Скачать или смотреть How to Fix an Async Function Returning an Empty Array in JavaScript

Learn how to effectively handle promises in JavaScript and prevent your async function from returning an empty array.
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This video is based on the question https://stackoverflow.com/q/67635882/ asked by the user 'Hiperfly' ( https://stackoverflow.com/u/4353331/ ) and on the answer https://stackoverflow.com/a/67636439/ 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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Troubleshooting: Async Function Returning an Empty Array

If you're working with JavaScript, you've probably encountered some frustration with promises and asynchronous functions at some point. One common issue that many developers face is when an async function unexpectedly returns an empty array. This can be especially bewildering when you're expecting it to hold valuable data. Today, we'll dive into this problem and explore how to address it effectively.

The Problem: What’s Happening?

In your scenario, you’re trying to create a matrix from pairs of data using an async function called fetchData(). Here’s a breakdown of the problem:

The function attempts to iterate over an array (info_array) and create rows using pairs generated by calling fetchDataPair(i, j).

However, the function returns matrix before the asynchronous calls complete, resulting in an empty array being logged and returned.

Here's a simplified snippet of your original function for context:

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

The Main Issue

The core issue lies in how you handle the promises created by fetchDataPair(i, j). You’re pushing unresolved promises to row and then trying to handle their resolution afterward, which doesn’t work as expected in asynchronous programming.

The Solution: Using Promise.all Correctly

To fix this issue, we need to ensure that all the promises are resolved before we attempt to return the matrix.

Step-by-Step Breakdown of the Fix

Creating an Array of Promises: Instead of trying to push the result of Promise.all(row) into matrix, you should push the unresolved promises directly into an array.

Waiting for All Promises to Resolve: Once you've created rows of promises, you’ll want to call Promise.all() on these rows as well to ensure that your matrix is populated only after all the promises have resolved.

Returning the Final Result: Finally, you'll return the resolved matrix.

Here’s a revised version of your code that implements these changes:

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

Key Points About This Solution

Promise.all: This is a method that takes an array of promises and returns a single promise that fulfills when all of the promises in the array have completed. This ensures that matrix is only resolved after all the internal promises for rows are complete.

Asynchronous Control Flow: Always remember that the function will not wait for promises to resolve unless you explicitly handle this with await or Promise.all().

Conclusion

Debugging asynchronous functions can be tricky, especially when promises are involved. By correctly handling promise resolution, you can avoid returning empty arrays and ensure that your data processing logic works as intended. The solution outlined above should help you get your function back on track and return the expected matrix. Happy coding!