Develop Reference

getting undefined error in Promise Chaining if then has multiple lines of code

Iam doing a vary basic promise demo code.
Can anybody please explain me the reason why the followng code block gives undefined? Any help would be highky appreciated.
This is my promise Definition code. I have created three functions each returning a promise. For now, iam using setTimeout in the example
// promise-definitions.js
const definitions = {
getUser : (id) => {
return new Promise((resolve, reject) => {
setTimeout(()=> {
console.log('Getting User data...');
resolve({id: id, userName: 'Shamik Roy'})
}, 2000);
})
},
getRepos : (user) => {
return new Promise((resolve, reject) => {
setTimeout(()=> {
console.log('Getting repositories List...');
resolve(['repo1', 'repo2']);
}, 2000);
});
},
getCommits: (repoId) => {
return new Promise((resolve, reject) => {
setTimeout(()=>{
console.log('Getting commit List...');
resolve([{ id: 1, commit: "First Commit"},{ id:2, commit: "Second Commit"}]);
}, 2000);
})
}
module.exports = definitions;
// promise-consumer.js
The following approach gives undefined in this line : console.log('Repository Data', repos);
const promise = require('./promise-definitions');
promise.getUser(10)
.then((user)=> {
console.log('User Data', user);
promise.getRepos(user.userName);
})
.then((repos) => {
console.log('Repository Data', repos);
promise.getCommits(repos[0]);
})
.then(commits => {
console.log('Commits', commits)
})
However the follwing approach works fine:
const promise = require('./promise-definitions');
promise.getUser(10)
.then((user)=> {
console.log('User Data', user);
return promise.getRepos(user.userName);
})
.then((repos) => {
console.log('Repository Data', repos);
return promise.getCommits(repos[0]);
})
.then(commits => {
console.log('Commits', commits)
})
Iam using a return statement when calling the individual promises and it is working fine.Can anyone please point out the reason for this.

Can anyone please point out the reason for this.
At the highest level, it's simply because this is how promise chains are designed to work. When you implement it the way they are designed to work, you get an expected result. When you don't, you don't get the result you want.
A .then() handler on a promise chain just calls its callback and looks at the return value. If you supply a callback that returns nothing or returns a non-promise, then the promise chain just keeps right on going. The .then() infrastructure has NO idea at all what's going on inside the callback you supplied. It has no magic powers to know you started some asynchronous operation inside your .then() handler that it should magically wait for. Instead, the ONLY way it knows to wait for something to finish is if you return a promise from the .then() handler. This is how promises are designed.
In your first example, you call some function that returns a promise, but because you don't return the promise from the .then() handler, the parent promise chain does not know anything about your new promise.
You have effectively just created a new branched promise chain that is not connected in any way to the original one. This new promise chain proceeds on its own schedule and does not affect the original promise chain in any way.
In your second code block, you return the new promise from the .then() handler. That properly inserts that new promise into the existing promise chain and the parent promise chains waits for that newly returned promise to fulfill before proceeding with the rest of its chain. This is, obviously, how you properly chain promises together.

How to propagate resolve() in nested promises?

I'm writing a React application and in particular cases I have to resolve nested promises. The code works fine but I can't propagate the resolve() function up to the outer level, thus I'm not able to get the returning value.
This is the code:
writeData(data) {
this.store.dispatch({type: "START_LOADER"})
return new Promise((resolve, reject) => {
this.manager.isDeviceConnected(this.deviceId).then(res => {
this.manager.startDeviceScan(null, null, (error, device) => {
if (device.id === this.deviceId) {
resolve("test") // -> this is propagate correctly
device.connect().then((device) => {
this.store.dispatch({type: "SET_STATUS", payload: "Device is connected!\n"})
return device.discoverAllServicesAndCharacteristics()
}).then((device) => {
device.writeCharacteristicWithoutResponseForService(
data.serviceId,
data.charId,
data.dataToWrite
).then(res => {
resolve("test2") // -> this is not propagated
}).catch(error => {
reject(error.message)
})
}).catch((error) => {
reject(error.message)
});
}
});
}).catch(error => {
reject(error.message)
})
})
}
...
...
async writeAsyncData(data) {
await this.writeData(data)
}
When I call this function:
this.writeAsyncData({data}).then(response => {
// here I expect whatever parameter I have passed to resolve()
console.log(response)
})
In case I leave resolve("test") uncommented I can console.log it without any problem, but if I comment it, the resolve("test2") doesn't show in console.log and response is undefined.
How can I make sure that even the nested parameter of the inner resolve reach the console.log ?

