How to properly test an application with a database?

Question

Unit testing

There is an application with single-file components and getting data from the database.
To test working with the database, do you need a separate test version with fake data, or does jest
allow you to set the structure of the received data?

Answer 1

I don't know much about testing systems, but there are automated testing systems.
I suspect it's about the API.
For example, the same Postman provides this feature.
For example, there are 100 API methods
. You made some changes to one of them.
Now how do you know if this change has affected all other methods.
Yes, that's right, you need to call them in turn and check the response scheme.
Hands?
Yes you!
I made edits, pressed the button and checked everything in 10 seconds.
No database is standing next to it, this is another level of abstraction!