Understanding Promise Behavior with AsyncStorage in React Native

Learn how to properly handle asynchronous data retrieval in React Native using AsyncStorage. This blog explains the common issues and provides clear solutions for new developers.
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Understanding Promise Behavior with AsyncStorage in React Native

If you are new to React Native, you might encounter situations when working with async functions, particularly when trying to retrieve data using AsyncStorage. One common issue arises when a function returns a Promise instead of the expected value. In this guide, we'll explore a specific scenario that leads to the output Promise { "_U": 0, "_V": 0, "_W": null, "_X": null, } when trying to fetch a value from AsyncStorage.

The Problem

In the provided code snippet, the main focus is on retrieving the value of selectedLanguage from AsyncStorage. However, when invoking the getData() function, the console logs a Promise object instead of the actual value. This can be confusing, especially for those just starting with asynchronous programming in JavaScript.

Here’s what happens in the code:

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

This code leads to an output that looks like this:

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

To understand this output, we need to delve deeper into how asynchronous functions work in JavaScript.

Why Are You Seeing a Promise?

The getData() function is marked as async, which means it will always return a Promise. When you call getData() without awaiting its completion, you're simply getting the Promise object itself, not the resolved value.

What’s Going On?

Async Function Return Value: When an async function completes, it returns a Promise. If you do not wait for that promise to resolve, you will see the Promise object itself in the console, as in your case.

Console Output: The console logs the promise state (like _U, _V, etc.) instead of the value you expect because the promise has not yet been resolved at the time of logging.

The Solution

To resolve the promise and get the actual value from AsyncStorage, you need to utilize .then() to handle the asynchronous result or employ the await keyword if you’re in an async context. Here’s how you can do both:

Using .then()

Modify your call to getData() as follows:

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

Using await

Alternatively, you can also await the result if you are within an async function:

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

Complete Example

Here’s how you can integrate the solution:

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

Conclusion

Understanding the behavior of Promise objects in JavaScript is essential for working effectively with async functions, especially in frameworks like React Native. By using either the .then() method or the await keyword, you can avoid confusion and retrieve the values you need from AsyncStorage seamlessly.

If you find yourself struggling with similar asynchronous patterns, remember that patience and practice are key as you navigate the intricacies of JavaScript. Happy coding!