node js handling asynchronous if-else - javascript

I'm runningif-else block of code as shown below :
if (condition){
var id = data1.id ;
}
else
var id = data2.id ;
And i'm using this id for some other operation as :
if (id==some value){
do something ;
}
The above operation is performed when a user registers to the application or if the already logged in user wants views a certain page. Since, node.js is asynchronous, when the user register and the result is stored in database, the id field is undefined and in the 2ndif condition the value of id becomes undefined. But if the user is already logged in this works perfectly as there is not much time consuming operation done. So can anyone suggest me some way to resolve the above issue. Any help or guidance will be highly appreciated.

If any part of your function or if statement is asynchronous, then you need to design the whole function to be asynchronous and return a result either via a callback or via a returned promise. Then, even if the if statement doesn't need to do something asynchronous, you still return the value asynchronously so the caller always gets the response the same way regardless of how it was retrieved. Promises are great for this.
Here's a typical function that handles either a synchronous result or an asynchronous result by designing for an asynchronous result by returning a promise that resolves with the value.
let cache = {};
function getValue(key) {
// check cache to see if we already have the value
let val = cache[key];
if (val !== undefined) {
// already have value, return promise that resolves with this value
return Promise.resolve(val);
} else {
return new Promise(function(resolve, reject) {
db.getValue(key, function(err, val) {
if (err) {
reject(err);
} else {
// save value in cache
cache[key] = val;
resolve(val);
}
});
});
}
}
getValue(someKey).then(function(val) {
// use val here whether it was in the cache or not
}).catch(function(err) {
// handle error here
});

Related

Javascript asynchronous function not returning

I'm trying to filter documents inside an array. I have a asynchronous getter function that gets a user, and inside that i should return the document if the user is the same as the current session user.
The problem is that the return function doesn't work inside that getter, since it returns a promise. I don't understand Synch and Async very well but if i put it outside like this:
documents = documents.filter(document => {
if(document.user.id != this.user.id) {
return document
}
})
It returns just fine
documents = documents.filter(document => {
if(document.user.id != this.user.id) {
getter.getNextUserToSign(document.id).then(res => {
if( res != null) {
//if next user == session user
if(res.id == this.user.id){
/*This log shows*/
console.log("is equal to " + res.id)
/*but this return doesnt work*/
return document
}else{
console.log(res.id + " is not equal")
}
}else{
console.log("is null")
}
})
}
})
It goes until the log saying "is equal to {id}" but the document is not returned.
As #JClassic said, you're returning from the inner anonymous function, not the main function. If getNextUserToSign is async and returns a promise, as it looks like it does, then the filter (which is synchronous) will continue iterating even before the first call to that is complete.
What you can do instead is a bit of refactoring. Instead of directly filtering your document array, you can map it onto those getNextUserToSign promises, and then use Promise.all() to wait for all of those promises to resolve (i.e. complete). The .then callback on the Promise.all will be passed an array that's basically your res values for each of the documents in the same order, and then you can filter the main array based on its corresponding values in the results array.
So a refactor like that might look something like this (I took out your console.logs for clarity, you can put them back if you want them):
documents = documents.filter(document => document.user.id !== this.user.id);
const promises = documents.map(document => getter.getNextUserToSign(document.id));
Promise.all(promises).then(results => {
documents = documents.filter((document, i) => {
return results[i] && results[i].id === this.$session.get('user').id;
});
});
So first it filters out the documents that have the current user's ID, since that doesn't require async. Then it maps the remainders to their promises. Then, when all the promises have resolved, it filters the documents array based on the results from those promises.
This is not a scoping issue but more of a trying to handle an asynchronous operation in synchronous manner.
You are using a getter which returns a promise which is an asynchronous operation and while you're expecting your getter to return you your document, it returns a promise instead.
In order to solve it you can use a combination of async/await and promises to resolve your document, something like this probably!
documents = documents.filter(async document => {
return await new Promise((resolve, reject) => {
if (document.user.id !== this.user.id) {
getter.getNextUserToSign(document.id).then(res => {
if (res !== null) {
if (res.id == this.$session.get("user").id) {
resolve(document);
}
}
reject(res.id + " is not equal");
});
}
});
});

