Too many redirects for handling error where side doesn't equal 'question' or 'answer'?

I don't know why none of the other commenters caught this, but this conditional:

  } else if (side !== 'question' || side !== 'answer') {
    return res.redirect(`/cards/${id}?side=question`);

will always trigger causing a loop. The or statement makes sure of that, because side only needs to not be equal to question OR not be equal to answer. Meaning that even if it is equal to question or answer, it will not be equal to the other, and therefore the condition will be triggered as true. This could be simply solved by changing the logical or operator to a logical and operator as so:

  } else if (side !== 'question' && side !== 'answer') {
    return res.redirect(`/cards/${id}?side=question`);

You're likely getting caught in an infinite loop. You should add a default condition in the event that side doesn't equal "question" or "answer". So it would be something like:

if (!side) {
    return res.redirect(`/cards/${id}?side=question`);
  } else if (side !== 'question' || side !== 'answer') {
    return res.redirect(`/cards/${id}?side=question`);
  } else {
    // do something
  }

I would add "console.log(side);" to the else condition so you can see what the value of "side" is.

I think there is an alternative solution without any if statement or redirection.

const side = req.query.side || 'question';

Here I assign the string 'question' if the side property of the query string is undefined.