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How does promise work in Node.js and AWS lambda functions

I am very new to Javascript and Node.js in general . As such i am trying to get my head around the concept of asynchronous programming in Node.js and also how to use the same in AWS lambda.
I came across this blog post on Amazon blogs which explains the support for async/await in Node.js lambda functions : https://aws.amazon.com/blogs/compute/node-js-8-10-runtime-now-available-in-aws-lambda/
So far as i knew the concept of a function generating a promise is as in the below code snippet:
const func1 = () => {
console.log("Starting with execution of func1...")
return new Promise((resolve, reject) => {
setTimeout(() => {
resolve("First promise ended after 3 secs")
}, 3000)
})
}
So in here the function explicitly generates a Promise and returns the same.
But in the above AWS blog post i see a function definition as such :
let AWS = require('aws-sdk');
let lambda = new AWS.Lambda();
exports.handler = async (event) => {
return await lambda.getAccountSettings().promise() ;
};
Now i have checked the AWS SDK for Node.js documentation (https://docs.aws.amazon.com/AWSJavaScriptSDK/latest/AWS/Lambda.html#getAccountSettings-property) and i see that the function getAccountSettings accepts a callback as the third parameter.
I am confused with the .promise() syntax of generating promises .How does a function ensure that using that syntax it would give back a promise object . Because from the docs there is no mention that if i use .promise() it would return a promise instead. I am assuming there might be a rule of thumb here in this aspect.
Also instead of return await lambda.getAccountSettings().promise() if i just write return await lambda.getAccountSettings() what difference would it make .
Is there any documentation on this that i can refer to ?
Request you to please throw some more light on this new way of getting promise object back.
Thanks for any help in advance.

If you would like to understand why the .promise() method was made available, and why the promise was not just returned just-like-that, note how such an API evolves over time, and backwards compatibility must be maintained.
Let's build something similar, but much simplified. Let's make a function that provides 1/x for a given number, but an error object when x=0. This will be done asynchronously.
Also, we want the function to return synchronously an object that allows one to register a listener for when an error occurs, another for when there is success, and yet another when either of the two happens. This is a much simplified idea from what AWS returns: there you get a very rich Request object.
So imagine we are in 2012 and promises are not yet widely available/used in JavaScript. So we provide the following API for our asynchronous 1/x function:
// Implementation in the first version of the API (without promises):
// * returns an object with which you can register a listener
// * accepts an optional callback
function getAsyncInverse(num, callback) {
var onSuccess = [], // callbacks that are called on success
onError = [], // callbacks that are called on failure
onComplete = []; // callbacks that are called in both situations
if (callback) onComplete.push(callback);
function complete(err=null) {
var result = null;
if (num === 0) err = new Error("Division by Zero");
else result = 1/num;
// Communicate the result/error to the appropriate listeners:
if (err) for (var i = 0; i < onError.length; i++) onError[i](err);
else for (var i = 0; i < onSuccess.length; i++) onSuccess[i](result);
for (var i = 0; i < onComplete.length; i++) onComplete[i](err, result);
}
var timeoutId = setTimeout(complete, 100);
var request = {
on: function (type, callback) {
if (type === "success") onSuccess.push(callback);
else if (type === "error") onError.push(callback);
else if (type === "complete") onComplete.push(callback);
return request;
},
abort: function () {
clearTimeout(timeoutId);
complete(new Error("aborted"));
return request;
}
}
return request;
}
// How version 1 is used, by registering a listener via the returned object
var num = 2;
var request = getAsyncInverse(num); // let's not pass a callback here
request.on("success", function (result) { // ... but use the request object
console.log("The result is:", result);
}).on("error", function (err) {
console.log("There was an error:", err);
});
But then promises become more popular and users of your API are pushing for a promise API. You want to ensure backwards compatibility, and so decide to just extend the returned request-object with one additional property, a method: promise()
Here is how the above implementation would be altered to make that happen:
// Implementation in the second version of the API (with promise), but backwards-compatible
// * returns an object with which you can register a listener, or get the promise object
// * accepts an optional callback
function getAsyncInverse(num, callback) {
let onSuccess = [], // callbacks that are called on success
onError = [], // callbacks that are called on failure
onComplete = []; // callbacks that are called in both situations
if (callback) onComplete.push(callback);
let request;
// New: create a promise, and put the logic inside the promise-constructor callback
let promise = new Promise(function (resolve, reject) {
function complete(err=null) {
let result = null;
if (num === 0) err = new Error("Division by Zero");
else result = 1/num;
// Communicate the result/error to the appropriate listeners:
if (err) for (let callback of onError) callback(err);
else for (let callback of onSuccess) callback(result);
for (let callback of onComplete) callback(err, result);
// New: also call resolve/reject
if (err) reject(err);
else resolve(result);
}
let timeoutId = setTimeout(complete, 100);
request = {
on: function (type, callback) {
if (type === "success") onSuccess.push(callback);
else if (type === "error") onError.push(callback);
else if (type === "complete") onComplete.push(callback);
return request;
},
abort: function () {
clearTimeout(timeoutId);
complete(new Error("aborted"));
return request;
},
promise: function () { // <--- added feature!
return promise;
}
};
});
return request; // We return the same as in version-1, but with additional promise method
}
// How version 2 is used, by getting the new promise method
let num = 2;
let promise = getAsyncInverse(num).promise();
promise.then(function (result) {
console.log("The result is:", result);
}).catch(function (err) {
console.log("There was an error:", err);
});
As you can see, it would not be a good idea to omit the request object, and have the function return the promise. This would break existing code using your API (no backwards compatibility).