Problem with Promises and code timeout when inserting data into a collection

I am trying to verify if a object already exists in a wix collection and if it does cancel the inset() call to the database
import wixData from "wix-data";
export function Memberships_beforeInsert(item, context) {
var name = item.firstName + item.lastName;
name = name.toLowerCase();
wixData.query(context.collectionName)
.find()
.then((res) => {
var members = res.items;
var len = res.length;
console.log(len)
for (var i = 0; i < len; i++) {
let member = members[i];
let memberName = member.firstName + member.lastName;
memberName = memberName.toLowerCase();
if (memberName === name) {
let toUpdate = {
'_id': member._id
}
wixData.update(context.collectionName, toUpdate)
return null;
}
return item;
}
});
//toHere
}
Im fairly new to and wixCode but I was expecting this to wait until the .then() is called then return as follows, but due to wixCode utilizing promises, the code goes immediately to the //toHere section of the code in which it dosent find a return and dismisses the call. Which adds the data to the database instead of returning null.
OK so when we look at your code it seems that you want to find a matching record to the one you are trying to insert to Memberships and then abort the insert and execute an update if one exists. The better way to do this is to look for the specific record match using the query .eq() function. If this finds a matching record then you can update otherwise continue with the insert. See below.
So quickly what is a promise?
In layman's terms think of a Promise like a FedEx tracking code.
When you call a promise based function to ask it to do something you are given back a Promise that what you asked for will be done or that you will be told if a problem occurred.
Just like a fed ex tracking code - you don't know if something has arrived (your function has done what you want it to) unless you check the tracking status using the code OR the fedex delivery van arrives and you receive the package. At which point the tracking code is updated to say the package was delivered and you might get a text or email saying it has been delivered.
A Promise function either completes successfully and you get a result in the then function call OR if an error occurs a catch() event is triggered. So All Promise based functions are similar to if then else conditional test only they look like this:
sendFedEx(package) // Call doesn't resolve immediately you have to wait for fedEx!
.then((fedExDeliveryInfo) => {
//Your package arrived!
})
.catch((fedExLostPackageInfo) => {
//Your package got lost :-(
});
In your code you do a case insensitive compare outside of the data collection. This is probably going to get resource intensive with a large data collection. A better way would be to store the case insensitive string:
(item.firstName + item.lastName).toLowerCase() in the data record and then use it for your query using .eq. That way you let the data collection do its job and simplify your code.
Note: This makes use of the fact that beforeInsert() returns a Promise.
Syntax:
function beforeInsert(item: Object, context: HookContext): Promise
Here is a modified suggestion for you with lots of comments!
import wixData from "wix-data";
export function Memberships_beforeInsert(item, context) {
// Memberships_beforeInsert returns a promise. So does
// wixData.query...find() so we simply return it to maintain the Promise
// chain.
var compareName = (item.firstName + item.lastName).toLowerCase();
// Add a compareName column value to item for future querying
// This will be inserted into the data collection
item.compareName = compareName;
//-------------------------------------------------------//
// This is the head of the Promise chain we need to return
//-------------------------------------------------------//
return wixData.query(context.collectionName)
.eq('compareName', item.compareName) // Query the compareName
.find()
.then((res) => {
var members = res.items;
var len = res.length;
console.log(len);
// Should only have one record or no records otherwise we have a
// problem with this code :-)
// So if we have too many we throw and error. This will be caught in
// an outer catch if we have one
if (len > 1) {
throw Error(`Internal Error! Too many records for ${item.firstName} ${item.lastName}`);
}
// If we get here we have a valid record OR we need to return a result
// To do this we will use a return variable set to the item which
// assumes we will insert item. This will be overridden with the save
// Promise if we have a record already
var result = item;
if (len === 1) {
// We have a record already so we need to update it and return null
// to the caller of beforeInsert to halt the insert. This is a
// Simple case of adding the _id of the found record to the item we
// have been given.
item['_id'] = member._id;
// Again remember we are using promises so we need to return the
// wixData.update result which is a promise.
result = wixData.update(context.collectionName, toUpdate)
.then((savedRecord) => {
// Now we have chained the update to the beforeInsert response
// This is where we can tell the Insert function to abort by
// returning null.
return null;
});
}
// Now we can return the result we have determined to the caller
return result;
});
}
This should do what you are trying to accomplish.

