对数据库和服务器使用Promise

Question

Im still learning js, but im trying to write a basic server in nodejs. The server is supposed to take a query and then return a JSON from a db.

Im trying to use promises but can get my head around this. My server returns an empty reply. So what am i doing wrong?

My server file so far (simplified):

http.createServer(function (request, response) {
  if (request.method === "GET") {
    ...
    var dbRes = dbFunc.getFromDB(query);
    response.statusCode = 200;
    response.setHeader('Content-Type', 'application/json');
    response.end(JSON.stringify(dbRes));
  }
}).listen(port);

My db file so far (simplified):

exports.readHandeler = function (query) {

  var promise = new Promise(function(resolve, reject) {
    db.collection('data').find(query).toArray(function(err, res) {
      if (err) {
        throw err;
        reject(-1);
      }
      resolve(res);     
    }    
  });

  promise.then(function(result) {
    return result;
  }, function(err) {
    return -1;
  });
}

Answer 1

My server returns an empty reply. So what am i doing wrong?

Your dbRes will be undefined since you're expecting a response from async promise execution. You need to send response only after promise resolves.

Making this change to your code

http.createServer(function (request, response) {
  if (request.method === "GET") {
  ...
    dbFunc.getFromDB(query).then((dbRes) => {
      response.statusCode = 200;
      response.setHeader('Content-Type', 'application/json');
      response.end(JSON.stringify(dbRes));
    }).catch((err) => {
      console.log('err', err.stack);
    });

  }
}).listen(port);

Also, you need to return Promise in db execution part.

exports.readHandeler = function (query) {
  return new Promise(function (resolve, reject) {
    db.collection('data').find(query).toArray(function (err, result) {
      if (err) {
        return reject(err); // or -1 based on your need
      }
      return resolve(result);
    });
  });
};