To nest promises properly, you do NOT wrap them in yet another manually created promise. That is an anti-pattern. Instead, you return the inner promises and that will then chain them. Whatever the inner-most promise returns will be the resolved value for the whole chain.
In addition, when you have any asynchronous operations that return callbacks, you must promisify them so that you are doing all your asynchronous control flow with promises and can consistently do proper error handling also. Do not mix plain callbacks with promises. The control flow and, in particular, proper error handling gets very, very difficult. One you start with promises, make all async operations use promises.
While this code is probably simplest with async/await, I'll first show you how you properly chain all your nested promises by returning every single inner promise.
And, to simplify your nested code, it can be flattened so that rather than each level of promise making deeper indentation, you can just return the promise back to the top level and continue processing there.
To summarize these recommendations:
1. Don't wrap existing promises in another manually created promise. That's a promise anti-pattern. Besides being unnecessary, it's very easy to make mistakes with proper error handling and error propagation.
2. Promisify any plain callbacks. This lets you do all your control flow with promises which makes it a lot easier to avoid errors or tricky situations where you don't know how to propagate errors properly.
3. Return all inner promises from within the .then() handlers to properly chain them together. This allows the inner-most return value to be the resolved value of the whole promise chain. It also allows errors to properly propagate all the way up the chain.
4. Flatten the chain. If you have multiple promises chained together, flatten them so you are always returning back to the top level and not creating deeper and deeper nesting. One case where you do have to make things deeper is if you have conditionals in your promise chain (which you do not have here).
Here's your code with those recommendations applied:
// Note: I added a timeout here so it will reject
// if this.deviceId is never found
// to avoid a situation where this promise would never resolve or reject
// This would be better if startDeviceScan() could communicate back when
// it is done with the scan
findDevice(timeout = 5000) {
return new Promise((resolve, reject) => {
const timer = setTimeout(() => {
reject(new Error("findDevice hit timeout before finding match device.id"));
}, timeout);
this.manager.startDeviceScan(null, null, (error, device) => {
if (error) {
reject(error);
clearTimeout(timer);
return
}
if (device.id === this.deviceId) {
resolve(device);
clearTimeout(timer);
}
});
});
}
writeData(data) {
this.store.dispatch({type: "START_LOADER"});
return this.manager.isDeviceConnected(this.deviceId).then(res => {
return this.findDevice();
}).then(device => {
return device.connect();
}).then(device => {
this.store.dispatch({type: "SET_STATUS", payload: "Device is connected!\n"})
return device.discoverAllServicesAndCharacteristics();
}).then(device => {
return device.writeCharacteristicWithoutResponseForService(
data.serviceId,
data.charId,
data.dataToWrite
);
}).then(res => {
return "test2"; // this will be propagated
});
}
Here's a version using async/await:
findDevice(timeout = 5000) {
return new Promise((resolve, reject) => {
const timer = setTimeout(() => {
reject(new Error("findDevice hit timeout before finding match device.id"));
}, timeout);
this.manager.startDeviceScan(null, null, (error, device) => {
if (error) {
reject(error);
clearTimeout(timer);
return
}
if (device.id === this.deviceId) {
resolve(device);
clearTimeout(timer);
}
});
});
}
async writeData(data) {
this.store.dispatch({type: "START_LOADER"});
let res = await this.manager.isDeviceConnected(this.deviceId);
let deviceA = await this.findDevice();
let device = await deviceA.connect();
this.store.dispatch({type: "SET_STATUS", payload: "Device is connected!\n"})
await device.discoverAllServicesAndCharacteristics();
let res = await device.writeCharacteristicWithoutResponseForService(
data.serviceId,
data.charId,
data.dataToWrite
);
return "something"; // final resolved value
}
Note: In your original code, you have two overriding definitions for device. I left that there in the first version of the code here, but changed the first one to deviceA in the second one.
Note: As your code was written, if this.manager.startDeviceScan() never finds a matching device where device.id === this.deviceId, your code will get stuck, never resolving or rejecting. This seems like a hard to find bug waiting to happen. In the absolute worst case, it should have a timeout that would reject if never found, but probably the implementation of startDeviceScan needs to communicate back when the scan is done so the outer code can reject if no matching device found.
Note: I see that you never use the resolved value from this.manager.isDeviceConnected(this.deviceId);. Why is that? Does it reject if the device is not connected. If not, this seems like a no-op (doesn't do anything useful).
Note: You call and wait for device.discoverAllServicesAndCharacteristics();, but you never use any result from it. Why is that?