Avoiding javascript callback and promise hell

I have many asynchronous methods to execute and my program flows can change a lot depending on each method return. The logic below is one example. I could not write it in a easy-to-read way using Promises. How would you write it?
Ps: more complex flows are welcome.
Ps2: is_business is a predefined flag where we say whether we are writing a "business user" or a "person user".
begin transaction
update users
if updated
if is_business
update_business
if not updated
insert business
end if
else
delete business
end if
else
if upsert
insert user
if is_business
insert business
end if
end if
end if
commit transaction
The nice thing about promises is that they make a simple analogy between synchronous code and asynchronous code. To illustrate (using the Q library):
Synchronous:
var thisReturnsAValue = function() {
var result = mySynchronousFunction();
if(result) {
return getOneValue();
} else {
return getAnotherValue();
}
};
try {
var value = thisReturnsAValue();
console.log(value);
} catch(err) {
console.error(err);
}
Asynchronous:
var Q = require('q');
var thisReturnsAPromiseForAValue = function() {
return Q.Promise(function() {
return myAsynchronousFunction().then(function(result) {
if(result) {
// Even getOneValue() would work here, because a non-promise
// value is automatically cast to a pre-resolved promise
return getOneValueAsynchronously();
} else {
return getAnotherValueAsynchronously();
}
});
});
};
thisReturnsAPromiseForAValue().then(function(value) {
console.log(value);
}, function(err) {
console.error(err);
});
You just need to get used to the idea that return values are always accessed as arguments to then-callbacks, and that chaining promises equates to composing function calls (f(g(h(x)))) or otherwise executing functions in sequence (var x2 = h(x); var x3 = g(x2);). That's essentially it! Things get a little tricky when you introduce branches, but you can figure out what to do from these first principles. Because then-callbacks accept promises as return values, you can mutate a value you got asynchronously by returning another promise for an asynchronous operation which resolves to a new value based on the old one, and the parent promise will not resolve until the new one resolves! And, of course, you can return these promises from within if-else branches.
The other really nice thing illustrated in the example above is that promises (at least ones that are compliant with Promises/A+) handle exceptions in an equally analogous way. The first error "raised" bypasses the non-error callbacks and bubbles up to the first available error callback, much like a try-catch block.
For what it's worth, I think trying to mimic this behavior using hand-crafted Node.js-style callbacks and the async library is its own special kind of hell :).
Following these guidelines your code would become (assuming all functions are async and return promises):
beginTransaction().then(function() {
// beginTransaction() has run
return updateUsers(); // resolves the boolean value `updated`
}).then(function(updated) {
// updateUsers() has "returned" `updated`
if(updated) {
if(isBusiness) {
return updateBusiness().then(function(updated) {
if(!updated) {
return insertBusiness();
}
// It's okay if we don't return anything -- it will
// result in a promise which immediately resolves to
// `undefined`, which is a no-op, just like a missing
// else-branch
});
} else {
return deleteBusiness();
}
} else {
if(upsert) {
return insertUser().then(function() {
if(isBusiness) {
return insertBusiness();
}
});
}
}
}).then(function() {
return commitTransaction();
}).done(function() {
console.log('all done!');
}, function(err) {
console.error(err);
});
The solution is a mix of #mooiamaduck answer and #Kevin comment.
Using promises, ES6 generators and co library makes the code much clearer. I found a good example when reading a postgresql node library example (pg). In the example below pool.connect and client.query are asynchronous operations that returns Promises. We can easily add an if/else after geting result and then make more async operations keeping code looking like synchronous.
co(function * () {
var client = yield pool.connect()
try {
yield client.query('BEGIN')
var result = yield client.query('SELECT $1::text as name', ['foo'])
yield client.query('INSERT INTO something(name) VALUES($1)', [result.rows[0].name])
yield client.query('COMMIT')
client.release()
} catch(e) {
// pass truthy value to release to destroy the client
// instead of returning it to the pool
// the pool will create a new client next time
// this will also roll back the transaction within postgres
client.release(true)
}
})

