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Extend Express's Request Object with Typescript Declaration Merging.

kwabenberko profile image Kwabena Bio Berko ・Updated on ・2 min read

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. πŸ˜‹

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.

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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. πŸ˜ƒ

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",
    ]                    
...
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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
        }
    }
}

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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.

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Happy Coding, everyone!

Discussion (27)

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stevemu profile image
Steve Mu

Thank you so much!!!

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kwabenberko profile image
Kwabena Bio Berko Author

You're welcome. 😊

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tjsarkka profile image
TJSarkka

Thank you! I spent the last hours scratching my head and trying different combinations of config files and .d.ts files provided in other solutions.. and this was the only one that worked for me for both ts-node and tsc. :)

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kwabenberko profile image
Kwabena Bio Berko Author

You're welcome.
Glad to be of help.

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mxgit1090 profile image
Mark Tsai • Edited

Similar methods to define Request types can also be found in the following libraries:

I also spent so much time resolving this issue, thank you for your article!

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kwabenberko profile image
Kwabena Bio Berko Author

You're welcome
Really glad you found it useful.

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aashishclikfin profile image
aashishClikFin • Edited

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.

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damjuve profile image
damjuve

Hello,

Thanks for this very usefull tips.
I still have a question about exporting this.
I would like to extend express Request with UserModel, exactly like you did. But I am doing it in a homemade node module.
Then I install this node module in a node project and I don't have access to the extended request. How can i do this ?

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alphakennybudy profile image
Adil • Edited

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?

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georgenorris profile image
George C. Norris

Getting a Augmentations for the global scope can only be directly nested in external modules or ambient module declarations. ??

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vkyrychenko profile image
Vitalii

Nice article! You saved my day πŸ˜ŽπŸ‘

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 πŸ€·β€β™‚οΈ.

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asgamboa12 profile image
Asgamboa12

Thank you very much, this problem was killing me and I could not find a way to solve it until I saw your post, thank you very much !!!

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sellonen profile image
Sakari Ellonen

I created an account here just to say thank you, this was such a nice solution that a typescript newbie like me would not have come up on my own.

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kwabenberko profile image
Kwabena Bio Berko Author

I'm really glad you found it useful.

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hanslissi profile image
riejoc16

Oh wow! I was stuck for a while now trying to achieve exactly that. I couldn't find what I had done wrong. Only with this post I was able to get it done, thanks!

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kwabenberko profile image
Kwabena Bio Berko Author

You're welcome. πŸ˜„

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renemaldonado profile image
Rene Maldonado Fimbres

Bro, have been having problems with this for 2 days.

Thanks for the info, I was missing the @ sign .

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kwabenberko profile image
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georgenorris profile image
George C. Norris

I've tried SOO many variations to make this work, and NOTHING seems to work. πŸ€·πŸ»β€β™‚οΈ

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andersonluis87 profile image
Anderson Luis Silva

Thank you very much!
Very well explained.

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kwabenberko profile image
Kwabena Bio Berko Author

Thank you. 😊

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terabytes profile image
terabytes

VS code doesn't show error in editor but during compilation i get:
currentUser does not exist on type 'Request'.
Its Ts error.

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mehmetnyarar profile image
Mehmet N Yarar

same here. not only that, testing (jest/ts-jest) also fails unless I do:

declare module 'express' {
  export interface Request {
    i18n: import('i18next').i18n
    language: string
  }
}
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pendar747 profile image
Pendar

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

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jonn_joo profile image
Ioannis • Edited

Could you add a GitHub link please. It does not work for me as well.

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jpolstre profile image
jpolstre • Edited

Thanks, in my case work whitout: declare global {

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kira56 profile image
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.