Testing in Javascript while mocking/stubbing a function

I am trying to test a function that makes an external call using sinon but I can't figure out how I can muck/stub a function inside it that tries to make an external call.

I hope one of you can give me some insight in this taking the following function as an example.

function Foo (num) {
  let externalResult = externalCall(num);
  if(externalResult == 15) {
    return true;
  }
  return false;
}

What I would like to stub/moch, is the function externalCall. I would like it to return (for example) 14 every time it is called. I also want to verify that it is called.

I've been searching but have not been able to find anything. I might have overlooked things as all I have is a little background in Java (student). But all I have so far is: it('Should give the right boolean based on the input to the external call.',

async () => {
    const testingExService = await ExService.new() // the object to test

  });