How do I type hint the user argument when calling passport.serializeUser in TypeScript?

user_4152785 0
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Using TypeScript, I'm writing an API using Express and Mongoose, and I'm trying to set up authentication using Passport, but I'm getting a "Property does not exist" error when trying to pass a serializer function to passport.

According to the documentation, I need to call passport.serializeUser with a function that takes my user object and calls done with that user's ID.

I'm getting the type definitions from @types/passport, and the declaration file types the user argument for the serializeUser function as Express.User:

serializeUser<TID>(fn: (user: Express.User, done: (err: any, id?: TID) => void) => void): void;

But Express.User - also defined by @types/passport - is an empty interface:

declare global {
    namespace Express {
        // tslint:disable-next-line:no-empty-interface
        interface AuthInfo {}
        // tslint:disable-next-line:no-empty-interface
        interface User {}

Here's my serializer. TypeScript complains that Property '_id' does not exist on type 'User'.

passport.serializeUser((user, done) => {
  done(null, user._id);
});

To work around this, I've added my type definition that add the _id property to Express.User:

declare namespace Express {
  interface User {
    _id?: string;
  }
}

This eliminates the TypeScript error, but it doesn't seem like this is the correct way to solve the problem. Is there a different way to provide a serializer function that doesn't require me to override the Express.User type definition?

passport.js typescript express node.js
1 Answers

The way Declaration Merging you've done is recommended by @types/express contributors. You can find how they test serializerUser with TypeScript here.

declare global {
  namespace Express {
    interface User {
      username: string;
      _id?: number;
    }
  }
}

Another way is to type User argument of callback function each time you use the function:

type User = {
  _id?: number
}

passport.serializeUser((user: User, done) => {
  done(null, user._id);
});

I think Declaration Merging is a bit smarter way.

user_55429520
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