Resolving Issues with Multiple File Reads in JavaScript: Understanding Promise.all

Описание: Learn how to properly handle multiple file reading operations in JavaScript using Promise.all, ensuring your async calls run smoothly without resolution issues.
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This video is based on the question https://stackoverflow.com/q/63857597/ asked by the user 'DevLoverUmar' ( https://stackoverflow.com/u/7344164/ ) and on the answer https://stackoverflow.com/a/63861122/ provided by the user 'Bergi' ( https://stackoverflow.com/u/1048572/ ) at 'Stack Overflow' website. Thanks to these great users and Stackexchange community for their contributions.

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Resolving Issues with Multiple Files Reads in JavaScript

As developers, we often face challenges with asynchronous operations, especially when dealing with file management. Your task may involve reading multiple files and processing their data simultaneously. This guide addresses a specific scenario related to asynchronous file reads that may leave you scratching your head: why is Promise.all neither resolving nor rejecting when reading multiple files?

In this article, we will break down a prevalent issue and provide a structured solution to ensure all file reading tasks operate as expected using JavaScript. Let's dive into it!

Problem Overview

You are trying to read data from an Excel file and then, based on its contents, read face image files associated with each user. Here's how the planned workflow should function:

Read User Data: Load user information from an Excel sheet and create an array of user objects.

Read User Faces Simultaneously: For each face image corresponding to a user, read the file and store the Base64 encoded string in the corresponding user object.

Expect Successful Resolution: Once all face images are read without errors, the final array of users with their face images should be returned.

However, you encounter a frustrating issue: the Promise.all function does not seem to resolve, leaving you with unprocessed promises and resulting data that is incomplete or incorrect.

Understanding the Solution

The problem arises due to improper promise handling in image reading and the conditional logic that prevents promises from resolving as intended. Let's improve your code by creating a robust solution step-by-step.

Step 1: Simplify File Reading

Instead of creating multiple default promises within your file reading functions, consider using a reusable function that handles file reading. This function will encompass all aspects of file reading, including error handling.

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Step 2: Updating User Data Loading

Now, refactor your loadUsersData function to utilize the new readFile function. This streamlines the code by removing the unnecessary new Promise construction.

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Step 3: Simplifying Image File Reading

Next, modify your readFace function to process the face image files using the same readFile function. This eliminates the conditional promise resolution that was causing your issues.

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Step 4: Finalizing the Load Users Function

Finally, when invoking these functions to load users, make sure that you're properly utilizing Promise.all to await the results from your face reading operations. This ensures that all operations are completed before proceeding.

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Conclusion

By refactoring your code to include a dedicated file reading utility function and restructuring your promises, you can successfully perform asynchronous file reading operations in JavaScript. Focus on proper error handling and avoid conditional promise resolutions to maintain the integrity of your operations. This approach will transform your handling of file reading into an efficient and effective process, allowing you to achieve your desired outcome seamlessly.

Remember, effective handling of promises is essential in creating responsive applications in JavaScript. Happy coding!