after making an httpinvoke call, I need to load couchDB but the promise is not passing on.
createDB: function() {
var db = new PouchDB('options');
db.info().then(function (info){
if(info.doc_count <= 10000) {
var db = new PouchDB('options');
db.destroy().then(function(info){
httpinvoke('http://localhost:9080/secure/sync.form','GET');
}).then(function (res){
console.log(JSON.stringify(res)); //This never gets called but if I move this then() block to httpinvoke(xxx).then() it does get called
}).catch(function(err){
console.log(JSON.stringify(err));
});
}
});
}
Promises chain by return values. If you want to make anything meaningful with a promise you have to return it. A promise represents a value + time, your httpinvoke call doesn't return is similar to a synchronus function not returning.
createDB: function() {
var db = new PouchDB('options');
db.info().then(function (info){
if(info.doc_count <= 10000) {
var db = new PouchDB('options');
db.destroy().then(function(info){
return httpinvoke('...','GET'); // NOTE THE RETURN
}).then(function (res){
console.log(res); // console already shows JSON
}); // no need to `catch anyway`, errors are errors let's not suppress.
}
});
}
Also note that promise
Related
I'm learning NodeJS and I have the following code:
var test = '';
function test2(name,callback) {
UserData
.findOne({'token': name})
.then(function(user) {
test = user.token;
console.log('current: '+user.token);
return callback(user.token);
})
.catch(function(err) {
console.log(err);
});
}
var isAuthenticated = function(req,res,next){
test2(req.cookies.remember_me, function(user) {test=user; });
console.log('test:::: '+test);
var isLog = false;
if(req.session.user!= undefined && req.session.user===test){
isLog=true;
}
if(req.cookies.remember_me ===test){
console.log('test'+test);
isLog=true;
}
if(isLog){
return 1;
}else
{
console.log('not auth');
return -1;
}
}
and the result is :
test:::: P9Ysq2oSCHy1RVyWsePxJhpEYLD81qOiIayTyiNJCnOkmllvEspwrDAW8tD9rmfJ
not auth
current: k8LJcCty6568QpXNS3urBedlJ0MDfEYlbOqo9Q7tQi9EOyeSkyesgHHzUjBhDgZx
I know it's bcause if the async nature of NodeJS but how can i make test to be always the same as 'current';
Thank you.
You are making a pretty classic mistake of expecting code to run in the order written, but it doesn't because of the async nature of javascript. For example:
test2(req.cookies.remember_me, function(user) {test=user; });
console.log('test:::: '+test);
Here your console.log() will run before the callback because the callback only happens after you've heard back from the DB. (Although it's not clear where that test value ('P9Ysq2oSCH...') is coming from.
As long as you're learning Node, you should start by trying to avoiding mixing callbacks and promises. Your findOne() function returns a promise, which is why you can call then() on it. You should just return this promise and then call then() in the calling function:
function test2(name) {
// return is important - you're returning the promise which you will use later.
return UserData.findOne({'token': name})
.then(function(user) {
return user.token;
})
.catch(function(err) {
console.log(err);
});
}
function isAuthenticated(req,res,next){
return test2(req.cookies.remember_me)
.then(function(token) {
console.log('test:::: '+token);
var isLog = false;
if(req.session.user!= undefined && req.session.user===token){
isLog=true;
}
if(req.cookies.remember_me ===token){
isLog=true;
}
return isLog
})
}
// Use the function
isAuthenticated(req,res,next)
.then(function(isLoggedin){
if(isLoggedin) {
// user logged in
} else {
// not logged in
}
})
Here is my Code:
Checking if user is folowing official Twitter Account (here I've returned new Promise
var isFollowing = function(sender) {
return new promise(function(resolve, reject) {
var following = false;
var params = {
source_id: sender,
target_screen_name: config.get('twitter.official_account')
};
TwitObj.get('friendships/show', params, function(err, data) {
if (!err) {
following = data.relationship.source.following;
resolve(following);
} else {
reject(err);
}
});
});
};
Validation:
var validateMsg = function(dm) {
var sender = getSender(dm);
var result = [];
isFollowing(sender.id)
.then(function(response) {
if (!response) {
result = interactiveMessage(false, lang.__('FOLLOW_MAIN', sender.name));
console.log(result); // Displays as expected
return result; // Not returning value Do I need to return promise again? If yes, WHY?
}
});
};
Main Implementation:
var direct_message = function(msg) {
verifyAccount().catch(function(err) {
console.error(err);
});
var dm = msg.direct_message;
var result = validateMsg(dm);
console.log(result);
};
Questions is how should I force validateMsg function to return result variable inside then function.
