Develop Reference

Calling a function with a promise into a foreach

I'm getting this error:
"lambdaCallbackWith": [
"TypeError: #<Promise> is not a function",
"at Array.forEach (<anonymous>)",
When i ran the code below:
var resizeVolume = function* (event, context) {
var co = require('co');
volumes.forEach(co(function *(volume) {
yield eC2.setNewVolume(volume.VolumeId, volumeSize);
}));
}
The error occurred in foreach, i added the CO but no success.
The code without CO show an error saying the yield is a reserved word.

yield can only be used in a generator function. That's why
volumes.forEach(() => {
yield ...
})
Won't work.
You will need a library to use a generator in a forEach loop such as co-foreach.
If you don't want to you can use a regular for loop like this:
const co = require('co');
var resizeVolume = function* (event, context) {
co.wrap(function *() {
for (let i = 0 ; i < volumes.lenght ; i++) {
const volume = volumes[i];
yield eC2.setNewVolume(volume.VolumeId, volumeSize);
}
});
}

request-promise + co API trigger in protractor test

I want to call module.api.create from my protractor test. Referring this solution:-
Chain multiple Node http request I am using request-promise + co like this:-
//api/module1.js
var co = require('co');
var rp = require('request-promise');
exports.create = co(function* def() {
var response, token;
urlLogin.body.username = username;
response = yield rp(urlLogin);
//extract token and run other APIs
...
}).catch(err => console.log);
And
//api/api.js
var module1= require('./module1'),
exports.module1= function (){
return module1;
};
In my Spec/Test I am adding
api = require('../../api/api');
api.module1.create;
Issue i am facing is than even without calling the "api.module1.create;" line, the require line "api = require('../../api/api');" is calling the create automatically every-time the test is executed

From the co README:
co#4.0.0 has been released, which now relies on promises. It is a stepping stone towards the async/await proposal. The primary API change is how co() is invoked. Before, co returned a "thunk", which you then called with a callback and optional arguments. Now, co() returns a promise.
I believe you're looking for co.wrap, which returns a function that executes the generator and returns a promise (this function may also be known as a thunk). Using just co eagerly executes the generator and returns the result of executing the generator.
const co = require('co')
co(function* () {
// this will run
console.log('hello from plain co!')
})
co.wrap(function* () {
// this won't run because we never call the returned function
console.log('hello from wrapped co 1!')
})
const wrappedfn = co.wrap(function* () {
// this runs because we call the returned function
console.log('hello from wrapped co 2!')
})
wrappedfn()
You can also wrap a function by yourself, which does the same thing as co.wrap and lets you do more stuff afterwards.
exports.create = function() {
return co(function* () {
// this will run only when exports.create is called
console.log('hello from plain co!')
})
// If you want to do stuff after and outside the generator but inside the enclosing function
.then(...)
}

Strapi.io Controller.find().exec(...)

I try to use the controller.find().exec(...) method and under the exec callback function I try to create the body content. But I have no idea what should I do. I think I have to use the yield keyword there but it drops an error if I try to use yield in the callback function. My code looks something like this:
let value;
Firstcontroller.find().exec(function (error, result) {
value = yield result;
});
this.body = value;
I already tried with yield and without, with let and without. And some of the options. Please let me share if you have any idea. I have to use exec because I have to use the result and pass it to a different find method.

Please try this:
// Retrieve the value
const value = yield Firstcontroller.find();
// Set the value to the body
this.body = value;

Generators in KOA

How does work app.use in KOA?
When I set some generator inside app.use, everything works perfect.
How can I do the same elsewhere?
When I just execute generator manual:
var getRelationsList = function *() {
var res = yield db.relations.find({});
console.log({'inside: ': res});
}
console.log({'outside: ': getRelationsList().next()});
getRelationsList().next();
I'm getting just { 'outside: ': { value: [Function], done: false } }
This what I expect is:
{ 'outside: ': { value: {object_with_results}, done: false } }
{ 'inside: ': {object_with_results}
EDIT
I changed my code like that:
var getRelationsList = function *() {
var res = yield db.relations.find({});
console.log({'inside: ': res});
}
console.log({'outside ': co(getRelationsList)});
Now inside console log show's me good results but outside console log shows me just empty object.

