NodeJS get async return value (callback) [duplicate]

Answer 1

Callbacks can't return a value as the code they would be returning to has already executed.

So you can do a couple things. One pass a callback function and once your async function gets the data call the callback and pass the data. Or pass the response object and use it in your async function

Passing a callback

exports.getJobInfoByID = function(req, res) {
    var jobIDParam = req.params.id;
    asyncJobInfo(jobIDParam,null,function(data){
       res.send(data);
    });
}

var asyncJobInfo = function(jobID, next,callback) {
    //...
    oozie.get(command, function(error, response) {
       //do error check if ok do callback
       callback(response);
    });
};

Passing response object

exports.getJobInfoByID = function(req, res) {
    var jobIDParam = req.params.id;
    asyncJobInfo(jobIDParam,null,res);
}

var asyncJobInfo = function(jobID, next,res) {
    //...
    oozie.get(command, function(error, response) {
       //do error check if ok do send response
       res.send(response);
    });
};

Answer 2

In async world you can't return values. Whatever needs to be done when the value is ready should be executed inside the callback. Another alternative is using promises. You'll need the es6-promise package:

var Promise = require('es6-promise').Promise;

var asyncJobInfo = function(jobID) {
  var oozie = oozieNode.createClient({config: config});
  var command = 'job/' + jobID + '?show=info';
  console.log("running oozie command: " + command);
  // Creates a new promise that wraps
  // your async code, and exposes two callbacks:
  // success, and fail.
  return new Promise(function(success, fail) {
    oozie.get(command, function(error, response) {
      if (error) {
        fail(error);
      } else {
        success(response);
      }
    });
  });
};

Now you can use the promise and pass the callbacks that will run once it is resolved:

exports.getJobInfoByID = function(req, res) {
  asyncJobInfo(req.params.id).then(function(data) {
    res.send(data)
  }).catch(function(error) {
    console.error(error);
  });
};

The above can be shortened:

exports.getJobInfoByID = function(req, res) {
  asyncJobInfo(req.params.id)
    .then(res.send.bind(res))
    .catch(console.error);
};