Update: While debugging, I got to know that, console.log(response) in
validation function is displaying later after throwing undefined in
"then" function which means program is not able to get response
immediately and due to async nature, I/O is not getting blocked. How
to tackle this?
You're not actually returning anything in the validateMsg function. You're returning a synchronous value ( result ) in the .then function which is a separate function.
The second thing to consider is that you're expecting the validateMsg function, which is asynchronous to behave synchronously.
var result = validateMsg(dm);
The way to achieve what I believe you're looking to do involves making the following changes:
1) Return the promise chain in the validateMsg function.
var validateMsg = function(dm) {
var sender = getSender(dm);
var result = [];
return isFollowing(sender.id)
.then(function(response) {
if (!response) {
result = interactiveMessage(false, lang.__('FOLLOW_MAIN', sender.name));
return result;
}
// Not sure what you want to do here if there is a response!
// At the moment you're not doing anything if the validation
// is successful!
return response;
});
};
2) Change your function call, since you're now returning a promise.
validateMsg(dm).then( function( result ){
console.log( result );
})
3) Consider adding a catch somewhere if only to help you debug :)
validateMsg(dm).then( function( result ){
console.log( result );
})
.catch( function( err ){
console.log( "Err:::", err );
});
The validationMsg function does not have a return value, thus the result in the main function has the value undefined. Try to put return in front of isFollowing so the function returns a promise, then treat validationMsg as a promise.
Let's say I have 3 files.
index.js makes a call to the backend like this
$.post('/test/', data, function(response) {
//handle success here
})
routes.js handles the route like this
app.post('/test/', function(req, res){
item.getItems(function(response){
res.json(response);
});
});
items.js is the model which accesses the database and makes a POST request for each item
function getItems(callback) {
database.query('SELECT * from items', function(result){
result.forEach(function(item){
request.post('/api/', item, function(req, res) {
//finished posting item
});
});
});
//callback here doesnt wait for calls to finish
}
where/when should I call the callback passed to getItems() to handle a success/failure in index.js?
Because your request.post() operations are async, you have to use some method of keeping track of when they are all done and then you can call your callback. There are multiple ways of doing that. I'll outline a few:
Manually Keeping Track of Count of Request Operations
function getItems(callback) {
database.query('SELECT * from items', function(result){
var remaining = result.length;
result.forEach(function(item){
request.post('/api/', item, function(err, res) {
--remaining;
//finished posting item
if (remaining === 0) {
callback();
}
});
});
});
}
The main problem with doing this manually is that propagating error in nested async operations is difficult when you get to actually figuring out how you're going to handle errors. This is much easier in the other methods shown here.
Using Promises
// load Bluebird promise library
var Promise = require('bluebird');
// promisify async operations
Promise.promisifyAll(request);
function queryAsync(query) {
return new Promise(function(resolve, reject) {
// this needs proper error handling from the database query
database.query('SELECT * from items', function(result){
resolve(result);
});
});
}
function getItems(callback) {
return queryAsync('SELECT * from items').then(function(result) {
return Promise.map(result, function(item) {
return request.postAsync('/api/', item);
});
});
}
getItems.then(function(results) {
// success here
}, function(err) {
// error here
})
It seems strange that you're making an API request in your server-side code, unless this is some sort of middle tier code that interacts with the API... but you're interacting with a database, so I'm still confused on why you can't just do a database insert, or have a bulk insert API call?
Anyway, if you must do it the way you're asking, I've done this in the past with a recursive method that trims down the result array... I really don't know if this is a good approach, so I'd like to hear any feedback. Something like this:
function recursiveResult(result, successfulResults, callback) {
var item = result.shift();
// if item is undefined, then we've hit the end of the array, so we'll call the original callback
if (item !== undefined) {
console.log(item, result);
// do the POST in here, and in its callback, call recursiveResult(result, successfulResults, callback);
successfulResults.push(item);
return recursiveResult(result, successfulResults, callback);
}
// make sure callback is defined, otherwise, server will crash
else if (callback) {
return callback(successfulResults);
}
else {
// log error... callback undefined
}
}
function getItems(callback) {
var successfulResults = [];
var result = [1, 2, 3, 4];
recursiveResult(result, successfulResults, callback);
}
console.log('starting');
getItems(function(finalResult) {
console.log('done', finalResult);
});
Sorry for my Title, I don't know what can I put.
