如何在查询加载后让 mysql nodejs 发送变量

Question

I have looked up promises but cant figure it out, and i dont understand them. I know everything else works and sends data to the client, just empty. and its because i cant assign the variable in time when sql finished?

app.post('/api', urlencodedParser,function(req, res,next) {

  console.log(req.body);
  var data = req.body.productObjs;
var bool;


  data.forEach(function(product) {

    connection.query(`SELECT * FROM productInfo where specId = ${product.productSpec}`,function(error,rows){
      
(error,rows)=>{

      if (error) {
        next('error making the query: ' + error.message)
      }
       else {
        console.log(rows[0]['specId'])
        console.log(rows[0]['productPrice'])
        if (product.productPrice === rows[0]['productPrice']) {
      bool=true
        }
        if (product.productPrice !== rows[0]['productPrice']) {
bool=false
        }
      }
    })

});
 //res.send({"bool":bool})
})
})

Answer 1

So the very first clear issue is that you have a callback within a callback. That'd be my first refactoring attempt:

app.post('/api', urlencodedParser, function (req, res, next) {
  console.log(req.body);
  const data = req.body.productObjs;
  let bool;

  data.forEach((product) => {
    connection.query(`SELECT * FROM productInfo where specId = ${product.productSpec}`, (error, rows) => {
      if (error) {
        next('error making the query: ' + error.message);
      } else {
        console.log(rows[0]['specId']);
        console.log(rows[0]['productPrice']);
        bool = product.productPrice === rows[0]['productPrice'];
      }
    });
  });
});

1. I're removed double callbacks
2. Simplified condition to assign your value to bool variable
3. Changed types for modern JS syntax