How to check if an object has a key?

Question

I have a questionnaire which shows questions with prompt() where each subsequent question depends on the user's answer. And each question has two answers and a question id to show next. And the code shows the question depending on the answer.

And here come the my question. Not every one will have two possible answers. How can I show answer options in prompt depending on the presence of answer_2 . For example, if there is answer_2 , then:

const answer = prompt(`${question}`, `${answer_1.text} / ${answer_2.text}`)

If answer_2 is missing then:

const answer = prompt(`${question}`, `${answer_1.text}`)

Here my code:

 const questions = [{ id: "1", question: "q1", answer_1: { text: "a1", next_question: "2", }, answer_2: { text: "a2", next_question: "3", }, }, { id: "2", question: "q2", answer_1: { text: "a1", next_question: "", }, }, { id: "3", question: "q3", answer_1: { text: "a1", next_question: "", }, answer_2: { text: "a2", next_question: "4", }, }, { id: "4", question: "q4", answer_1: { text: "a1", next_question: "", }, }, ]; //=== question display function quiz({ question, answer_1, answer_2 }) { const answer = prompt(`${question}`, `${answer_1.text} / ${answer_2.text}`); let nextQuestion; if (answer == answer_1.text) { nextQuestion = answer_1.next_question; } else if (answer == answer_2.text) { nextQuestion = answer_2.next_question; } else { alert("Invalid answer"); } if (nextQuestion) { quiz(questions.find((q) => q.id == nextQuestion)); } } quiz(questions[0]);

Answer 1

For questions with no answer_2 you get undefined as value in quiz function. With that you can use an if (answer_2) statement and output what you want, like so:

 const questions = [ { id: "1", question: "q1", answer_1: { text: "a1", next_question: "2" }, answer_2: { text: "a2", next_question: "3" } }, { id: "2", question: "q2", answer_1: { text: "a1", next_question: "" } }, { id: "3", question: "q3", answer_1: { text: "a1", next_question: "" }, answer_2: { text: "a2", next_question: "4" } }, { id: "4", question: "q4", answer_1: { text: "a1", next_question: "" } } ]; //=== question display function quiz({ question, answer_1, answer_2 }) { let answer; if (answer_2) { answer = prompt(`${question}`, `${answer_1.text} / ${answer_2.text}`); } else { answer = prompt(`${question}`, `${answer_1.text}`); } let nextQuestion; if (answer == answer_1.text) { nextQuestion = answer_1.next_question; } else if (answer_2 && answer == answer_2.text) { nextQuestion = answer_2.next_question; } else { alert("Invalid answer"); } if (nextQuestion) { quiz(questions.find((q) => q.id == nextQuestion)); } } quiz(questions[0]);

Answer 2

Since a number divided by one is that same number, 1 can be considered your default value.

const a1 = answer_1.text;
const a2 = answer_2?.text || 1;