How to Extend the Express Request Object Type with TypeScript • CeamKrier

In modern web development, TypeScript has become a staple for developers who want to add static type-checking to their JavaScript code. This additional layer of type safety can help catch errors early in the development process and make code more maintainable. When working with Express.js, one common scenario developers encounter is the need to extend the request object to include additional properties, such as user information added by middleware. In this blog post, we’ll explore how to extend the request object’s type using TypeScript.

Understanding the Request Object

In an Express.js application, the request object is a central piece of middleware functions. It provides access to the HTTP request and contains information such as the request method, URL, headers, and more. However, there are times when you need to augment this object with additional properties. For instance, after authenticating a user, you might want to attach the user’s ID to the request object for further processing in subsequent middleware or route handlers.

By default, TypeScript doesn’t know about these additional properties, which can lead to errors when trying to access them. To solve this, we need to extend the type definition of the request object.

Extending the Request Object’s Type

To extend the request object, we need to modify the global Express namespace. Here’s how we can achieve this:

Create a Custom Type Definitions Directory

First, create a new directory at the root of your project to hold custom type definitions. A common convention is to name this directory @types.
```
mkdir @types
```
Extend the Express Package

Within the @types directory, create a subdirectory named express. This will allow us to extend the express package’s types.
```
mkdir @types/express
```
Create an Index File for Type Definitions

Inside the express directory, create a file named index.d.ts. This file will contain the type extensions for the Express request object.
```
// @types/express/index.d.ts

import * as express from 'express'

declare global {
	namespace Express {
		interface Request {
			userId?: string
			isSubscriber?: boolean
         ...any other field you might attach to request object
		}
	}
}
```
In this code snippet, we’re extending the Request interface to include two optional properties: userId and isSubscriber. The declare global syntax is used to add these properties to the global Express namespace, making them available throughout your application.
Configure TypeScript to Use Custom Type Definitions

Finally, update your tsconfig.json file to include the @types directory in the typeRoots array. This tells TypeScript to look in this directory for additional type definitions.
```
{
	"compilerOptions": {
		// ...other options...
		"typeRoots": ["@types"]
	}
}
```
By default, TypeScript only looks in node_modules/@types for type definitions. Adding our custom @types directory ensures that TypeScript recognizes the additional type definitions we’ve created.

Why Extend the Request Object?

Extending the request object offers several benefits:

Type Safety: By extending the request object’s type, TypeScript will know about the additional properties and can provide compile-time checks. This reduces the risk of runtime errors caused by accessing undefined properties.
Improved Developer Experience: With type definitions in place, editors can offer better IntelliSense, making it easier for developers to discover and use the additional properties.
Maintainability: Clearly defining the shape of the request object helps maintain consistency across the application, especially in large codebases where multiple developers are working together.

Conclusion

Extending the request object’s type in an Express application using TypeScript is a straightforward process that enhances the robustness and maintainability of your code. By following the steps outlined in this post, you can easily add custom properties to the request object and take full advantage of TypeScript’s type-checking capabilities. As you continue to develop your application, these practices will help you catch errors early and improve the overall quality of your code.