Like the OP from this question, I want to do a for loop, and do something when all the actions have finished.
I checked the answer, and the async library, but all the solutions involve iterating over an array. I don't want to do something "forEach" element of an array, I don't have an array.
What if I just want to do an operation n times ? For example, say I want to insert n random entries in my database, and do something afterwards ? For now I'm stuck with something like :
function insertMultipleRandomEntries(n_entries,callback){
var sync_i=0;
for(var i=0;i<n_entries;i++){
insertRandomEntry(function(){
if(sync_i==(max-1)){
thingDoneAtTheEnd();
callback(); //watched by another function, do my stuff there
}
else{
sync_i++;
console.log(sync_i+" entries done successfully");
thingDoneEachTime();
}
});
}
}
Which is absolutely horrendous. I can't find anything like a simple for in async, how would you have done this ?
You can use Promises, supported without a library in node.js since version 4.0.
If the callback function of insertRandomEntry has a parameter, you can pass it to resolve. In the function given to then, you receive an array of parameters given to resolve.
function insertMultipleRandomEntries(n_entries,callback){
var promises = [];
for(var i=0;i<n_entries;i++) {
promises.push(new Promise(function (resolve, reject) {
insertRandomEntry(function (val) {
thingDoneEachTime(val);
resolve(val);
});
}));
}
Promise.all(promises).then(function (vals) {
// vals is an array of values given to individual resolve calls
thingDoneAtTheEnd();
callback();
});
}
Related
I have a function that processes an array of data (first parameter) and, once the procesing is finished, it invokes only one time a callback function (second parameter). I'm using forEach to process data item by item, consisting the processing of each item in some checkings and storing the param in database. The function storeInDB() does the storing work and uses a callback (second parameter) when the item has been stored.
A first approach to the code is the following:
function doWork(data, callback) {
data.forEach(function (item) {
// Do some check on item
...
storeInDB(item, function(err) {
// check error etc.
...
callback();
});
});
}
However, it's wrong, as the the callback function will be invoked several times (as many as element in the data array).
I'd like to know how to refactor my code in order to achieve the desired behaviour, i.e. only one invocation to callback once the storing work is finished. I guess that async could help in this task, but I haven't find the right pattern yet to combine async + forEach.
Any help is appreciated!
You can use a library such as async to do this, although I would recommend using promises if possible. For your immediate problem you can use a counter to determine how many storage calls have completed and call the callback when the total number are completed.
let counter = 0;
data.forEach(function (item) {
// Do some check on item
...
storeInDB(item, function(err) {
// check error etc.
counter++
if (counter == data.length) {
callback();
}
});
});
you can also utilize the three parameters passed to the function to execute on each array method
function doWork(data, callback) {
data.forEach(function (value,idx,arr) {
// Do some check on item
...
storeInDB(arr[idx], function(err) {
// check error etc.
...
if ( (idx + 1) === arr.length ) {
callback();
}
});
});
}
If storeInDB function returns a promise, you can push all async functions to an array and use Promise.all. After all tasks run successfully, It will invokes callback function.
Hope this helps you.
function doWork(data, callback) {
let arr = [];
data.map(function(itm) {
// Do some check on item
...
arr.push(storeInDB(item));
});
Promise.all(arr)
.then(function(res) {
callback();
});
}
I have a problem, but I have no idea how would one go around this.
I'm using loopback, but I think I would've face the same problem in mongodb sooner or later. Let me explain what am I doing:
I fetch entries from another REST services, then I prepare entries for my API response (entries are not ready yet, because they don't have id from my database)
Before I send response I want to check if entry exist in database, if it doesn't:
Create it, if it does (determined by source_id):
Use it & update it to newer version
Send response with entries (entries now have database ids assigned to them)
This seems okay, and easy to implement but it's not as far as my knowledge goes. I will try to explain further in code:
//This will not work since there are many async call, and fixedResults will be empty at the end
var fixedResults = [];
//results is array of entries
results.forEach(function(item) {
Entry.findOne({where: {source_id: item.source_id}}, functioN(err, res) {
//Did we find it in database?
if(res === null) {
//Create object, another async call here
fixedResults.push(newObj);
} else {
//Update object, another async call here
fixedResults.push(updatedObj);
}
});
});
callback(null, fixedResults);
Note: I left some of the code out, but I think its pretty self explanatory if you read through it.
So I want to iterate through all objects, create or update them in database, then when all are updated/created, use them. How would I do this?
You can use promises. They are callbacks that will be invoked after some other condition has completed. Here's an example of chaining together promises https://coderwall.com/p/ijy61g.
The q library is a good one - https://github.com/kriskowal/q
This question how to use q.js promises to work with multiple asynchronous operations gives a nice code example of how you might build these up.
This pattern is generically called an 'async map'
var fixedResults = [];
var outstanding = 0;
//results is array of entries
results.forEach(function(item, i) {
Entry.findOne({where: {source_id: item.source_id}}, functioN(err, res) {
outstanding++;
//Did we find it in database?
if(res === null) {
//Create object, another async call here
DoCreateObject(function (err, result) {
if (err) callback(err);
fixedResults[i] = result;
if (--outstanding === 0) callback (null, fixedResults);
});
} else {
//Update object, another async call here
DoOtherCall(function (err, result) {
if(err) callback(err);
fixedResults[i] = result;
if (--outstanding === 0) callback (null, fixedResults);
});
}
});
});
callback(null, fixedResults);
You could use async.map for this. For each element in the array, run the array iterator function doing what you want to do to each element, then run the callback with the result (instead of fixedResults.push), triggering the map callback when all are done. Each iteration ad database call would then be run in parallel.
