Extend Express's Request Object with Typescript Declaration Merging.

Express has been my go to server side node web framework for the past few years. Its fast, unopinionated and so easy to get up and running. I really love using it along with Typescript too, I should say. It enhances code quality and understand-ability. Refactoring your code in Typescript is also much more easier and faster. Plus, you get the added advantage of code completion and IntelliSense when using modern text editors like Visual Studio Code. 😋

One of the concepts of Typescript which I've recently began using is Declaration Merging.
Declaration Merging allows you to merge two or more distinct declaration or types declared with the same name into a single definition. This concept allows you to attach your own custom property onto another Typescript interface type. Lets take a look at a typical Express middleware.

The above code is an Express middleware that is used to ensure that a user is authenticated when he or she tries to access a protected resource. It decodes the user's token from the authorization property of the request headers and attaches the user to the Request object. But see that red squiggly line?
Thats because the property currentUser does not exist on Express's Request interface type. Let's fix that. 😃

The first thing we need to do is to create a new declaration file @types > express > index.d.ts in the root of our project.

You would notice this is the exact same file name and path in our node_modules/@types folder. For Typescript declaration merging to work, the file name and its path must match the original declaration file and path.

Next we need to make some few changes in the project's tsconfig.json file. Let's update the typeRoots value to the following:

...
"typeRoots": [
      "@types",
      "./node_modules/@types",
    ]                    
...

By default, the Typescript compiler looks for type definitions in the node_modules/@types folder. The above code instructs the compiler to look for type definitions in this folder as well as our custom @types folder in our project root.

It's now time to add our custom currentUser property to Express's Request interface type by modifying the index.d.ts file we created earlier:

import { UserModel } from "../../src/user/user.model";

declare global{
    namespace Express {
        interface Request {
            currentUser: UserModel
        }
    }
}

Lets take a look again at our middleware file and we immediately notice that the red squiggly line is gone! This is because the Typescript compiler now recognizes the currentUser property as a valid property on the Request type interface.

Happy Coding, everyone!

Comments

[aashishClikFin]

First of all thanks a lot for your post, I finally resolved the problem I faced since the last 3 months.

Just would like to add one more bit, since I was using passport alongside passport-jwt hence your solution didn't exactly work for me.

Here is what I used in the index.d.ts file

declare module 'express-serve-static-core' {
export interface Request {
user?: yourCustomType;
}
}

It might help someone else as well, but thanks a lot for guiding me in the right direction.

[Jinendra Jain]

Thanks Bro

[playground]

Great article, thanks for sharing. Good to know there is another way of doing things.
I have been doing it the inverse by extending Request

For example:
export interface IRequest extends Request {
type: string;
sensorId: string;
timestamp: ITime;
}

in the server.ts
...
app.get("/test", (req: IRequest, res: IResponse) => {
...
})

[Adil]

Thank you, it helped. But as far as I understood, it adds currentUser field to all Requests? What if I want it to be present only in the current file and not declared in the other files where I use Request?

[Luca S. Martins]

You could declare currentUser as optional and use it when needed

[Gabriel]

NOTE that you have to add typeRoots inside compilerOptions in the tsconfig.json file.

[Vitalii]

Nice article! You saved my day 😎👍

I see a lot of libraries using this approach.

But It looks a bit strange for me. Just imagine that you have a lot of middleware that attaches something to the request object. In the end, you have a bloated interface and you lose the power of TypeScript because you don't know which middlewares were executed before handler/controller. So you don't know if the required data was attached and need to double-check if it's there 🤷‍♂️.

[Peter Kitonga]

If anyone is still having errors with this, don't forget to import Express at the top of your file.

import { Express } from 'express'

For some reason, I couldn't get it to work without the import

[Luca S. Martins]

Thanks! Same problem here.

[Nguyễn Công Trọng]

Thanks!

[Pendar]

This was awesome! I was missing the step with having to create the same folder path structure and adding the "typeRoots"

[Qi Mu]

Thank you so much!!!

[Kwabena Bio Berko]

You're welcome. 😊

[Dan Maia]

The best article about that I've found today in the Internet.

[Diego CS]

Thanks!!!!!!!! I was looking for information, but all that I found was not useful, but your explanation goes straight to the point. Than you very much.