Can you help me please, I would like to print data from a "then" in a "then" ?
Thank you
models.book.find()
.then( function (content) {
var i = 0;
while (content[i]) {
models.author.findOne({"_id": content[i].author_id}, function(err, data) {
console.log(data); //here, it' good
content[i] = data;
MY_DATA = content;
return MY_DATA;
});
i++;
};
})
.then(function (result) {
console.log(result); // here I would like to print MY_DATA
});
There are a number of problems with your code, and I don't think it's behaving as you're expecting it to.
Chaining Promises
In order to effectively chain promises how you're expecting, each promise callback needs to return another promise. Here's an example with yours changed around a bit.
var promise = models.book.find().exec(); // This returns a promise
// Let's hook into the promise returned from
var promise2 = promise.then( function (books) {
// Let's only get the author for the first book for simplicity sake
return models.author.findOne({ "_id": books[0].author_id }).exec();
});
promise2.then( function (author) {
// Do something with the author
});
In your example, you're not returning anything with your callback (return MY_DATA is returning within the models.author.findOne callback, so nothing happens), so it's not behaving as you're expecting it to.
model.author.findOne is asynchronous
model.author.findOne is asynchronous, so you can't expect to call it multiple times in the callback without handling them asynchronously.
// This code will return an empty array
models.book.find( function (err, books) {
var i = 0, results = [];
while (books[i]) {
models.author.findOne({ "_id": books[i].author_id}, function (err, data) {
// This will get called long after results is returned
results.push(data);
});
i++;
};
return results; // Returns an empty array
});
Handling multiple promises
Mongoose uses mpromise, and I don't see a method to handle multiple promises together, but here's a way your case could be done.
var Promise = require('mpromise');
models.book.find().exec()
.then( function (books) {
var i = 0,
count = 0,
promise = new Promise(),
results = [];
while (books[i]) {
models.author.findOne({ "_id": books[i].author_id }, function (err, author) {
results.push(author);
count++;
// Keep doing this until you get to the last one
if (count === books.length) {
// Fulfill the promise to get to the next then
return promise.fulfill(results);
}
return;
});
}
return promise;
})
.then( function (results) {
// Do something with results
});
I don't know if this will work exactly like it is, but it should give you an idea of what needs to be done.
I am using bluebird for promises.
I am trying to promisify the download module.
Here is my implementation:
Promise = require('bluebird'),
download = require('download');
var methodNameToPromisify = ["download"];
function EventEmitterPromisifier(originalMethod) {
// return a function
return function promisified() {
var args = [].slice.call(arguments);
// Needed so that the original method can be called with the correct receiver
var self = this;
// which returns a promise
return new Promise(function(resolve, reject) {
// We call the originalMethod here because if it throws,
// it will reject the returned promise with the thrown error
var emitter = originalMethod.apply(self, args);
emitter
.on("response", function(data) {
resolve(data);
})
.on("data ", function(data) {
resolve(data);
})
.on("error", function(err) {
reject(err);
})
.on("close", function() {
resolve();
});
});
};
};
download = { download: download };
Promise.promisifyAll(download, {
filter: function(name) {
return methodNameToPromisify.indexOf(name) > -1;
},
promisifier: EventEmitterPromisifier
});
Then using it:
return download.downloadAsync(fileURL, copyTo, {});
My problem is that it doesn't download all of the files (I have a list sent to this function), what am I doing wrong?
An emitter does emit multiple data events, one for every chunk it receives. However, a represents only one future value, in your case you want that to be the complete response.
resolve is supposed to be called only once, to fulfill the promise with the passed value, which is then settled. Further calls will have no effect - and that's why you get only the first parts of your list.
Instead, you will need to accumulate all the data, and when the stream ends you can fulfill the promise with all of it.
var Promise = require('bluebird'),
download = require('download'),
Buffer = require('buffer'); // should be global anyway
exports = {
downloadAsync: function promisifiedDownload() {
var args = arguments, self = this;
return new Promise(function(resolve, reject) {
// We call the originalMethod here because if it throws,
// it will reject the returned promise with the thrown error
var emitter = download.apply(self, args);
var buffers = [];
emitter.on("data", function(data) {
buffers.push(data);
}).on("error", function(err) {
reject(err);
}).on("close", function() {
resolve(Buffer.concat(buffers));
});
});
};
};
Notice it's quite nonsensical to use promisifyAll when you only want to promisify a single method. I've omitted it for simplicity
You might also listen for the incoming response object, and attach the data listener directly to it. You can then use the end event instead of close.