That’s not how filter works: it needs to return true or false. You don’t return a value, what you do is return a new array containing only the elements that return true for the condition you give it.
Also, you aren’t trying to get the index, you’re trying to get the ID, which is not the same thing (it is an array of answers, index 0 is id 1, index 1 is id 2 and so on). So that createAnswerKey function is mapping over the quiz questions to give you an array of the ids of the correct answers.
quizQuestions.map((quizQuestion) => {
return quizQuestion.answers.find((answer) => answer.correct === true)[id];
})
So map over the questions. For each question object, find the answer where the correct field is true, and grab its ID. This assumes there will always be one and only one correct answer for each question.
To make it more concise, use destructuring + the fact correct is a boolean:
quizQuestions.map(({answers}) => answers.find(({correct}) => correct)[id]);
To find the index instead of the ID:
quizQuestions.map(({answers}) => answers.findIndex(({correct}) => correct));
Note that this is one possible solution, there are others (for example using reduce, or using an imperative loop inside the function).