the answer https://stackoverflow.com/a/71289206/ provided by the user 'Jorge Santos' ( https://stackoverflow.com/u/11962428/ ) at 'Stack Overflow' website. Thanks to these great users and Stackexchange community for their contributions.

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Understanding the Issue: Why is the Result Undefined?

It's a common scenario that many developers face: you've successfully sent a POST request using the fetch API, and the response contains the expected JSON data. However, when you attempt to log the result, all you see is undefined. What could be causing this? Let’s break it down and find a solution.

The Problem Explained

Consider the following code snippet from your POST request:

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

In this code, you're using the fetch API to make a network request. The request should ideally return a JSON object, but when you try to log the result of that request, you get undefined. The cause of this lies in the first .then() function where you're processing the response.

The Solution: Return the JSON Object

To resolve this issue, it's essential to return the result of response.json() in the first .then() statement. The response.json() method returns a promise that needs to be resolved. If you don’t return it, the second .then() receives nothing (undefined), which is why you're seeing that result.

Let's Fix the Code

Here’s the corrected version of your code:

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

Key Changes Made:

Return Statement: Add a return statement to response.json() so that the promise can be resolved and passed to the next then().

Chaining Promises: Ensure that each then() properly forwards the promise chain.

Conclusion: Debugging Fetch Requests

When dealing with network requests in JavaScript, it's crucial to understand how promises work and ensure you're returning values correctly. By returning the parsed JSON from the initial then(), you ensure that the subsequent chain receives the expected data instead of undefined.

Next time you encounter a similar issue with undefined results in your fetch operations, remember this simple but effective practice!

Happy Coding!

By following these guidelines, you can handle fetch requests more effectively and avoid pitfalls like undefined results. If you have further questions or need additional clarification, feel free to explore more resources or reach out to the community!