Mongo has a function called upsert.
http://docs.mongodb.org/manual/reference/method/db.collection.update/
It does exactly what you ask for without needing the checks. You can fire all three requests asnc and just validate the result comes back as true. No need for additional processing.
I have a for-loop statement and an async MongoDB inside loop body. What I want to do is to make a find query from my MongoDB database, and push the result into an Array.
Here is the code:
function() arrResult() {
var arr = [];
for(...) {
collection.find({ foo: i }, function (err, cursor) {
arr.push(cursor);
}
}
return arr;
}
But it's obvious that the return value of the function would be an empty Array.
I want to tackle this problem using Q module. Is there any solutions?
I want to tackle this problem using Q module. Is there any solutions?
Yes, promises are a very easy abstraction to deal with this. You can execute the queries in parallel, and collect their results with all.
In particular, with Q it would look like this:
function arrResult(…) {
var promises = [];
for (…)
promises.push( Q.ninvoke(collection, "find", {foo: i}) );
return Q.all(promises);
}
arrResult(…).then(function(arr) {
…
}, function(err) {
// first error, if any occured
});
You need a sync mechanism that acts like a process gate.
Each returning query has to arrive at the gate, e.g. decrements some counter and deposit its result.
When all arrived at the gate, a final callback does return the collected results.
Using Node.js and the node-postgres module to communicate with a database, I'm attempting to write a function that accepts an array of queries and callbacks and executes them all asynchronously using the same database connection. The function accepts a two-dimensional array and calling it looks like this:
perform_queries_async([
['SELECT COUNT(id) as count FROM ideas', function(result) {
console.log("FUNCTION 1");
}],
["INSERT INTO ideas (name) VALUES ('test')", function(result) {
console.log("FUNCTION 2");
}]
]);
And the function iterates over the array, creating a query for each sub-array, like so:
function perform_queries_async(queries) {
var client = new pg.Client(process.env.DATABASE_URL);
for(var i=0; i<queries.length; i++) {
var q = queries[i];
client.query(q[0], function(err, result) {
if(err) {
console.log(err);
} else {
q[1](result);
}
});
}
client.on('drain', function() {
console.log("drained");
client.end();
});
client.connect();
}
When I ran the above code, I expected to see output like this:
FUNCTION 1
FUNCTION 2
drained
However, the output bizarrely appears like so:
FUNCTION 2
drained
FUNCTION 2
Not only is the second function getting called for both requests, it also seems as though the drain code is getting called before the client's queue of queries is finished running...yet the second query still runs perfectly fine even though the client.end() code ostensibly killed the client once the event is called.
I've been tearing my hair out about this for hours. I tried hardcoding in my sample array (thus removing the for loop), and my code worked as expected, which leads me to believe that there is some problem with my loop that I'm not seeing.
Any ideas on why this might be happening would be greatly appreciated.
The simplest way to properly capture the value of the q variable in a closure in modern JavaScript is to use forEach:
queries.forEach(function(q) {
client.query(q[0], function(err, result) {
if(err) {
console.log(err);
} else {
q[1](result);
}
});
});
If you don't capture the value, your code reflects the last value that q had, as the callback function executed later, in the context of the containing function.
forEach, by using a callback function isolates and captures the value of q so it can be properly evaluated by the inner callback.
A victim of the famous Javascript closure/loop gotcha. See my (and other) answers here:
I am trying to open 10 websocket connections with nodejs, but somehow my loop doesnt work
Basically, at the time your callback is executed, q is set to the last element of the input array. The way around it is to dynamically generate the closure.
It will be good to execute this using async module . It will help you to reuse the code also . and will make the code more readable . I just love the auto function provided by async module
Ref: https://github.com/caolan/async
Trying to find the best-use example of returning an array of data in Node.js with Q library (or any similar library, I'm not partial) when using Firebase .on("child_added");
I've tried using Q.all() but it never seems to wait for the promises to fill before returning. This is my current example:
function getIndex()
{
var deferred = q.defer();
deferred.resolve(new FirebaseIndex( Firebase.child('users').child(user.app_user_id).child('posts'), Firebase.child('posts') ) );
return deferred.promise;
}
function getPost( post )
{
var deferred = q.defer();
deferred.resolve(post.val());
return deferred.promise;
}
function getPosts()
{
var promises = [];
getIndex().then( function (posts) {
posts.on( 'child_added', function (_post) {
promises.push( getPost(_post) );
});
});
return q.all(promises);
}
The problem occurs in getPosts(). It pushes a promise into your array inside an async function--that won't work since q.all is called before the promise objects have been added.
Also, child_added is a real-time event notification. You can't use that as a way to grab "all of the data" because there is no such thing as "all"; the data is constantly changing in real-time environments. FirebaseIndex is also using child_added callbacks internally, so that's not going to work with this use case either.
You can grab all of the posts using the 'value' callback (but not a specific subset of records) as follows:
function getPosts() {
var def = q.defer();
Firebase.child('users').once('value', function(snap) {
var records = [];
snap.forEach(function(ss) {
records.push( ss.val() );
});
def.resolve(records);
});
return def.promise;
}
But at this point, it's time to consider things in terms of real-time environments. Most likely, there is no reason "all" data needs to be present before getting to work.
Consider just grabbing each record as they come in and appending them to whatever DOM or Array where they need to be stored, and working from an event driven model instead of a GET/POST centered approach.
With luck, you can bypass this use case entirely.