Develop Reference

Synchronizing in Node JS [duplicate]

When using a simple callback such as in the example below:
test() {
api.on( 'someEvent', function( response ) {
return response;
});
}
How can the function be changed to use async / await? Specifically, assuming 'someEvent' is guaranteed to be called once and only once, I'd like the function test to be an async function which does not return until the callback is executed such as:
async test() {
return await api.on( 'someEvent' );
}

async/await is not magic. An async function is a function that can unwrap Promises for you, so you'll need api.on() to return a Promise for that to work. Something like this:
function apiOn(event) {
return new Promise(resolve => {
api.on(event, response => resolve(response));
});
}
Then
async function test() {
return await apiOn( 'someEvent' ); // await is actually optional here
// you'd return a Promise either way.
}
But that's a lie too, because async functions also return Promises themselves, so you aren't going to actually get the value out of test(), but rather, a Promise for a value, which you can use like so:
async function whatever() {
// snip
const response = await test();
// use response here
// snip
}

It's annoying that there isn't a straightforward solution, and wrapping return new Promise(...) is fugly, but I have found an ok work-around using util.promisify (actually it also kinda does the same wrapping, just looks nicer).
function voidFunction(someArgs, callback) {
api.onActionwhichTakesTime(someMoreArgs, (response_we_need) => {
callback(null, response_we_need);
});
}
The above function does not return anything, yet. We can make it return a Promise of the response passed in callback by doing:
const util = require('util');
const asyncFunction = util.promisify(voidFunction);
Now we can actually await the callback.
async function test() {
return await asyncFunction(args);
}
Some rules when using util.promisify
The callback must be the last argument of the function that is gonna be promisify
The supposed-callback must be in the form (err, res) => {...}
Funny thing is we do not need to ever specifically write what's the callback actually is.

async/await is magic. You can create a function asPromise to handle this kind of situations:
function asPromise(context, callbackFunction, ...args) {
return new Promise((resolve, reject) => {
args.push((err, data) => {
if (err) {
reject(err);
} else {
resolve(data);
}
});
if (context) {
callbackFunction.call(context, ...args);
} else {
callbackFunction(...args);
}
});
}
and then use it when you want:
async test() {
return await this.asPromise(this, api.on, 'someEvent');
}
the number of args is variable.

You can achieve this without callbacks , use promise async await instead of callbacks here how I would do this. And also here I have illustrated two methods to handle errors
clickMe = async (value) => {
// begin to wait till the message gets here;
let {message, error} = await getMessage(value);
// if error is not null
if(error)
return console.log('error occured ' + error);
return console.log('message ' + message);
}
getMessage = (value) => {
//returning a promise
return new Promise((resolve, reject) => {
setTimeout(() => {
// if passed value is 1 then it is a success
if(value == 1){
resolve({message: "**success**", error: null});
}else if (value == 2){
resolve({message: null, error: "**error**"});
}
}, 1000);
});
}
clickWithTryCatch = async (value) => {
try{
//since promise reject in getMessage2
let message = await getMessage2(value);
console.log('message is ' + message);
}catch(e){
//catching rejects from the promise
console.log('error captured ' + e);
}
}
getMessage2 = (value) => {
return new Promise((resolve, reject) => {
setTimeout(() => {
if(value == 1)
resolve('**success**');
else if(value == 2)
reject('**error**');
}, 1000);
});
}
<input type='button' value='click to trigger for a value' onclick='clickMe(1)' />
<br/>
<input type='button' value='click to trigger an error' onclick='clickMe(2)' />
<br/>
<input type='button' value='handling errors with try catch' onclick='clickWithTryCatch(1)'/>
<br/>
<input type='button' value='handling errors with try catch' onclick='clickWithTryCatch(2)'/>

const getprice = async () => {
return await new Promise((resolve, reject) => {
binance.prices('NEOUSDT', (error, ticker) => {
if (error) {
reject(error)
} else {
resolve(ticker);
}
});
})}
router.get('/binanceapi/price', async function (req, res, next) {
res.send(await binanceAPI.getprice());});

Getting asynchronous function in external library with callback to work synchronously

