Exercise 1 - What is the Big O of the below function?

function funChallenge(input) {
  let a = 10; // consttime
  a = 50 + 3; // consttime
 
  for (let i = 0; i < input.length; i++) { // lineartime(input.length)
    anotherFunction(); // ??? well I would say it depends
    let stranger = true; // consttime
    a++; // consttime
  }
  return a; // consttime
}

Considering that the only critical part of the Big O analysis for this code is the loop, I would say that this functions runs in linear time O(n), n being the size of the input array. However I’m not so sure how to analyse the anotherFunction() since I have no clue what it does. I guess I could add its own Big O time? Like O(funChallenge) = O(n*O(anotherFunction()))

So final answer: $O(n\ast O(anotherFunction()))$

Correct answer: O(n)

My answer is not exactly wrong given my reasoning, but there is a misuse of notation here. Since Big O is already upper bound, there is no need to use the nested O(anotherFunction), it can be just:

$O(n\ast T)$, where T = time complexity of anotherFunction()