How to Ensure Promise.all Waits for All Fetch Requests to Complete in JavaScript

A complete guide on handling nested promises in JavaScript, ensuring all requests are completed before proceeding. Discover how to implement an effective loading state.
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This video is based on the question https://stackoverflow.com/q/67319355/ asked by the user 'Filipe Costa' ( https://stackoverflow.com/u/15634392/ ) and on the answer https://stackoverflow.com/a/67319486/ provided by the user 'David784' ( https://stackoverflow.com/u/1233305/ ) at 'Stack Overflow' website. Thanks to these great users and Stackexchange community for their contributions.

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Understanding the Problem: Why Isn't Promise.all Waiting?

If you’re working with asynchronous operations in JavaScript, particularly with nested promises, you may encounter situations where it seems that Promise.all isn’t waiting for all fetch requests to complete. This can be quite frustrating, especially when setting up a loading state is critical for UX. In this post, we will explore a common issue related to promises in JavaScript and how to effectively solve it using Promise.all.

The Scenario

Imagine you have two arrays: one representing different countries and another representing different markets. You want to fetch data related to each combination of country and market. The initial attempt to handle this nested structure led to confusion about how to correctly use Promise.all to wait for all requests to finish. Here’s a simplified version of the code that illustrates the issue:

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

In this setup, you may feel like your loading state isn’t functioning as intended because promises are wrapped inside another asynchronous mapping.

The Solution: Correctly Nesting Your Promises

To ensure that you are waiting for all fetch requests to complete—not just within the market for each country but also across the countries—you need to return the outer promises in a way that allows you to use Promise.all effectively. Here’s how to do it correctly:

Restructuring the Code

Return the results from the countries map:

You need to modify the existing code to return the inner promises for each country.

Use Promise.all on the outer promises:

This enables you to know when all requests for all countries have finished.

Here’s the updated code:

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

Breakdown of the Updated Structure

Creating the Promise Arrays:

const prCountries = countries.map(...) creates an array of promises that represents all combinations of countries and markets.

Fetching Data:

Inside the market map, we are still using fetch, which returns a promise for each request.

Combining Results:

Promise.all(prCountries) ensures that you only continue your logic (like stopping a loading state) once all fetch calls have completed.

Conclusion: Mastering Asynchronous Patterns in JavaScript

Handling multiple asynchronous requests can be challenging, especially when they are nested. By understanding and restructuring your use of Promise.all, you can effectively manage loading states in your application. This pattern is not only useful for fetching data but can also be applied to various scenarios in your JavaScript programming journey. Mastering these patterns will lead to cleaner, more efficient code and a better user experience.

If you found this post helpful, feel free to explore more about asynchronous programming in JavaScript to enhance your skills!

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