I am using a library call to connect to my vendor. The libary call requires a callback in the call. Without a callback in the function, I can easily make this synchronous. With the Callback, everything I do is stuck in the callback and never bubbles it way out.
I have literally tried 100 different ways to get this to work.
function removeFromDNC(emailAddress, accessToken_in)
{
return new Promise( function(resolve, reject)
{
try{
const options =
{
auth: {
accessToken: accessToken_in
}
, soapEndpoint: 'https://webservice.XXX.exacttarget.com/Service.asmx'
};
var co = {
"CustomerKey": "DNC",
"Keys":[
{"Key":{"Name":"Email Address","Value": emailAddress}}]
};
var uo = {
SaveOptions: [{"SaveOption":{PropertyName:"DataExtensionObject",SaveAction:"Delete"}}]
};
const soapClient = new FuelSoap(options);
//again, I don't control the structure of the next call.
let res = soapClient.delete('DataExtensionObject', co, uo, async function( err, response ) {
if ( err ) {
// I can get here, but my reject, or if I use return, does nothing
reject();
}else{
// I can get here, but my reject, or if I use return, does nothing
resolve();
}
});
console.log("res value " + res); // undefined - of course
}catch(err){
console.log("ALERT: Bad response back for removeFromDNC for email: " + emailAddress + " error: " + err);
console.log("removeFromDNC promise fulfilled in catch");
reject();
}
});
}

Both methods resolve and reject expect parameters, which are res and err in your case.
As far as removeFromDNC returns a Promise instance, you should call it using either async/await syntax:
const res = await removeFromDNC(...);
or chaining then/catch calls:
removeFromDNC(...)
.then((res) => { ... }) // resolve
.catch((err) => { ... }) // reject
EDIT:
If you want to avoid usage of callbacks inside removeFromDNC, consider promisifying of soapClient.delete call. Refer to util.promisify() if you working in Node.js or use own implementation.
Here is the example for demonstration:
const promisify = (fun) => (...args) => {
return new Promise((resolve, reject) => {
fun(...args, (err, result) => {
if(err) reject(err);
else resolve(result);
})
})
}
const soapClient = {
delete: (value, cb) => {
setTimeout(() => cb(null, value), 10);
}
};
async function removeFromDNC(emailAddress, accessToken_in) {
const soapDelete = promisify(soapClient.delete.bind(soapClient));
const res = await soapDelete('Soap Responce');
//You can use res here
return res;
}
removeFromDNC().then(res => console.log(res))

Resolve promise after callback it's executed

I have the following code on which I am trying to block the execution of the method _saveAddress multiple time, so I made a promise for this method.
const [pressEventDisabled, setPressEventDisabled] = useState(false);
<TouchableOpacity style={style.button_container} activeOpacity={1} disabled={pressEventDisabled} onPress={async () => {setPressEventDisabled(true); await _saveAddress(); setPressEventDisabled(false);}} >
The problem is that I want to resolve the promise after the callback method it's executed. It's there any way to wait for the dispatch function to execute or to resolve the promise inside the callback method?
This is the method for saving the address:
const _saveAddress = () => new Promise(async (resolve) => {
var valid = _validate();
if (valid) {
const address = createAddressJson();
if (addressId) {
var addressIdProperty = {
id: addressId
};
const newAddress = Object.assign(addressIdProperty, address);
dispatch(editAddress(newAddress, _onAddressSaveEditCallback));
} else {
dispatch(addAddress(address, _onAddressSaveEditCallback));
}
} else {
//notify
notifyMessage(strings.fill_required_inputs_validation);
resolve();
}
});
This is the callback method:
const _onAddressSaveEditCallback = async (success: boolean, apiValidations: any, address ? : Address, ) => {
if (success) {
if (typeof callback == 'function') {
callback(address);
}
await Navigation.pop(componentId);
} else {
setDataValidations(apiValidations);
}
};

Just do exactly what you say in the title. Nothing more, nothing less:
if (addressId) {
var addressIdProperty = {id: addressId};
const newAddress = Object.assign(addressIdProperty, address);
dispatch(editAddress(newAddress, async (s,v,a) => {
await _onAddressSaveEditCallback(s,v,a);
resolve();
}));
} else {
dispatch(addAddress(address, async (s,v,a) => {
await _onAddressSaveEditCallback(s,v,a);
resolve();
}));
}
Of course, since you are passing async () => {} to addAddress instead of _onAddressSaveEditCallback you have to call _onAddressSaveEditCallback yourself since addAddress will be calling the async () => ...
But mixing promises and callbacks like this isn't great. It leads to weird looking and sometimes confusing code. A better solution is to promisify addAddress:
function addAddressPromise (address) {
return new Promise((resolve, reject) => {
addAddress(address, (success, validations, address) {
if (success) return resolve(address);
else reject(validations)
});
});
}
Now you can wait for addAddress:
const _saveAddress = async () => {
// Don't create new promise here, we do it in addAddress..
// ...
let result = await addAddressPromise(address);
dispatch(result);
await _onAddressSaveEditCallback();
// ...
}

How to alter result of callback/promise function?