Understanding explicit promise construction anti pattern

CertainPerformance highlighted in my previous post advised me to avoid the explicit Promise construction antipattern with reference to to following question in stackoverflow
Frankly, Speaking, I am new to JS and node and I haven't used promise a lot. I went and read those article but either I was unable to comprehend or unable to relate or maybe somewhere my understanding of promises have been vague/wrong all together
So I decided to ask this question in a new thread and seek for help.
So what am I doing and why am I doing it
I am creating helper/common function which I could use to keep my code tidy and if in case I want to change anything inside function at anytime, I don't have to manually change every function.
So these are the functions I have made
//Find user by email Address
const findUserByEmail = (emailAddress) => {
return new Promise((resolve, reject) => {
User.findOne({email: emailAddress}).then(response => {
resolve(res)
}).catch(error => {
reject("Error in findUserByEmail", error);
})
})
}
//Create User
const createNewUser = (newUserDetails) => {
return new Promise((resolve, reject) => {
new User({
fullName: newUserDetails.fullName,
email: newUserDetails.email,
image: newUserDetails.image,
gender: newUserDetails.gender,
age: newUserDetails.age
}).save().then((response) => {
resolve(response)
}).catch((error) => {
reject("Problem in Creating New User", error)
})
})
}
Question 1
Now, I am assuming CertainPerformance said the excessive use of promises because I am creating new promise return new Promise((resolve, reject) => { when I am already using promises with mongoose User.findOne({email: emailAddress}).then(response => { ?
But the reason for me to create those promise was, when I call these helper function from anywhere in my app after importing
const { findUserByEmail } = require("./my_db_query");
I would probably want it return a response or throw an error in case of error
findUserByEmail("test#example.com").then(/*...*/).catch(/*...*/);
If I change my above code snippet without adding new promise
function findUserByEmail (email) {
return User.findOne({email: email}).then(currentUser => currentUser).catch(error => error)
}
Question 2
Then I won't probably be able to .then and .catch in findUserByEmail("test#example.com")?
And In API route of App, where I would be calling the findUserByEmail("test#example.com") function, I would want to do something else if there is an error (which would be different for different case and hence I cannot use it in my helper function).
Question 3
Does, it make sense now for doing return new Promise((resolve, reject) => { instead of doing just one return User.findOne( or am I missing something?

