How should data be presented when testing?

Question

Software testing

Good afternoon, I asked myself the question of testing, and in this regard I am completely green, I have not even been born yet. The question is, when testing individual functions, blocks, data is given to the testing function, or the entire application is brought to the required form (filling the application itself with data, and not submitting ready-made data to the input.), Which is better?

Answer 1

Depends on the purpose of testing and what we are testing.
If we are testing a single function, then it makes no sense to fill the entire application with data that the function under test does not use. (Unit testing)
And vice versa, it makes no sense to test the processing of a network request error and at the same time slip knowingly correct data (Functional testing)