Collections.employees undefined for the MEAN stack tutorial

This question refers to How to Use MEAN Stack: Build a Web Application from Scratch.

I have followed the steps as the tutorial instructs, including creating employee.routes.ts and database.ts.

As the tutorial shows, the database.ts file includes:
collections.employees = employeesCollection

I have also appended all the code as shown in the tutorial. In employee.routes.ts ,
const employees = await collections.employees. appears 5 times.

Problem: VS code is showing ‘collections.employees’ is probably undefined.

When I run the app (npx ts-node src/server.ts), the webpage shows “Unable to GET” . This is probably associated with the reference error of “Collections.employees”.

If all the instructions in the tutorial are correct, can anyone tell what is going on and how to fix it?