Issue with Nodejs promise

I have a sequence of function calls, connected with ES6 promises. Apparently, there is something wrong with this implementation, as API calls to the endpoint are not returning anything and the browser is stuck waiting for a response.
Please advise.
module.exports.insertTreatmentDetails = function (req, res) {
var doctorId = 10000
var departmentId = 10000
var procedureid = 10000
var hospitalSchema = new hospitalModel();
var p = new Promise(function (resolve, reject) {
counterSchema.getNext('Treatment.doctor.doctorId', collection, function (doctorId) {
doctorId = doctorId;
})
counterSchema.getNext('Treatment.departmentId', collection, function (departmentId) {
departmentId = departmentId
})
counterSchema.getNext('Treatment.procedureid', collection, function (procedureid) {
procedureid = procedureid
})
}).then(function () {
setData()
}).then(function (){
hospitalSchema.save(function (error, data) {
if (error) {
logger.error("Error while inserting record : - " + error)
return res.json({ "Message": error.message.split(":")[2].trim() });
}
else {
return res.json({ "Message": "Data got inserted successfully" });
}
});
});
};

The short answer is that you aren't calling resolve or reject inside the first promise in your chain. The promise remains in a pending state. Mozilla has a good basic explanation of promises.
How to Fix
It appears that you want to retrieve doctorId, departmentId, and procedureId before calling setData. You could try to wrap all three calls in one promise, checking whether all three have returned something in each callback, but the ideal is to have one promise per asynchronous task.
If it's feasible to alter counterSchema.getNext, you could have that function return a promise instead of accepting a callback. If not, I would recommend wrapping each call in its own promise. To keep most true to what your code currently looks like, that could look like this:
const doctorPromise = new Promise((resolve, reject) =>
counterSchema.getNext('Treatment.doctor.doctorId', collection, id => {
doctorId = id;
resolve();
}));
Then you could replace the first promise with a call to Promise.all:
var p = Promise.all([doctorPromise, departmentPromise, procedurePromise])
.then(setData)
.then(/* ... */);
Promises allow you to pass a value through to the next step, so if you wanted to get rid of your broadly-scoped variables (or set them in the same step where you call setData), you could just pass resolve as your callback to counterSchema.getNext and collect the values in the next step (also how you'd want to do it if you have counterSchema.getNext return a promise:
Promise.all([/* ... */])
.then(([doctorID, departmentID, procedureID]) => {
// If you aren't changing `setData`
doctorId = doctorID;
departmentId = departmentID;
procedureid = procedureID;
setData();
// If you are changing `setData`
setData(doctorID, departmentID, procedureID);
}).then(/* ... */).catch(/* I would recommend adding error handling */);

NodeJS Multiple function promises

Let's say I have some code that looks like this:
var doSomething = function(parameter){
//send some data to the other function
return when.promise(function(resolveCallback, rejectCallback) {
var other = doAnotherThing(parameter);
//how do I check and make sure that other has resolved
//go out and get more information after the above resolves and display
});
};
var doAnotherThing = function(paramers){
return when.promise(function(resolveCallback, rejectCallback) {
//go to a url and grab some data, then resolve it
var s = "some data I got from the url";
resolveCallback({
data: s
});
});
};
How do I ensure that var other has completely resolved before finishing and resolving the first doSomething() function? I'm still wrapping my head around Nodes Async characteristic
I really didn't know how else to explain this, so I hope this makes sense! Any help is greatly appreciated
EDIT: In this example, I am deleting things from an external resource, then when that is done, going out the external resource and grabbing a fresh list of the items.
UPDATED CODE
var doSomething = function(parameter){
//send some data to the other function
doAnotherThing(parameter).then(function(){
//now we can go out and retrieve the information
});
};
var doAnotherThing = function(paramers){
return when.promise(function(resolveCallback, rejectCallback) {
//go to a url and grab some data, then resolve it
var s = "some data I got from the url";
resolveCallback({
data: s
});
});
};

