Скачать или смотреть Accessing Generic Function Types in TypeScript

Explore how to access the type of generic functions in TypeScript, even without directly exporting them. Learn effective methods using `Parameters` and custom generics.
---
This video is based on the question https://stackoverflow.com/q/74058228/ asked by the user 'Ilja' ( https://stackoverflow.com/u/911930/ ) and on the answer https://stackoverflow.com/a/74058635/ provided by the user 'Darryl Noakes' ( https://stackoverflow.com/u/15261914/ ) at 'Stack Overflow' website. Thanks to these great users and Stackexchange community for their contributions.

Visit these links for original content and any more details, such as alternate solutions, latest updates/developments on topic, comments, revision history etc. For example, the original title of the Question was: How to access type of functions generics?

Also, Content (except music) licensed under CC BY-SA https://meta.stackexchange.com/help/l...
The original Question post is licensed under the 'CC BY-SA 4.0' ( https://creativecommons.org/licenses/... ) license, and the original Answer post is licensed under the 'CC BY-SA 4.0' ( https://creativecommons.org/licenses/... ) license.

If anything seems off to you, please feel free to write me at vlogize [AT] gmail [DOT] com.
---
Accessing Generic Function Types in TypeScript: A Comprehensive Guide

When working with TypeScript, you may find yourself needing to access generic types from functions, particularly when those types aren't exported. This can be a daunting task if you're not familiar with the techniques available to work with generics. In this post, we'll explore a common challenge when dealing with function generics and provide you with solutions to tackle this issue effectively.

The Problem: Accessing Function Generics

Imagine you have a library that exposes functions with generic types, such as:

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

You want to access and store the type of Data (which extends BaseData) in your own type MyType, but the BaseData itself is not exported. This makes it challenging to utilize those types directly in your code.

Understanding the Limitation

The primary limitation here is that if the generic type Data is not explicitly used in a way that you can infer it, there's no direct way to access that type. If you merely call getData<Data extends BaseData>(), you won't be able to extract BaseData directly.

Example Setup

Here's a sample declaration to illustrate the issue:

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

To address this, you have a couple of options.

Solutions for Accessing Generics

Option 1: Using Parameters

You can leverage TypeScript's Parameters utility type. This approach allows you to extract the type of specific parameters from the generic function. Here’s how you can do this:

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

This method is straightforward and effective when you want to access specific parameter types.

Option 2: Creating a Custom Generic

If the types are wrapped in additional structures or you need finer control, you can create a custom generic utility. Here's how you can do this:

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

This approach can be particularly useful if your parameter has a more complex structure:

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

Handling Return Types

If the generic type is found in the return type of a function, you can apply similar as demonstrated with the parameters:

Define the function.

Utilize a utility type to extract the return type.

While this specific post focused on accessing parameter types, the same principles can be applied to return types as well.

Conclusion

Accessing generic types in TypeScript can initially seem complex, especially when dealing with non-exported base types. However, with the techniques we've covered—using Parameters and custom generics—you can effectively extract these types for your own use. This not only enables better type inference in your own code but also enhances maintainability and readability.

Key Takeaway

Always remember that if a type isn't directly accessible, TypeScript offers alternative utility types and techniques that allow you to infer and utilize those types, making your code cleaner and more robust.

Now that you're equipped with the knowledge to tackle generic types in TypeScript, feel free to explore further and implement these solutions in your projects!