Sample question/answer kata

Evaluation.ts

import { makeAutoObservable } from "mobx";

type Choice = {
  id: number;
  text: string;
};

type Question = {
  id: number;
  text: string;
  choices: Choice[];
};

interface EvaluationApi {
  submitAnswer: (
    questionId: number,
    answer: Choice["id"]
  ) => Promise<"Good answer" | "Wrong answer">;
  getNextQuestion: (previousQuestionId?: number) => Promise<Question>;
}

enum EvaluationStatus {
  WAITING_FOR_NEXT_QUESTION = "Waiting for the next question",
  WAITING_FOR_ANSWER = "Answer this question !",
  SUBMIT_ERROR = "Cannot answer a question before it was asked !",
  WRONG_ANSWER = "❌ Wrong answer, try again",
  GOOD_ANSWER = "✅ Good answer",
}

export class Evaluation {
  // Reactive states
  currentQuestion?: Question;
  statusMessage: string = EvaluationStatus.WAITING_FOR_NEXT_QUESTION;

  private goodAnswers = new Set<number>();

  constructor(private api: EvaluationApi) {
    makeAutoObservable(this); // Make it reactive with this single line
    this.getNextQuestion();
  }

  // Action
  async submitAnswer(choiceId: Choice["id"]) {
    if (
      !this.currentQuestion ||
      !this.currentQuestion.choices.find((choice) => choice.id === choiceId)
    ) {
      this.setStatusMessage(EvaluationStatus.SUBMIT_ERROR);
      return;
    }
    await this.api
      .submitAnswer(this.currentQuestion.id, choiceId)
      .then(this.checkAnswer(this.currentQuestion.id));
  }

  // Derived
  get nbOfGoodAnswers() {
    return this.goodAnswers.size;
  }

  // Internal actions

  private async getNextQuestion() {
    this.currentQuestion = await this.api.getNextQuestion(
      this.currentQuestion?.id
    );
    this.setStatusMessage(EvaluationStatus.WAITING_FOR_ANSWER);
  }

  private checkAnswer =
    (questionId: number) => (result: "Good answer" | "Wrong answer") => {
      if (result === "Wrong answer") {
        this.setStatusMessage(EvaluationStatus.WRONG_ANSWER);
        return;
      }
      this.setStatusMessage(EvaluationStatus.GOOD_ANSWER);
      this.goodAnswers.add(questionId);
      this.getNextQuestion();
    };

  private setStatusMessage(status: EvaluationStatus) {
    // TODO : Manage i18n here ?
    this.statusMessage = String(status);
  }
}