[typescript] Add the possibility to extend `NextApiRequest` with custom fields

Feature request

It’s often the case, when we add a middleware for example, that we need to add extra informations in the req object.

const handler = async (req: NextApiRequest, res: NextApiResponse) => {
  req.session.set('user', user)
}

This emit a TS error:

Property 'session' does not exist on type 'NextApiRequest'.

I would like a way to extend the type NextApiRequest with session here for example.

Express tackles this problem with declaration merging: see here.

As a temporary workaround, I created a custom type, but that’s not ideal because it forces me to import this type instead of the “builtin” type next provides.

export type NextApiRequestWithSession = NextApiRequest & {
  session: Session
}

[Kerumen]

Nice! Thanks 😃

Looking forward for #12186

[balazsorban44]

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