Generators are a powerful tool for organizing asynchronous code, but they don't magically wait for asynchronous code to run.
What's Happening
Let's walk through your code so you can see what is happening:
getRelationsList is a generator function, that when called returns a new generator. At this point, no code in your generator function has been called (although if you were passing params they would be set). You then call .next on your generator to start execution of the generator function. It will execute up until it hits the first yield statement and return an object with the yielded value and the completion status of the generator.
It seems you understand most of that so far, but generators do not magically transform the yielded out values. When you yield out db.relations.find({}), you'll get the return value of the find function which I'm assuming is a Promise or some type of thenable:
so your 'outside' value is { value:Promise, done:false }
The reason your inside console.log never ran is that you're actually creating a new generator each time you call getRelationsList(), so when you call getRelationsList().next() again after the outside console.log you're creating a new generator and calling next, so it only executes up to the first yield, just like the call on the previous line.
In order to finish execution you must call next twice on the same instance of your generator: once to execute up to the yield and once to continue execution to the end of the function.
var gen = getRelationsList()
gen.next() // { value:Promise, done:false }
gen.next() // { value:undefined, done:true } (will also console.log inside)
You'll notice, however, if you run this, the inside console.log will be undefined. That's because the value of a yield statement is equal to the value passed to the following .next() call.
For example:
var gen2 = getRelationsList()
gen2.next() // { value:Promise, done:false }
gen2.next(100) // { value:undefined, done:true }
Outputs
{ inside:100 }
Because we passed 100 to the second .next() call and that became the value of the yield db.relations.find({}) statement which was then assigned to res.
Here's a link demoing all of this: http://jsfiddle.net/qj1aszub/2/
The Solution
The creators of koa use a little library called co which basically takes yielded out promises and waits for them to complete before passing the resolved value back into the generator function (using the .next() function) so that you can write your asynchronous code in a synchronous style.
co will return a promise, which will require you to call the .then method on to get the value returned from the generator function.
var co = require('co');
var getRelationsList = function *() {
var res = yield db.relations.find({});
console.log({'inside: ': res});
return res
}
co(getRelationsList).then(function(res) {
console.log({'outside: ': res })
}).catch(function(err){
console.log('something went wrong')
});
co also allows you to yield out other generator functions and wait for their completion, so you don't have to wrap things with co at every level and deal with promises until you're at some sort of 'top level':
co(function *() {
var list = yield getRelationsList()
, processed = yield processRelations(list)
, response = yield request.post('/some/api', { data:processed })
return reponse.data
}).then(function(data) {
console.log('got:', data)
})

Your problem is that you call getRelationsList() function multiple times which is incorrect.
Change your code to following
var g = getRelationsList();
console.log('outside: ', g.next());
g.next(); //You get your console.log('inside: .... here

Generators must be acted upon by outside code.
Under the hood koa use the co library to 'Run' the generator.
Here is how you might achieve what your wanting outside of koa:
var co = require('co');
var getRelationsList = function *() {
var res = yield db.relations.find({});
console.log({'inside: ': res});
}
co(getRelationsList).catch(function(err){});
I did a short screencast on JavaScript generators that should help you understand what's going on:
http://knowthen.com/episode-2-understanding-javascript-generators/
++ EDIT
If your using generators to program in more of an synchronous style (eliminating callbacks), then all your work needs to be done in the generator and you should use a library like co to execute the generator.
Here is a more detailed example of how you would interact with a generator, manually. This should help you understand the results your getting.
function * myGenerator () {
var a = yield 'some value';
return a;
}
var iterator = myGenerator();
// above line just instantiates the generator
console.log(iterator);
// empty object returned
// {}
var res1 = iterator.next();
// calling next() start the generator to either the
// first yield statement or to return.
console.log(res1);
// res1 is an object with 2 attributes
// { value: 'some value', done: false }
// value is whatever value was to the right of the first
// yield statment
// done is an indication that the generator hasn't run
// to completion... ie there is more to do
var toReturn = 'Yield returned: ' + res1.value;
var res2 = iterator.next(toReturn);
// calling next(toReturn) passes the value of
// the variable toReturn as the return of the yield
// so it's returned to the variable a in the generator
console.log(res2);
// res2 is an object with 2 attributes
// { value: 'Yield returned: some value', done: true }
// no further yield statements so the 'value' is whatever
// is returned by the generator.
// since the generator was run to completion
// done is returned as true

Using yield to wait until async code is done

I am trying to learn how to use generators and yield, so I tried the following but it doesn't seem to be working.
I am using the following function, which contains 2 async calls:
var client = require('mongodb').MongoClient;
$db = function*(collection, obj){
var documents;
yield client.connect('mongodb://localhost/test', function*(err, db){
var c = db.collection(collection);
yield c.find(obj).toArray(function(err, docs){
documents = docs;
db.close();
});
});
return documents.length;
};
Then to make the call original call, I am doing this:
var qs = require("querystring");
var query = qs.parse("keywords[]=abc&keywords[]=123");
var total = $db("ads", {"details.keywords": {$in: query["keywords[]"]}});
console.log(total);
When I get my output back in the console, I get this:
{}
I was expecting a number such as 200. What is it that I am doing wrong?

