Скачать или смотреть Resolving Promise void Issues When Returning Data from then() in TypeScript

Learn how to fix `Promise void ` return types while working with async functions in TypeScript, ensuring that your API data is properly retrieved and returned.
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Resolving Promise<void> Issues When Returning Data from then() in TypeScript

When working with TypeScript, especially in conjunction with APIs, you may come across frustrating issues related to Promises and data retrieval. One such problem is when you find that returning data within a .then() block can yield a Promise<void> type instead of the expected Promise<data>. In this guide, we will explore why this happens and how to properly return your data from asynchronous functions in TypeScript.

The Problem: Unexpected Promise<void>

Consider the following scenario: you've written an asynchronous function to fetch data from an API, and inside this function, you're using Axios to perform the request. However, when you retrieve the data in another function, you find it shows up as Promise<void> instead of the defined data type. This can be extremely confusing when your intent was to return structured data.

The core of the issue lies in how you're handling the return values from your Promise. Let's break down your existing code to identify the problematic areas.

Examining the Existing Code

Here's a look at your function getSymbolStats, which fetches data and processes it:

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

Why Are You Getting Promise<void>?

When using .then() inside getSymbolStats, the data you return from the then block doesn't actually get returned from the parent function (getSymbolStats). That's why, when you call this function from another, it resolves to a Promise<void>. The data is effectively lost because it wasn't propagated back out of the asynchronous function.

The Solution: Use await or Explicitly Return the Promise

Option 1: Using await for Simplified Code

One of the best ways to simplify your code and properly return your data is to utilize the await keyword. This allows you to handle asynchronous operations in a more straightforward manner. Here’s how you can rewrite getSymbolStats:

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

Option 2: Explicitly Returning from .then()

If you prefer to stick with .then(), you must make sure to return the Promise properly. Modify your function as follows:

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

Conclusion

In summary, the confusion regarding Promise<void> often stems from not properly returning values in asynchronous functions. By either using await or ensuring your Promise chains are correctly returning values, you can avoid these pitfalls. Next time you find your TypeScript implementation not working as expected, remember this advice to ensure you're returning the right data types from your asynchronous functions.

By applying these best practices in your TypeScript code, you'll create cleaner and more maintainable code while preventing frustrating issues down the road.