TypeError：无法读取 TypeScript 中未定义的属性“currentQuestions”

Question

我有以下代码，我想做的是从数据库中获取一些数据（这似乎是成功的，我试图记录它），然后排列这些数据并发送一个数组作为响应。我的班级：

export class Controller {

    currentQuestions: IQuestionArranged[] = []; // This should be my array that I am trying to send as response

    constructor(){ }

    public async startNewGame(req: express.Request, res: express.Response): Promise<void> {
        let factory: RepositoryFactory = new RepositoryFactory();
        let repository: RepositoryInterface = factory.createRepository();

        let questionsFetched: IQuestionFetched[] = await repository.fetchQuestions();    // array of questions as they were fetched
        // let currentQuestions: IQuestionArranged[] = []; // <<<<<

        // forEach through fetched questions to arrange them
        questionsFetched.forEach(currentQuestion => {
            // check if the current question already exists in the array
            if (this.currentQuestions.filter(e => e.id == currentQuestion.questionId).length != 0) {
                // if yes -> find the question and add the new answer with its id
                this.currentQuestions
                    .filter(e => e.id == currentQuestion.questionId)
                    .map(e => e.answers.push({
                        id: currentQuestion.answerId,
                        answer: currentQuestion.answer
                    }));
            } else {
                // if no -> add the new question with the new answer
                this.currentQuestions.push({
                    id: currentQuestion.questionId,
                    question: currentQuestion.question,
                    answers: [{
                        id: currentQuestion.answerId,
                        answer: currentQuestion.answer
                    }]
                });
            }
        });
        res.send(this.currentQuestions);
    }
}

但是我收到了这个错误，上面写着“UnhandledPromiseRejectionWarning: TypeError: Cannot read property 'currentQuestions' of undefined”（完整的错误描述 here）。如果我使用局部变量（标有 5 个左箭头作为注释的变量）而不是类属性，它似乎工作得很好，这让我觉得我没有正确使用它，但我不知道我的错误在哪里...

Answer 1

你能用函数表达式试试箭头函数吗？例如 =>

public startNewGame = async (req: express.Request, res: express.Response) => {
 // Your code goes here.
}

Answer 2

我找到了一个更清洁的解决方案。我必须做的是像这样绑定“this”变量：

constructor() {
   this.startNewGame = this.startNewGame.bind(this);
}