The return of doAnotherThing appears to be a promise. You can simply chain a then and put your callback to utilize other. then also already returns a promise. You can return that instead.
// Do stuff
function doSomething(){
return doAnotherThing(parameter).then(function(other){
// Do more stuff
return other
});
}
// Usage
doSomething().then(function(other){
// other
});

Below is how to accomplish what you're trying to do with bluebird.
You can use Promise.resolve() and Promise.reject() within any function to return data in a Promise that can be used directly in your promise chain. Essentially, by returning with these methods wrapping your result data, you can make any function usable within a Promise chain.
var Promise = require('bluebird');
var doSomething = function(parameter) {
// Call our Promise returning function
return doAnotherThing()
.then(function(value) {
// Handle value returned by a successful doAnotherThing call
})
.catch(function(err) {
// if doAnotherThing() had a Promise.reject() in it
// then you would handle whatever is returned by it here
});
}
function doAnotherThing(parameter) {
var s = 'some data I got from the url';
// Return s wrapped in a Promise
return Promise.resolve(s);
}

You can use the async module and its waterfall method to chain the functions together:
var async = require('async');
async.waterfall([
function(parameter, callback) {
doSomething(parameter, function(err, other) {
if (err) throw err;
callback(null, other); // callback with null error and `other` object
});
},
function(other, callback) { // pass `other` into next function in chain
doAnotherThing(other, function(err, result) {
if (err) throw err;
callback(null, result);
})
}
], function(err, result) {
if (err) return next(err);
res.send(result); // send the result when the chain completes
});
Makes it a little easier to wrap your head around the series of promises, in my opinion. See the documentation for explanation.

Nested Promises Stuck

The following code gets stuck:
var Promise = require('promise');
var testPromise = function(){
return new Promise(function(fulfill, reject){
element.all(by.repeater('item in menu.items')).first().then(function(el){
console.log('test f');
fulfill(el);
console.log('test fe');
});
});
};
... called by the following:
testPromise().then(function(el){
console.log('test successful '+el);
});
The console prints
test f
test fe
And get stuck no more code is executed. It never reaches the then although fulfill has been called.
if using nested promises is an anti pattern then how do I do the following without a nested promise:
var getMenuItemEl = function(itemName){
return new Promise(function(fulfill, reject){
var elFound;
element.all(by.repeater('item in menu.items')).then(function(els){
async.each(els, function(el, callback){
el.getText().then(function(text){
console.log('getMenuItemEl:'+text);
if(text === itemName){
elFound = el;
}
callback();
});
}, function(err){
console.log('complete '+elFound);
if(elFound){
console.log('fulfill');
fulfill(elFound);
console.log('after fulfill');
}else{
reject('no item found');
}
});
});
});
};
This also gets stuck after the fulfill has been called

if using nested promises is an anti pattern then how do I do the following without a nested promise
You would not use the async library. Since all your methods (element.all(), el.getText() etc) do already return promises, you don't need to go back to node-style error callbacks. From my "rules for promises",
every async function (even if it is a callback) should return a promise, and use your libraries' methods to compose them. You really don't need to call the Promise constructor on your own. That iteration you are doing there can be easily done by a map over the els, then collecting all the single promises together with Promise.all.
function getMenuItemEl(itemName) {
return element.all(by.repeater('item in menu.items'))
.then(function(els){
return Promise.all(els.map(function(el) {
return el.getText()
.then(function(text){
console.log('getMenuItemEl:'+text);
return (text === itemName) ? el : null;
});
}));
})
.then(function(foundEls) {
for (var i=0; i<foundEls.length; i++)
if (foundEls[i] != null) {
console.log('fulfill');
return foundEls[i];
}
throw new Error('no item found');
});
}

The following code solves my problem, mainly due to my lack of knowledge of the protractor api
var doClickMenuItemEl = function(itemName){
return element.all(by.repeater('item in menu.items')).filter(function(elem, index) {
return elem.getText().then(function(text) {
return text === itemName;
});
}).then(function(els){
els[0].click();
});
};
It also says on https://github.com/angular/protractor/issues/379 that
Yes, this is inherited from webdriver promises, which are not quite promise A+ compliant, unfortunately.

