Unit testing functions with side effects?

Question

Let's say you're writing a function to check if a page was reached by the appropriate URL. The page has a "canonical" stub - for example, while a page could be reached at stackoverflow.com/questions/123, we would prefer (for SEO reasons) to redirect it to stackoverflow.com/questions/123/how-do-i-move-the-turtle-in-logo - and the actual redirect is safely contained in its own method (eg. redirectPage($url)), but how do you properly test the function which calls it?

For example, take the following function:

function checkStub($questionId, $baseUrl, $stub) {
  canonicalStub = model->getStub($questionId);
  if ($stub != $canonicalStub) {
    redirectPage($baseUrl . $canonicalStub);
  }
}

If you were to unit test the checkStub() function, wouldn't the redirect get in the way?

This is part of a larger problem where certain functions seem to get too big and leave the realm of unit testing and into the world of integration testing. My mind immediately thinks of routers and controllers as having these sorts of problems, as testing them necessarily leads to the generation of pages rather than being confined to just their own function.

Do I just fail at unit testing?

Answer 1

You say...

This is part of a larger problem where certain functions seem to get too big and leave the realm of unit testing and into the world of integration testing

I think this is why unit testing is (1) hard and (2) leads to code that doesn't crumble under its own weight. You have to be meticulous about breaking all of your dependencies or you end up with unit tests == integration tests.

In your example, you would inject a redirector as a dependency. You use a mock, double or spy. Then you do the tests as @atk lays out. Sometimes it's not worth it. More often it forces you to write better code. And it's hard to do without an IOC container.