Скачать или смотреть Resolving TypeScript Type Errors: A Guide to Promise void | Object vs Promise Object

Learn how to handle TypeScript type errors effectively, particularly the `Promise void | Object ` and `Promise Object ` mismatch in asynchronous operations.
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This video is based on the question https://stackoverflow.com/q/65110715/ asked by the user 'omastik' ( https://stackoverflow.com/u/4682574/ ) and on the answer https://stackoverflow.com/a/65111017/ 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 TypeScript Type Errors in Promises

As developers, we often face challenges when working with asynchronous code in TypeScript. One common type error that arises is related to the mismatch between Promise<void | GetDocumentResult> and Promise<GetDocumentResult>. This issue can lead to frustration and confusion, especially when you expect your promise to return a specific type but end up with a union type that includes void.

In this guide, we'll break down this problem and provide a clear solution to help you get back on track with your TypeScript code.

The Problem: Type Mismatch in Promises

Here's the scenario: You’re implementing a service to cache documents in base64 format. The idea is to fetch documents from sessionStorage, and if they are not available, retrieve them from a service (IRequestService) and store them. However, when trying to return the document, you encounter a type error:

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

Code Snippet Causing the Issue

To better understand the problem, let's take a look at the original code snippet you provided:

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

The issue arises in the last line where you call then. Since the callback doesn't return anything (void), TypeScript infers the type as Promise<void | GetDocumentResult>, which conflicts with your expected return type Promise<GetDocumentResult>.

The Solution: Ensure the Callback Returns a Value

To resolve this type error, you need to ensure that the callback in the then method returns a value. Here's the revised version of your method:

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

Key Changes Made

Return Value from then Callback: By adding return value; at the end of the then callback, we explicitly indicate that the promise should resolve with GetDocumentResult.

Optimizing Cache Handling: I recommend extracting cache-related logic (like setDocument) into a separate service, keeping the document map in memory. This will enhance performance by minimizing JSON parsing each time getDocument is called.

Conclusion

Type errors in TypeScript can be troublesome, especially when dealing with promises and their resulting types. Understanding how to manage types ensures that your asynchronous operations run smoothly. By making sure that your promise callbacks return the expected values, you can prevent common pitfalls and maintain clean, clear code.

Next time you encounter a Promise<void | Object> type error, you'll know exactly how to tackle it. Happy coding!