Typescript & Mongoose - "this" not available in instance methods

Question

I am currently converting my API from JS to TS. However, I'm having some difficulties with mongoose & typescript. Specifically, this is not available inside my instance methods.

My code:

AccountSchema.methods.comparePassword = async function (candidatePassword: string) {
  const isMatch = await bcrypt.compare(candidatePassword, this.password);
  return isMatch;
};

Which generates the following error as this refers to the function rather than the actual document:

Argument of type 'Function' is not assignable to parameter of type 'string'.

However, according to Mongoose's documentation, there shouldn't be an error as this should refer to the actual document.

What this actually refers to:

this: {
    [name: string]: Function;
}

How I defined my schema:

const AccountSchema = new mongoose.Schema<IAccount>({...})

My approach worked just fine with JS, so I know it has something to do with TypeScript. Mongoose documentation confirms that my implementation is the right way to define instance methods, so I'm quite confused why it doesn't work.

Is there a specific TS way of declaring & using instance methods in mongoose that I don't know of?

Any help would be greatly appreciated!

Answer 1

All I needed was a this parameter on that function. That's special in TS, and specifies the type of this within the function.

Like so:

async function (this: IAccount, candidatePassword: string) { ... }

this parameters are not passed explicitly, so the new parameter doesn't change how the function is called.