What does JavaScript do when it sees this if(this.getCurrentQuestion().isCorrectAnswer(answer))?

answer is being passed into the function that is being called. It is a string. It is being checked against the current question which is why 'this' is used. Read left to right, 'this' is what is being checked, it calls the method getCurrentQuestion() so we know which question we are dealing with and, it then calls the method isCorrectAnswer() to check to see if the parameter supplied, 'answer', is the correct answer to the given question.

Question.prototype.isCorrectAnswer(guess) {
        return this.answer === guess
};

This prototype is a method that formulates the last part of what is being checked. We send in answer and, get a true or false of whether the answer we retrieved is the same as the correct answer.

Quiz.prototype.getCurrentQuestion = function() {
         return this.questions[this.currentQuestionIndex];
};

This prototype gets the question we are currently on based on the current index.

So again, we are using 'this' to refer to where we are actually at and what we are specifically dealing with. We then call the method created by the prototype to get the current question. And finally, when we have the question, we need to check the answer so we call the method created again by another prototype and send in the users answer as a parameter. This is what is actually returning the true or false.

If the answer is correct (true) the score is incremented and the index is increased. If false, we are simply increasing the index.