Because .findOne already returns a Promise, there's no need to construct a new one with new Promise - instead, just chain onto the existing Promise chain with .then and .catch as needed. Such Promise chains can have any number of .thens and .catchs - just because you consume a Promise with one .then doesn't prevent you from using the same resolve value elsewhere. To illustrate:
makePromise()
.then((result) => {
console.log(result);
// Returning inside a `.then` will pass along the value to the next `.then`:
return result;
})
.then((result) => {
// this `result` will be the same as the one above
});
In other words - there's no need to construct a new Promise every time you want to be able to use another .then. So:
Then I won't probably be able to .then and .catch in findUserByEmail("test#example.com")
isn't correct - you can indeed chain onto the end of an existing Promise with as many .thens and .catches as you want.
Note that a .then which only returns its parameter and does nothing else (such as .then(currentUser => currentUser)) is superfluous - it won't do anything at all. Also note that a .catch will catch Promise rejections and resolve to a resolved Promise. So if you do
function findUserByEmail(email) {
return User.findOne({email: email})
.then(currentUser => currentUser)
.catch(error => error)
}
that catch means that callers of findUserByEmail will not be able to catch errors, because any possible errors were caught in findUserByEmail's catch. Usually, it's a good idea to allow errors to percolate up to the caller of the function, that way you could, for example:
someFunctionThatReturnsPromise('foobar')
.then((result) => {
// everything is normal, send the result
res.send(result);
})
.catch((err) => {
// there was an error, set response status code to 500:
res.status(500).send('there was an error');
})
So, unless your findUserByEmail or createNewUser helper functions need to do something specific when there's an error, it would probably be best just to return the Promise alone:
const findUserByEmail = email => User.findOne(email);
const createNewUser = newUserDetails => new User(newUserDetails).save();
If your helper functions do need to do something when there's an error, then to make sure that the error gets passed along properly to the caller of the function, I'd recommend either throwing the error inside the catch:
const findUserByEmail = email => User.findOne(email)
.catch((err) => {
// error handling - save error text somewhere, do a console.log, etc
throw err;
});
so that you can catch when something else calls findUserByEmail. Otherwise, if you do something like
const findUserByEmail = email => User.findOne(email)
.catch((err) => {
// do something with err
return err;
});
then the caller of findUserByEmail will have to check inside the .then if the result is actually an error, which is weird:
findUserByEmail('foo#bar.com')
.then((result) => {
if (result instanceof Error) {
// do something
} else {
// No errors
}
});
Better to throw the error in findUserByEmail's catch, so that the consumer of findUserByEmail can also .catch.

It never makes sense to create a promise with promise constructor when there's existing promise, that's why it's called promise construction antipattern.
This is a mistake, reject("Error in findUserByEmail", error). reject accepts only 1
argument, which is rejection reason. error will be ignored. It's conventionanl for an error to be Error object and not a string.
The function may be refactored to:
const findUserByEmail = (emailAddress) => {
return User.findOne({email: emailAddress})
.then(response => response) // noop
.catch(error => {
const readableError = new Error('Error in findUserByEmail');
readableError.originalError = error;
throw readableError;
});
})
}
etc.
Antipatterns don't necessary result in bad performance but they result in code smell. They make the code harder to read, maintain and test, also show that a developer may have a poor understanding of the subject.
Promise constructor has some insignificant performance impact. It introduces another level of nesting and contributes to callback hell - promises are supposed to help avoiding it.
If I change my above code snippet without adding new promise <...>
Then I won't probably be able to .then and .catch in findUserByEmail("test#example.com")?
No, a promise can be chained with then(...) and catch(...) (which is syntactic sugar for then(null, ...)) as many times as needed, that's the strong side of the pattern. Notice that catch(err => { return err }) and catch(err => { throw err }) is not the same thing, the former catches an error, the latter rethrows it.

Generate a few Promises without responding to all of them?

I have an action which creates a new Promise and return a resolve:
actions: {
myAction(context, data) {
return new Promise((resolve, reject) => {
this.$http("/api/something").then(response => {
resolve(response);
}, error => {
reject(error);
})
})
}
}
Now, I have two components calling this action (generating two new Promises), but only the second function needs to do another action after the resolve arrives.
firstCall: function() {
this.$store.dispatch("myAction");
}
secondCall: function() {
this.$store.dispatch("myAction").then(response => {
//Do something after receiving new data
}, error => {
console.error("Error")
})
}
Is this a mistake/bad practice to generate a Promise without responding to all of its resolves?

