在nodejs中的循环内等待承诺

Question

I know this question may already be asked. But I didn't understand how things are worked.that is why I am creating the new thread.

con.query(sql,[req.params.quizId],(err,rows,fields)=>{
    //rows contains questions
    if(err) throw err;
    else{
        let object={};

        rows.forEach((item,index)=>{
            object=item;
            //here iam passing question id to get choices a async function
            getChoices(item.id)
                .then(data=>{
                    object.choices=data;
                    //save the question array 
                    response.push(object);
                    //res.send(response);
                });
        })
      res.send(response) //return empty array

    }
});

function getChoices(questionId) {
    let sql='SELECT id,text FROM `question_choices` where question_id=?';
    return new Promise((resolve, reject) => {
        con.query(sql,[questionId],(err,rows,fields)=>{
                if(err) throw err;
                else {
                    resolve(rows);
                }
        })
    })

}

I tried several things but none is worked. I think for loop didn't wait for the promise to complete and it sends the response directly. Some async problems are happening there.

I can able to get all questions from database and for each question I need to get corresponding choices that I want.

something like this

[{id:'xx', text:'yy',choices:[{id:'c',text:'kk']},etc]

Answer 1

forEach runs synchronously . You're looking for Promise.all , which accepts an array of Promises , and resolves to an array of the resolved values once all of the Promises resolve. To transform your rows array to an array of Promises , use .map .

Also, when there's an error, you should call reject so that you can handle errors in the consumer of the Promise (the con.query callback), otherwise, when there's an error, it'll hang forever without you knowing about it:

con.query(sql,[req.params.quizId],(err,rows,fields)=>{
  if(err) throw err;
  Promise.all(rows.map((item) => (
    getChoices(item.id)
      .then((choices) => ({ ...item, choices }))
  )))
  .then((response) => {
    res.send(response);
  })
  .catch((err) => {
    // handle errors
  })
});

function getChoices(questionId) {
  const sql='SELECT id,text FROM `question_choices` where question_id=?';
  return new Promise((resolve, reject) => {
    con.query(sql,[questionId],(err,rows,fields)=>{
      if(err) reject(err);
      else resolve(rows);
    });
  });
}