JavaScript: Promise chaining in foreach loop

I'm new to javascript promises and am having difficulty using them with a collection of elements. Within the collection, I perform an operation which returns a promise. Once the entire operation (including all the Promises in the collection) has completed, I need to perform another set of operations. The promises within the collection need to go in sequence.
I have tried the following approach:
public cleanup(onCleanupComplete: any): void {
if (this._app == null) return; //this._app comes out of an external API
// Below line of code won't compile, it is just for illustration.
// I'm trying to show that I need a promise in return from method
this.removeConference(0).then(() => {
// Do additional clean up operation and call onCleanupComplete
onCleanupComplete(true, null);
});
}
private removeConference(i : number) {
if (this._app.conversationsManager.conversations == null
|| i === this.conversationLength)
return; // this.conversationLength equals initial
// number of elements in collection
// How do I return a promise here?
var conversation = this._app.conversationsManager.conversations(0);
console.log("app.cleanup.leave", `Leaving conversation ${conversation}`);
conversation.leave().then(() => {
console.log("app.cleanup.leave", `Conversation ${conversation} left successfully`);
this.app.conversationsManager.conversations.remove(conversation);
_ this.removeConference(i);
});
}
What should I return from removeConference once all the conversations in collection are removed?
So this is something that nabbed me early on in understanding promises. You need to get ALL of your code away from the practice of passing in a callback, and then simply using the promise to call that. Instead, to keep the continuous nature of promises, you want to just return promises to the calling function, unless your function is the one that should decide what to do afterward. So, your code should look something like this.
public cleanup(onCleanupComplete: any):Promise<any> {
if (this._app == null) return; //this._app comes out of an external API
// Below line of code won't compile, it is just for illustration.
// I'm trying to show that I need a promise in return from method
var promiseArray = [];
for (var i = 0; i < this.conversationLength; i++) {
promiseArray.push(removeConference(i));
}
return Promise.all(promiseArray);
}
private removeConference(i : number):Promise<any> {
if (this._app.conversationsManager.conversations == null
|| i === this.conversationLength)
return Promise.resolve(true);
var conversation = this._app.conversationsManager.conversations(0);
console.log("app.cleanup.leave", `Leaving conversation ${conversation}`);
return conversation.leave().then(() => {
console.log("app.cleanup.leave", `Conversation ${conversation} left successfully`);
this.app.conversationsManager.conversations.remove(conversation);
this.removeConference(i);
});
}
I'm not 100% sure this compiles correctly, but hopefully it helps conceptually. Promise.all is really the key function here - it takes an array of promises, and creates a matching "control promise" that only resolves when all of them have.

Issue in returning data retrieved from DB queries called in the loop

I making multiple mongoDB queries in loop. and want to send the all results as one data array.But when I simple use the return for send the data it simply return undefined and do not wait for results of all DB request. I also tried to use q.moulde but same issue.
Code:
var getPrayerInCat = function(data){
var result ;
var finalData = [];
if(data.length >0){
data.forEach(function(data2){
var id= data2.id;
Prayer.find({prayerCat:id},function(err,prayer){
var deferred = Q.defer()
if (err) { // ...
console.log('An error has occurred');
// res.send(err);
result= finalData = err
} else {
if(!prayer){
// console.log(data2.id+'--0');
data2.prayersCount = 0;
result = deferred.resolve(finalData.push(data2))
} else {
// console.log(data2.id+'--'+prayer.length);
data2.prayersCount = prayer.length;
// console.log(prayer)
result = deferred.resolve(finalData.push(data2))
} // else for data forward
}
deferred.promise;
})
// deferred.resolve(finalData);
})
/*if(finalData.length > 0) { return finalData;}*/
}
}
finalData is returned undefined.
Let's start with the general rule for using promises:
Every function that does something asynchronous must return a promise
Which functions are these in your case? It's getPrayerInCat, the forEach callback, and Prayer.find.
Hm, Prayer.find doesn't return a promise, and it's a library function so we cannot modify it. Rule 2 comes into play:
Create an immediate wrapper for every function that doesn't
In our case that's easy with Q's node-interfacing helpers:
var find = Q.nbind(Prayer.find, Prayer);
Now we have only promises around, and do no more need any deferreds. Third rule comes into play:
Everything that does something with an async result goes into a .then callback
…and returns the result. Hell, that result can even be a promise if "something" was asynchronous! With this, we can write the complete callback function:
function getPrayerCount(data2) {
var id = data2.id;
return find({prayerCat:id})
// ^^^^^^ Rule 1
.then(function(prayer) {
// ^^^^^ Rule 3
if (!prayer)
data2.prayersCount = 0;
else
data2.prayersCount = prayer.length;
return data2;
// ^^^^^^ Rule 3b
});
}
Now, we have something a bit more complicated: a loop. Repeatedly calling getPrayerCount() will get us multiple promises, whose asynchronous tasks run in parallel and resolve in unknown order. We want to wait for all of them - i.e. get a promise that resolves with all results when each of the tasks has finished.
For such complicated tasks, don't try to come up with your own solution:
Check the API of your library
And there we find Q.all, which does exactly this. Writing getPrayerInCat is a breeze now:
function getPrayerInCat(data) {
var promises = data.map(getPrayerCount); // don't use forEach, we get something back
return Q.all(promises);
// ^^^^^^ Rule 1
}
If we needed to do anything with the array that Q.all resolves to, just apply Rule 3.

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