I am trying to run a mongoose query and write it to the console without installing yet another library like bluebird. I have been unable to find this in the documentation.
Here is the query:
function activityH(){
return setRecords.find({'item.title': 'marzipan'}
, 'item.title item.quantity').exec();
}
And the context in which I am calling it:
....[a bunch of promises].then(activityF).then(activityG).then(activityH).then(function(doc){console.log(doc);});
All of the prior activities are completing, but nothing is logging even though my query should have results. I feel this is a very basic question but I have looked for hours trying to find a solution and if this is a duplicate, the original answer is very hard to search for!
Do I absolutely need bluebird to do this? E.g. this blog post
Thank you
You could write a little logging function wrapper to help you out. Something like
function log(data) {
console.log(data);
return data;
}
And then add it your Promise chain.
....[a bunch of promises]
.then(log)
.then(activityF)
.then(log)
.then(activityG)
.then(log)
.then(activityH)
.then(log)
If you want some default messaging you could also pass a message string
function log(msg) {
return function(data) {
console.log(msg, data);
return data;
}
}
And then would add to the chain like:
activityA()
.then(log('activityA'))
.then(activityB)
.then(log('activityB'))
Related
I'm trying to get my head around callbacks in Node.JS and it's pretty foreign compared to what I'm used to.
I have the following example from another post on here:
var someOutput;
var myCallback = function(data) {
console.log('got data: '+data);
someOutput = data;
};
var usingItNow = function(callback) {
callback('get it?');
};
//now do something with someOutput outside of the functions
I have added the someOutput variable - I want to get the text 'get it?' (obviously I don't) into there so that I can concatenate it on to a string, but it's not working in any combination I've tried.
I must be missing something fundamental here!
Thank you for the help.
You should call the function:
usingItNow(myCallback);
But anyway it is not a good practice to use callback to set variable and consume later. It will work for you now, but later if callback will be async, it will fail.
Consider using Promise and use someOutput in the then function.
A good Promise package is Bluebird
If you want to do it like a pro, consider using Promise.coroutine
http://bluebirdjs.com/docs/api/promise.coroutine.html
Or async + await if you are using an updated version of Node.js
Look on http://node.green/#ES2017-features-async-functions to check what avilable on your node version
As commented by emil:
"call usingItNow(myCallback);"
doh!
I'm having difficulty finding an answer to my question, perhaps because I don't know how to ask it (what search terms to use). I'm really struggling to understand promises, and have watched a number of tutorial videos and am still not getting some fundamental piece to make it click.
In Node, I am using the request-promise module, which returns a promise when I make a call to an rp() method. I am calling
return rp(service);
from within a function. But what I want to do, instead, is add a .then() handler to this to do some post-processing with the result of the service call, BEFORE returning the promise back to the caller so the caller can still have its own then handler.
How do I accomplish this?
It would be helpful to see your current code to better understand where exactly you are struggling with.
In general, you can think of promises as a placeholder for some future value that you can chain together.
If I understood your question correctly, you would like to make a request to a server, receive the response, do something with the response and return it back to the caller.
const callService = (url) => {
// make sure to return your promise here
return rp(url)
.then((html) => {
// do something with html
return html;
});
};
Now you can call it like so -
callService('http://www.google.com')
.then((html) => {
console.log(html);
}).catch((err) => {
// error handling
});
The common mistake is to omit the return statement in your top level function and subsequent then functions.
Feel free to provide additional details.
This question already has answers here:
Why is my variable unaltered after I modify it inside of a function? - Asynchronous code reference
(7 answers)
Waiting for async call
(1 answer)
Closed 8 years ago.
I have a User model function all that returns all users in an array.
User = {
all: function() {
var users = [];
globalLibrary.db.collection('users').find({}).toArray(function(err, items) {
test.equal(null, err);
users = items;
});
return users;
}
}
I want to ensure that the function doesn't finish or return before the mongodb query is complete.
Currently, this function is just returning [], and querying mongodb asynchronously. I want the function to wait until the function finish query is complete and it returns and array filled with users.
Note:
globalLibrary.db is just a cached mongodb connection.
