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Discover how to effectively handle an array of Promises mapped from Object entries when using JavaScript's async features.
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Solving the Problem of Awaiting an Array of Promises from Object Entries in JavaScript

Introduction

JavaScript has become a staple in web development, especially with the rise of modern libraries and frameworks like Playwright. However, developers often encounter challenges when working with asynchronous operations, particularly with Promises. One such problem arises when trying to await an array of Promises constructed from Object entries. In this post, we will explore how to solve this issue and ensure that your async code works as intended.

The Problem Explained

Many developers working with Playwright may face an issue where their Promise-based operations do not execute as expected. Specifically, in a scenario where promises are created from an Object's entries within an inner loop, you might find that an await Promise.all() does not handle the nested promises correctly.

For instance, consider a piece of code that attempts to evaluate table data from a web page:

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In the code above, you may notice that the outer await Promise.all does not seem to handle the inner promises correctly. So, what is going wrong here?

Understanding the Issue

The main issue arises because when you use map() to create promises, you end up with an array of arrays. Each invocation of map() creates a new array, and thus the outer Promise.all() is not awaiting the inner promises as you would expect.

The Solution: Using flatMap()

If you're targeting ES2019 or later, the solution is straightforward: you can replace your map() calls with flatMap(). This effectively flattens the structure, allowing you to aggregate all the promises into a single-level array, which Promise.all() can properly await.

Here's the modified code snippet using flatMap():

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Key Changes:

We replaced map() with flatMap(), which flattens the resultant array into a single array of promises.

Each promise in the flat array is now properly awaited by the outer Promise.all().

Conclusion

By utilizing flatMap(), you can simplify the handling of promises when working with Object entries in JavaScript. This approach prevents the potential pitfalls of nested arrays of promises and guarantees that your async code behaves as expected.

If you're working with modern JavaScript environments, this small adjustment can save you countless headaches and ensure that your web applications behave reliably.

Feel free to apply this solution to your own code and experience the benefits of clean and efficient async operations!