I have a function that can return result in both callback and promise:
function foo(args, cb) {
// do stuff
const promise = getSomePromise();
if (cb) {
promise.then((result) => {
cb(null, result);
}, (err) => {
cb(err);
});
} else {
return promise;
}
}
I want to alter the result of promise before returning it. How to do this in a way that introduces the least amount of spaghetti code?

Add a function to alter and return modified result
function alterResult(result){
const alteredResult = /// do something to result
return alteredResult;
}
Then add it in a then()to:
const promise = getSomePromise().then(alterResult);

You can write a function which returns something and pass it to the callback when initiating it like this
function foo(cb) {
const promise = getSomePromise(); // return some promise
promise.then((result) => { // result is here just "Hello"
cb(doSomething(result)); // return the altered value here to the callback
})
}
function getSomePromise() {
return new Promise((resolve, _) => resolve("Hello")) // make a new promise
}
function doSomething(res) { // define a function that alters your result
return res + ", World"
}
foo((val) => console.log(val)) // log it

Promise: Then before Resolve

Very first thing I've ever done in Node.js, I'm writing an AWS Lambda function, and I want to check whether a custom attribute on a User has a value before doing anything else. Since I'm told Promises are the way to handle asynchronous methods synchronously, I wrote the function:
var AWS = require('aws-sdk');
var s3 = new AWS.S3();
var cogId = new AWS.CognitoIdentityServiceProvider();
exports.handler = function (event, context) {
if (event != null)
{
var identityId = context.identity.cognitoIdentityId;
if (event.userId != null)
{
var userId = event.userId;
PromiseConfirmIdNotSet(userId)
.then(SetId(userId, identityId))
.catch();
}
}
context.done(null, 'Hello World'); // SUCCESS with message
};
function PromiseConfirmIdNotSet(userId)
{
console.log('Entering function');
return new Promise(function (resolve, reject) {
console.log('Entering Promise');
cogId.adminGetUser({
UserPoolId: myUserPool,
UserId: userId
},
function (err, data) {
console.log('err = ' + JSON.stringify(err));
console.log('data = ' + JSON.stringify(err));
if (data != null && data.UserAttributes.Name == null) {
console.log('Calling resolve');
resolve();
} else {
console.log('Calling reject');
reject();
}
});
});
console.log('Exiting Promise');
}
function SetId(userId, identityId)
{
cogId.updateUserAttributes();
}
But when I run it, the console log shows "Entering function", then "Entering Promise", then the execution goes to SetId without ever having called the callback specified in adminGetUser.
If I let the debugger continue after the main flow is done, eventually I do get the logs from the callback function, so it does eventually run.
Why is the Promise skipping to the then without the resolve ever getting called?

.then accepts a function as an argument. When you do
PromiseConfirmIdNotSet(userId)
.then(SetId(userId, identityId))
.catch();
PromiseConfirmIdNotSet is called, and synchronously, SetId is called, while the interpreter tries to construct a Promise chain from the function passed to .then. (But SetId doesn't return a function) Then, after that, PromiseConfirmIdNotSet's asynchronous code runs, and the Promise resolves - which isn't in the order you want.
Change it so that SetId is only called after the promise returned by PromiseConfirmIdNotSet resolves:
PromiseConfirmIdNotSet(userId)
.then(() => SetId(userId, identityId))
.catch();
The problem is similar to why
addEventListener('click', fn());
doesn't work - you'd change it to , fn); or , () => fn());.
If you additionally want context.done to occur only after a successful SetId, then put the context.done call inside the .then:
PromiseConfirmIdNotSet(userId)
.then(() => {
SetId(userId, identityId);
context.done(null, 'Hello World'); // SUCCESS with message
});

You can simply use async-await for neat asynchronous functions. Here is your code with async await. Please check and let me know if you find any more issues.
exports.handler = async function (event, context) {
if (event != null)
{
var identityId = context.identity.cognitoIdentityId;
if (event.userId != null)
{
var userId = event.userId;
await PromiseConfirmIdNotSet(userId);
await SetId(userId, identityId);
}
}
await context.done(null, 'Hello World'); // SUCCESS with message
};
function PromiseConfirmIdNotSet(userId)
{
console.log('Entering function');
return new Promise(function (resolve, reject) {
console.log('Entering Promise');
cogId.adminGetUser({
UserPoolId: myUserPool,
UserId: userId
},
function (err, data) {
console.log('err = ' + JSON.stringify(err));
console.log('data = ' + JSON.stringify(err));
if (data != null && data.UserAttributes.Name == null) {
console.log('Calling resolve');
resolve();
} else {
console.log('Calling reject');
reject();
}
});
});
console.log('Exiting Promise');
}
function SetId(userId, identityId)
{
cogId.updateUserAttributes();
}