Node.js and SQL: How can I get the value outside of the for loop?

Question

Using Node.js and a MySQL database I'm working on a small project.

I'm trying to loop through an array to get some values out of a MySQL database. I'm searching for the corresponding "medicine ID" to an "medicine name" that the user entered. My Code is working correctly and this is how it looks.

 var medizinArray = []; function appendArray(input) { medizinArray.push(input); } var sqlMedNameToId = "SELECT MedikamentId FROM Medikament WHERE Bezeichnung = ?" for (var i=0;i<medicineMontag.length;i++){ var montagsMedizin = medicineMontag[i]; mySqlConnection.query(sqlMedNameToId, montagsMedizin, function(err, rows, fields){ if(!err) { result = rows[0].MedikamentId; appendArray(result); } else { console.log(err); } }) } console.log(medizinArray);

The code is working but I can't get the medizinArray out of the for loop. In my console I get an empty array. When I put the console.log(medizinArray) inside the for loop I get the array that I want.

I'm currently not familiar with Promises. I read about it and saw some other questions but I can't figure out how to implement Promises in my code.

Answer 1

SQL operations are asynchronous, so to obtain the result outside of the callback you need to wrap them in a Promise and call the resolve() function when the operation is successful. Use any of the techniques below:

Async/await technique:

(async function(){
  var medizinArray = [];

function appendArray(input) {
  medizinArray.push(input);
}

var sqlMedNameToId = "SELECT MedikamentId FROM Medikament WHERE Bezeichnung = ?"
await new Promise(function(resolve, reject){
var e;
for (var i=0;i<medicineMontag.length;i++){
  var montagsMedizin = medicineMontag[i];
  mySqlConnection.query(sqlMedNameToId, montagsMedizin, function(err, rows, fields){
    if(e) return; 
    if(!err) {
      result = rows[0].MedikamentId;
      appendArray(result);
    } else {
      //console.log(err);
      e = true; 
      return reject(err);
      }
    if(i == medicineMontag.length-1) resolve(result);
  })
}
}
);

console.log(medizinArray);//now medizinArray shows up here
})().catch(function(err){console.log(err)});

Promise/then technique:

var medizinArray = [];

function appendArray(input) {
  medizinArray.push(input);
}

var sqlMedNameToId = "SELECT MedikamentId FROM Medikament WHERE Bezeichnung = ?"

new Promise(function(resolve, reject){
var e; 
for (var i=0;i<medicineMontag.length;i++){
  var montagsMedizin = medicineMontag[i];
  mySqlConnection.query(sqlMedNameToId, montagsMedizin, function(err, rows, fields){
    if(e) return;
    if(!err) {
      result = rows[0].MedikamentId;
      appendArray(result);
    } else {
      console.log(err);
      e = true; 
      return reject(err); 
      }
    if(i == medicineMontag.length-1) resolve(result);
  })
}
}
).then(function(result){
console.log(medizinArray);//now medizinArray shows up here
}).catch(function(err){
  console.log(err);
});