Resolving a deferred using Angular's $q.when() with a reason

I want to use $q.when() to wrap some non-promise callbacks. But, I can't figure out how to resolve the promise from within the callback. What do I do inside the anonymous function to force $q.when() to resolve with my reason?
promises = $q.when(
notAPromise(
// this resolves the promise, but does not pass the return value vvv
function success(res) { return "Special reason"; },
function failure(res) { return $q.reject('failure'); }
)
);
promises.then(
// I want success == "Special reason" from ^^^
function(success){ console.log("Success: " + success); },
function(failure){ console.log("I can reject easily enough"); }
);
The functionality I want to duplicate is this:
promises = function(){
var deferred = $q.defer();
notAPromise(
function success(res) { deferred.resolve("Special reason"); },
function failure(res) { deferred.reject('failure'); }
);
return deferred.promise;
};
promises.then(
// success == "Special reason"
function(success){ console.log("Success: " + success); },
function(failure){ console.log("I can reject easily enough"); }
);
This is good, but when() looks so nice. I just can't pass the resolve message to then().
UPDATE
There are better, more robust ways to do this. $q throws exceptions synchronously, and as #Benjamin points out, the major promise libs are moving toward using full Promises in place of Deferreds.
That said, this question is looking for a way to do this using $q's when() function. Objectively superior techniques are of course welcome but don't answer this specific question.

The core problem
You're basically trying to convert an existing callback API to promises. In Angular $q.when is used for promise aggregation, and for thenable assimilation (that is, working with another promise library). Fear not, as what you want is perfectly doable without the cruft of a manual deferred each time.
Deferred objects, and the promise constructor
Sadly, with Angular 1.x you're stuck with the outdated deferred interface, that not only like you said is ugly, it's also unsafe (it's risky and throws synchronously).
What you'd like is called the promise constructor, it's what all implementations (Bluebird, Q, When, RSVP, native promises, etc) are switching to since it's nicer and safer.
Here is how your method would look with native promises:
var promise = new Promise(function(resolve,reject){
notAPromise(
function success(res) { resolve("Special reason") },
function failure(res) { reject(new Error('failure')); } // Always reject
) // with errors!
);
You can replicate this functionality in $q of course:
function resolver(handler){
try {
var d = $q.defer();
handler(function(v){ d.resolve(v); }, function(r){ d.reject(r); });
return d.promise;
} catch (e) {
return $q.reject(e);
// $exceptionHandler call might be useful here, since it's a throw
}
}
Which would let you do:
var promise = resolver(function(resolve,reject){
notAPromise(function success(res){ resolve("Special reason"),
function failure(res){ reject(new Error("failure")); })
});
promise.then(function(){
});
An automatic promisification helper
Of course, it's equally easy to write an automatic promisification method for your specific case. If you work with a lot of APIs with the callback convention fn(onSuccess, onError) you can do:
function promisify(fn){
return function promisified(){
var args = Array(arguments.length + 2);
for(var i = 0; i < arguments.length; i++){
args.push(arguments[i]);
}
var d = $q.defer();
args.push(function(r){ d.resolve(r); });
args.push(function(r){ d.reject(r); });
try{
fn.call(this, args); // call with the arguments
} catch (e){ // promise returning functions must NEVER sync throw
return $q.reject(e);
// $exceptionHandler call might be useful here, since it's a throw
}
return d.promise; // return a promise on the API.
};
}
This would let you do:
var aPromise = promisify(notAPromise);
var promise = aPromise.then(function(val){
// access res here
return "special reason";
}).catch(function(e){
// access rejection value here
return $q.reject(new Error("failure"));
});
Which is even neater

Ok here's my interpretation of what I think you want.
I am assuming you want to integrate non-promise callbacks with a deferred/promise?
The following example uses the wrapCallback function to wrap two non-promise callbacks, successCallback and errCallback. The non-promise callbacks each return a value, and this value will be used to either resolve or reject the deferred.
I use a random number to determine if the deferred should be resolved or rejected, and it is resolved or rejected with the return value from the non-promise callbacks.
Non angular code:
function printArgs() {
console.log.apply(console, arguments);
}
var printSuccess = printArgs.bind(null, "success");
var printFail = printArgs.bind(null, "fail");
function successCallback() {
console.log("success", this);
return "success-result";
}
function errCallback() {
console.log("err", this);
return "err-result";
}
function wrapCallback(dfd, type, callback, ctx) {
return function () {
var result = callback.apply(ctx || this, arguments);
dfd[type](result);
};
}
Angular code:
var myApp = angular.module('myApp', []);
function MyCtrl($scope, $q) {
var dfd = $q.defer();
var wrappedSuccess = wrapCallback(dfd, "resolve", successCallback);
var wrappedErr = wrapCallback(dfd, "reject", errCallback);
var rnd = Math.random();
var success = (rnd > 0.5);
success ? wrappedSuccess() : wrappedErr();
console.log(rnd, "calling " + (success ? "success" : "err") + " callback");
dfd.promise.then(printSuccess, printFail);
}
Example output where the random number is less than 0.5, and so the deferred was rejected.
err Window /fiddlegrimbo/m2sgu/18/show/
0.11447505658499701 calling err callback
fail err-result