TL;DR
For the short answer, you're looking for a helper like co.
var co = require("co");
co(myGen( )).then(function (result) { });
But Why?
There is nothing inherently asynchronous about ES6 iterators, or the generators which define them.
function * allIntegers ( ) {
var i = 1;
while (true) {
yield i;
i += 1;
}
}
var ints = allIntegers();
ints.next().value; // 1
ints.next().value; // 2
ints.next().value; // 3
The .next( ) method, though, actually lets you send data back in to the iterator.
function * exampleGen ( ) {
var a = yield undefined;
var b = yield a + 1;
return b;
}
var exampleIter = exampleGen();
exampleIter.next().value; // undefined
exampleIter.next(12).value; // 13 (I passed 12 back in, which is assigned to a)
exampleIter.next("Hi").value; // "Hi" is assigned to b, and then returned
It might be confusing to think about, but when you yield it's like a return statement; the left hand side hasn't been assigned the value yet... ...and more importantly, if you had put the var y = (yield x) + 1; the parenthesis are resolved before the rest of the expression... ...so you return, and the +1 is put on hold, until a value comes back.
Then when it arrives (passed in, via the .next( )), the rest of the expression is evaluated (and then assigned to the left hand side).
The object that's returned from each call has two properties { value: ..., done: false }
value is what you've returned/yielded and done is whether or not it's hit the actual return statement at the end of the function (including implicit returns).
This is the part that can then be used to make this async magic happen.
function * asyncGen ( id ) {
var key = yield getKeyPromise( id );
var values = yield getValuesPromise( key );
return values;
}
var asyncProcess = asyncGen( 123 );
var getKey = asyncProcess.next( ).value;
getKey.then(function (key) {
return asyncProcess.next( key ).value;
}).then(function (values) {
doStuff(values);
});
There's no magic.
Instead of returning a value, I'm returning a promise.
When the promise completes, I'm pushing the result back in, using .next( result ), which gets me another promise.
When that promise resolves, I push that back in, using .next( newResult ), et cetera, until I'm done.
Can we do better?
We know now that we're just waiting for promises to resolve, then calling .next on the iterator with the result.
Do we have to know, ahead of time what the iterator looks like, to know when we're done?
Not really.
function coroutine (iterator) {
return new Promise(function (resolve, reject) {
function turnIterator (value) {
var result = iterator.next( value );
if (result.done) {
resolve(result.value);
} else {
result.value.then(turnIterator);
}
}
turnIterator();
};
}
coroutine( myGen ).then(function (result) { });
This isn't complete and perfect. co covers extra bases (making sure all yields get treated like promises, so you don't blow up by passing a non-promise value... ...or allowing arrays of promises to be yielded, which becomes one promise which will return the array of results for that yield ...or try/catch around the promise handling, to throw the error back into the iterator... yes, try/catch works perfectly with yield statements, done this way, thanks to a .throw(err) method on the iterator).
These things aren't hard to implement, but they make the example muddier than it needs to be.
This is exactly why co or some other "coroutine" or "spawn" method is perfect for this stuff.
The guys behind the Express server built KoaJS, using Co as a library, and Koa's middleware system just takes generators in its .use method and does the right thing.
But Wait, there's more!
As of ES7, it's very likely that the spec will add language for this exact use-case.
async function doAsyncProcess (id) {
var key = await getKeyPromise(id);
var values = await getValuesPromise(key);
return values;
}
doAsyncProcess(123).then(values => doStuff(values));
The async and await keywords are used together, to achieve the same functionality as the coroutine-wrapped promise-yielding generator, without all of the external boilerplate (and with engine-level optimizations, eventually).
You can try this today, if you're using a transpiler like BabelJS.
I hope this helps.

Yield and generators have nothing to do with asynchrony, their primary purpose is to produce iterable sequences of values, just like this:
function * gen() {
var i = 0;
while (i < 10) {
yield i++;
}
}
for (var i of gen()) {
console.log(i);
}
Just calling a function with a star (generator function) merely creates generator object (that is why you see {} in console), that can be interacted with using next function.
That said, you can use generator functions as an analogue of asynchronous functions, but you need a special runner, like co.

var client = require('mongodb').MongoClient;
$db = function*(collection, obj){
var documents;
yield client.connect('mongodb://localhost/test', function*(err, db){
var c = db.collection(collection);
yield c.find(obj).toArray(function(err, docs){
documents = docs;
db.close();
});
});
return documents.length;
};
var qs = require("querystring");
var query = qs.parse("keywords[]=abc&keywords[]=123");
var total = $db("ads", {"details.keywords": {$in: query["keywords[]"]}});
console.log(total);
As is, total is the iterator for the $db generator function. You would retrieve its yield values via total.next().value. However, the mongodb library is callback based and as such, its functions do not return values, so yield will return null.
You mentioned you were using Promises elsewhere; I would suggest taking a look at the bluebird in particular its promisify functionality. Promisification inverts the callback model so that the arguments to the callback are now used to resolve the promisified function. Even better, promisifyAll will convert an entire callback based API.
Finally, bluebird provides coroutine functionality as well; however its coroutines must return promises. So, your code may be rewritten as follows:
var mongo = require('mongodb');
var Promise = require('bluebird');
//here we convert the mongodb callback based API to a promised based API
Promise.promisifyAll(mongo);
$db = Promise.coroutine(function*(collection, obj){
//existing functions are converted to promised based versions which have
//the same name with 'Async' appended to them
return yield mongo.MongoClient.connectAsync('mongodb://localhost/test')
.then(function(db){
return db.collectionAsync(collection);})
.then(function(collection) {
return collection.countAsync();});
});
var qs = require("querystring");
var query = qs.parse("keywords[]=abc&keywords[]=123");
$db('ads',{"details.keywords": {$in: query["keywords[]"]}})
.then(console.log)