Proper way to get the result of MySQL from Node.js function callback?

You have to decide wether you want to provide result via callback or with return. Don't mix them, it's confusable.

Callback approach

var err, farms = GetAllFarms()
console.log("GetAllFarms:")
console.log(farms)
console.log(err)


function GetAllFarms(callback) {
    query = db.query("SELECT * FROM farms ", function (err, result) {
        console.log("DEBUG:QUERY//");
        console.log(query.sql);
        // console.log(result)

        if (err) {
            // console.log(err)
            return callback(err, null)
        } else {
            // console.log(result)
            return callback(null, result)
        }
    });

    // db.end()

    console.log("query")
    console.log(query.result)
}

// usage
GetAllFarms((error, result) => {
    if (error) {
      // handle error
    }
    // process result
})

Promise approach

var err, farms = GetAllFarms()
console.log("GetAllFarms:")
console.log(farms)
console.log(err)


function GetAllFarms() {
    return new Promise((resolve, rejct) => {
    db.query("SELECT * FROM farms ", function (err, result) {
          console.log("DEBUG:QUERY//");
          console.log(query.sql);

          if (err) {
              return reject(err)
          } else {
              return resolve(result)
          }
      });
  });
}

// usage
(async () => {
  const res = await GetAllFarms();
  // or 
  GetAllFarms().then(/* ... */).catch(/* ... */);
})