Скачать или смотреть Why Can't I Receive the Body from a POST Request in JavaScript While It Works in Postman?

Discover how to successfully handle POST request responses in JavaScript using the fetch API, even when responses appear to work in Postman but not in your code.
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This video is based on the question https://stackoverflow.com/q/67865134/ asked by the user 'Furkan Toprak' ( https://stackoverflow.com/u/8196438/ ) and on the answer https://stackoverflow.com/a/67865193/ provided by the user 'Ethan Snow' ( https://stackoverflow.com/u/4982094/ ) at 'Stack Overflow' website. Thanks to these great users and Stackexchange community for their contributions.

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Why Can't I Receive the Body from a POST Request in JavaScript While It Works in Postman?

If you've been developing with JavaScript and Flask, you might have encountered a frustrating situation: your POST requests work perfectly in Postman but fail to retrieve the body when executed in your JavaScript code using the fetch() API. This can leave you scratching your head, especially when everything seems to be in order. Let’s dig deeper into this problem and find a clear solution.

Understanding the Issue

When you send a POST request from your frontend JavaScript application to your backend Flask server using the fetch() API, you might notice that the response body appears to be undefined. In contrast, Postman allows you to view the response body without any issues. This can create confusion and can be attributed to how responses are processed differently in these tools.

What did you do wrong?

Your initial JavaScript code can be improved to ensure you are properly handling the response from the fetch() call. When you use fetch(), it does not automatically parse the body of the response. You typically need to specify how to handle this, especially if your server responds with JSON data.

Here's what your problematic code might look like:

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

In this code, while you are successfully sending the POST request, you are not utilizing the proper methods to handle the response body.

The Solution: Parsing the Response

To effectively retrieve the response body from your POST request, follow these steps to adjust your code:

1. Modify the Fetch Request

You need to chain a .then() after your fetch call to handle the response correctly. Here's the revised code:

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

2. Explanation of the Code

Response Handling: By calling response.json(), you tell JavaScript to parse the response body as JSON. If the response is actually JSON formatted, this will convert it into a JavaScript object or array, allowing you to work with it directly.

Error Handling: Although not shown in the code above, it's a good idea to handle possible errors in your fetch request. You can add a .catch() block to catch network errors or handle situations where the server returns an error status.

Additional Considerations

If your server might return a different type of content (like text), consider using response.text() instead.

Ensure that your payload (newUserBasicString) is correctly formatted as a JSON string, especially if your server expects JSON input.

Conclusion

In summary, the issue of receiving an undefined body when making a POST request using JavaScript’s fetch() API can be resolved by explicitly parsing the response. By properly handling the response with .then(response => response.json()), you can ensure that your code retrieves the data you need, similar to what you see in Postman. Always remember to keep error handling in mind to make your application robust and user-friendly.

If you're facing further issues or similar problems, don’t hesitate to reach out for more assistance! Happy coding!