app.ts

import { Session } from 'express'
let theSession: Session
theSession.user.login

index.d.ts

import { User } from "./user";
declare module 'express' {
  interface Session {
    user: User;
  }
}

user.ts

export class User {
  public login: string;
  public hashedPassword: string;

  constructor(login?: string, password?: string) {
    this.login = login || "";
  }
}