My solution using promises
Since some people closed the question as a duplicate, I'll write my answer here within the question. Hopefully someone else who is not familiar with asynchronous programming find this useful.
Problem
The point is that you need to use a callback - there's no way to block
on something asynchronous. (...) – Aaron Dufour
The function User.all() I wrote above will return empty array because nothing is stopping the process while the mongodb query is happening. There are some primitive ways to stop the process.
You can crank up some hacky stuff using setTimeout(). This way sucks though because you have to use some arbitrary time that might be higher than the actual time you need to query the mongodb. Simply put, it's slower.
You can also use some event based stuff that #AaronDufour linked in the comment (now deleted). So you can have something a pair of event emitter and a listener to replace setTimeout() way. #Someone points out though that you shouldn't use this in node.js for blocking function.
Now finally, the conventional way of dealing with this problem is using callbacks as pointed out by the answer below. This is fine, but callbacks can quickly get out of control once you start having multiple callbacks stacked inside one another.
I am using https://github.com/kriskowal/q for promises. While using promises doesn't solve all the woes of callbacks, but it looks most like simple synchronous programming style which I think is a huge plus.
First do npm install --save q to start using Q package.
Here's my new User.all() function.
var Q = require('q')
var Users = {
all: function() {
var deferred = Q.defer();
globalLibrary.db.collection('users').find({}).toArray(function(err, items) {
if (err) {
deferred.reject(new Error(err));
} else {
deferred.resolve(items);
}
});
return deferred.promise;
}
}
Now if you want to use User.all().
User.all()
.then(function (docs) {
// docs is the array of returned users from mongodb query.
console.log(docs);
}, function(error) {
// do something if there's an error.
}, function(progress) {
// do something while the query is running.
});
The preferred way of doing this in node.js is to embrace the async nature of it and pass in a callback to the function. A few decent tutorials (last one including mongodb examples):
http://justinklemm.com/node-js-async-tutorial/
http://msdn.microsoft.com/en-us/magazine/dn754378.aspx
If you feel you must go against the grain and go synchronous, I'd take a look at this sync library for node.js and mongodb:
https://www.npmjs.com/package/mongo-sync
I have the following problem with mongodb-native. I have a function whose purpose is to return some element from db.
function get(){
db.collection('test').findOne({...},{...},function(err, doc){
// my doc is here
});
// but here my doc is undefined
// so I can not return it
return doc;
}
So due to asynchroneous nature of node, I can not get my doc back. So how can I make it synchroneous (or is there any other way)?
I need for my single page app. So when my client does some action, ajax request is sent which is handled by get function. This function has to send back JSON.
The answer is to have your code work asynchroneous, that is the whole concept of JavaScript.
Thus if you have something like this in your synchroneous program:
function get() {
db.collection('test').findOne({...},{...},function(err,doc) {
});
return doc;
}
var doc = get();
console.log(doc);
You can refactor it into:
function printToConsole(err, doc) {
console.log(doc);
}
function get(callback) {
db.collection('test').findOne({...},{...},callback);
}
get(printToConsole);
It is not a unique solution, there are other workflows. Some like promises will make your code flatter.
But my personal suggestion, is to initially learn how to code asynchroneous code without supporting libraries. Initially it feels like a pain, just give it some time and you will start enjoying the idea.
This might be an awful question but I'm trying to use Promise-IO with nodeJS. I'm trying to use the seq function. I have something like the following.
seq = require("promised-io/promise").seq,
var functions= new Array(function1, function2);
seq(functions, startingDataObject)
In function1 I have something like the following.
function function1(startingDataObject) {
var deferred = new Deferred();
when(function3(startingDataObject),
function (returnedData) {
//Some logic
deferred.resolve(returnedData);
},
function (err) {
console.log(err);
throw err;
});
deferred.promise;
}
What i see happening is function2 is getting fired right after deferred.promise. I'm not sure if my google skills are lacking but I don't seem to see many examples on how to use this method. So my question is how do i use Promise-IO to make sync sequential calls.
https://github.com/kriszyp/promised-io#seq
Thanks
Found the issue. I was missing a return deferred.promise in function1. The example above worked perfectly.