How to Add Type Definitions for JavaScript Functions in TypeScript

Learn how to effectively add type definitions to JavaScript functions using TypeScript, focusing on the `react-native-ytdl` library.
---

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: Adding type definitions for javascript functions

If anything seems off to you, please feel free to write me at vlogize [AT] gmail [DOT] com.
---
How to Add Type Definitions for JavaScript Functions in TypeScript

When working with TypeScript, defining the types for your JavaScript functions can significantly enhance your code's reliability and readability. This is especially true if you are using libraries that do not come with type declarations pre-defined. In this guide, we will explore how to add type definitions for JavaScript functions, specifically for the react-native-ytdl library being used to fetch video links and details from YouTube.

Understanding the Problem

You already have a TypeScript class known as YoutubeVideo, which successfully fetches video links and basic information using the ytdl function from the react-native-ytdl library. However, to take full advantage of TypeScript’s type-checking capabilities, you need to define types for the getBasicInfo method. This guide will walk you through the steps required to create a type definition for the getBasicInfo function efficiently.

Step 1: Create a Type Declaration File

Instead of placing your type definitions inside the node_modules folder, which is generally not recommended, you can create a top-level declaration file. Here’s how to do this:

Step 2: Define the Types in the Declaration File

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

Explanation of the Code:

Module Declaration: The declare module 'react-native-ytdl' syntax allows TypeScript to recognize the module and its exports.

Type Definitions: We define a VideoFormat type and a BasicInfo type to represent the structure of video formats and basic info respectively.

Ytdl Interface: The Ytdl interface defines the function signature for ytdl and its method getBasicInfo, providing TypeScript with information about the property types.

Step 3: Using the Type Definitions

Now that you have defined your types, you can easily use the getBasicInfo method in your class. Here’s an example of how to update your YoutubeVideo class:

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

Now you can call getBasicInfo without any TypeScript errors, as it recognizes the types appropriately.

Conclusion

Adding type definitions not only helps prevent errors in your code but also clarifies the intent by documenting your JavaScript functions. With the react-native-ytdl library, you have seen how to define types for the various returned structures and methods, making your TypeScript implementation both robust and informative.

With these steps, you can confidently enhance your TypeScript projects by incorporating custom type definitions for any JavaScript functions you may encounter in various libraries. Implementing these practices will improve your code maintenance and readability over time.