Is this a mistake/bad practice to generate a Promise without responding to all of its resolves?
It's fine not to necessarily process resolutions, but not processing rejections is generally poor practice (and in fact, now generates warnings from up-to-date browsers; and NodeJS will soon be updated [unless it already has been] to terminate its process on an unhandled rejection).
So you want to be sure you catch any errors:
firstCall: function() {
this.$store.dispatch("myAction")
.catch(error => /* something here */);
}
(secondCall is already doing that, with the second argument to then.)
Unrelated, but that code exhibits the promise-creation-antipattern. You don't need a new promise, you already have one. Just:
actions: {
myAction(context, data) {
return this.$http("/api/something");
}
}
That does exactly what your code does, but more efficiently. Even if you're doing something in those then handlers that you've removed for the purposes of the question, since then returns a new promise, you wouldn't need new Promise.

NodeJS: Unhandled promise rejection

I'm having a little problem and after debugged all the app I noticed that this is the file that's causing the problem, returning me a UnhandledPromiseRejection
'use strict'
const connection = require('../models/'),
oracledb = require('oracledb'),
conexion = oracledb.getConnection(connection)
oracledb.outFormat = oracledb.OBJECT;
module.exports = {
index(req, res) {
conexion.then(con => {
return con.execute(
`SELECT id_application, name, description, creation_date ` +
`FROM application `
).then(bucket => {
return con.execute(
`SELECT id_definition, id_application, field_name_original, field_name_new,
column_name, position, id_type_data, field_size, creation_date,
description, filter, visible ` +
`FROM definition `
).then(definitions => {
res.status(200).json(creaJSON(bucket, definitions))
}).catch(error => { return res.status(500).json({'message': error}) })
}).catch(err => { return res.status(500).json({'message': err}) })
}).catch(err => { return res.status(500).json({'message': err}) })
},
create(req, res) {
},
update(req, res) {
}
}
const doRelease = (connection) => {
connection.close((err) => {
if(err) console.error(err.message);
})
}
const creaJSON = (buckets, definitions) => {
var df = new Array()
buckets['rows'].map(obj => {
definitions['rows'].map(def => {
if(obj['ID_APPLICATION'] == def['ID_APPLICATION']) df.push(def)
})
obj['Definitions'] = df
df = []
})
return buckets.rows
}
after the UnhandledPromiseRejection is being followed by: Error: ORA-12170: TNS:Connect timeout occurred
(node:1270) DeprecationWarning: Unhandled promise rejections are deprecated. In the future, promise rejections that are not handled will terminate the Node.JS process with a non-zero exit code.
I already looked for solutions, some says that promises are not catching correctly but I don't see any problem with them. Any other suggestion?
Any help will be welcome.
Thanks

const connection = require('../models/'),
oracledb = require('oracledb'),
conexion = oracledb.getConnection(connection)
is setting conexion to the promise returned by a call to .getConnection made when the entire source file is executed (in response to being required).
conexion has no handlers at this point. Handlers are only added later when the indexmethod of the exported {index, create, update} object is called.
Hence connection timeout in between the source file being required and index being called will produce an unhandled rejection error.
Obviously adding a catch clause such as
conexion = oracledb.getConnection(connection).catch( onRejected)
should fix this error, but how much recovery you want to put into coding onRejected is up to you.
Edit:
A less obvious approach to satisfying V8's version of how to handle uncaught promise rejection is to provide a dummy handler to thwart it:
conexion = oracledb.getConnection(connection);
conexion.catch(()=>undefined); // a do nothing catch handler.
Here the second line adds a handler to the conexion promise, making it "handled", which prevents it ever becoming an uncaught promise rejection. The promise returned by catch is superfluous and not recorded, but will be fulfilled if the no-operation catch handler is ever called.
Now the promise held in conexion can be rejected before index is called without generating an exception. Whether this is the best way to code promise topology for a particular application is a different question - you may very well wish to address a connection timeout earlier.