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Learn how to efficiently pass data between middlewares in your Express.js app using TypeScript without the need for `@ ts-ignore`. Discover step-by-step instructions and best practices.
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Passing Data Between Express.js Middlewares with TypeScript

If you're diving into the world of Express.js with TypeScript, you might encounter scenarios where you need to pass data between different middlewares. This is a common requirement, especially when you are managing authentication or user roles. In this guide, we’ll explore how to do this correctly, avoiding TypeScript errors like @ ts-ignore.

The Problem

In your Express.js app using TypeScript, you might come across code snippets such as:

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

The issue here is the line // @ ts-ignore. Although it helps in proceeding without TypeScript complaints, using @ ts-ignore is generally not recommended as it bypasses TypeScript's type safety.

The Solution

To pass data correctly to the next middleware without using @ ts-ignore, you can enhance the type definition of the Express request object. Here's how to do it:

Step 1: Create Type Definitions

You can extend the existing type definitions of Express by creating a declaration file. This allows you to define custom fields for the request type without compromising type safety.

Create a .d.ts file: In your root project folder, create a file named express.d.ts.

Add your custom type definition: Open the express.d.ts file and add the following code:

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

Explanation of the Code

Declare Namespace: The declare namespace Express construct lets TypeScript know you are working within the Express namespace.

Export Interface: By exporting an interface, you are extending the existing Request interface to include your custom property userRole.

Optional Property: The ? indicates that userRole is optional, meaning it might not always be present on the request.

Step 2: Use the Custom Property in Your Middleware

After updating your type definitions, you can now remove the @ ts-ignore directive from your middleware. Your verifyAccessToken function will look like this:

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

Benefits of This Approach

Type Safety: By defining the userRole property, you ensure that any interaction with it is type-checked by TypeScript, which prevents runtime errors.

Readability: Your middleware is cleaner and easier to understand without @ ts-ignore comments.

Conclusion

Passing data between middleware functions in an Express.js application using TypeScript can be accomplished smoothly with proper type definitions. By extending the existing Express request type, you ensure type safety while maintaining a clean and error-free codebase.

Now you can handle your Express.js middlewares effectively without relying on hacks like @ ts-ignore, making your codebase more robust and maintainable.

Happy coding!