Can't figure out promises with mongod and nodejs - javascript

I have a function getStuff() that needs to return the results of a query against MongoDB. I'm using nodejs and the mongod module. Here's the code I've got.
var mongo = require("mongod");
var url = "mongodb://localhost:27017/my-mongo";
function getStuff() {
var db = new mongo(url, ["stuff"]);
var promise = db.stuff.find()
.then(function (result) {
console.log(result);
return result;
}).done(function (data) {
console.log(data);
return data;
});
console.log(promise);
return promise;
}
exports.getStuff = getStuff;
I'm obviously missing something about how promises work. I'm getting good data in the first two console.log() calls. However the getStuff() function always returns undefined. This seems like it should be a super simple use case but I'm banging my head on my desk trying to get it to work properly. Any help would be greatly appreciated.

Your last log and return statement run as your .find.then.done chain are still being executed (that's Async for ya!). That's why promise is still undefined.
Try returning the entire promise, like instead of
var promise = db.stuff.find()...
try
return db.stuff.find()...

...that needs to return the results...
to get the data from getStuff
function getStuff(callback) {
var db = new mongo(url, ["stuff"]);
db.stuff.find()
.done(function (result) {
console.log(result);
callback(result);
}));
}
then you can call it like this
getStuff(function(data){
//here you will get your results
});

Related

jQuery Promises not working?

I have 2 csv's that I'm trying to read in, after which I use the data in those two to do stuff:
function getData() {
var deferredObject = $.Deferred(); //representation of some asynchronous work
d3.csv("./parse_shp.csv", function(data) {
console.log(data);
shp_array = data;
});
d3.csv("./fao_coutnry_shp.csv", function(data) {
console.log(data);
fao_array = data;
});
//once both of those are done, resolve the promise
deferredObject.resolve();
return deferredObject.promise();
}
function LevenshteinDistance() {
console.log("do stuff with the data");
}
//call LevenDistance after the promise has been resolved
getData().then(LevensteinDistance());
But that's not working... it'll print the line "do something with the data" before printing the data of the csv's.
What am I doing wrong? I used this link as an example.
I don't understand how connect deferredObject and getData()? Because even if I create the deferred object in the function, won't it just asynchronously do the csv reads anyway and then erroneously call defferedObject.resolve()?
Anyway, I'm new to promises so any help would be greatly appreciated!!
The argument to .then() must be a function. You're calling the function immediately, because you have () after the function name. It should be:
getData().then(LevenshteinDistance);

How do I return Vue resource without putting to data?

I want to return result directly (Normally we try to put to data and access that data yeah. but now how I want is directly). Like when we call hello() function I want to return result variable.
I try like the following, but it doesn't return yeah. How do I try?
hello: function() {
var result = "";
this.$http.get(window.location.href).success(function(data) {
result = data;
}).error(function (data, status, request) {
});
return result;
}
Looks like it doesn't return the data because the request is async. Your method sets up the request and then returns immediately, synchronously, before the request had a chance to assign any data to the result variable.
You can try to change your code slightly:
hello: function() {
var result = {};
this.$http.get(window.location.href).success(function(data) {
result.data = data;
result.ready = true;
}).error(function (data, status, request) {
});
return result;
}
That should enable you to access the data elsewhere (as result.data), but only after the async query has succeeded. The result.ready flag would tell you if and when that has happened.
In my opinion, it would definitely be better to work with promises, though, e.g. using jQuery Deferreds and promises, or ES6 promises along with a polyfill.

Structuring promises within angularjs

I have done a lot of reading around this, but ultimately the tutorials and guides I have found differ too much for me to get a decent grasp on this concept.
This is what I want to achieve:
1) Simple http request from our server [Any API for demonstration]
2) Run a function with data from (1). [Remove a property from the object]
3) Use result and length of (2) to run a loop of $http requests to our server. [Or any server]
4) This will result in 6 different objects. Run a function on these 6 objects. [Add a property]
5) Once ALL of this is done, run a separate function [Log "finished"]
How can this be achieved using promises? How do I pass data from (1) via a promise to (2)? Is this the right way to achieve what I need to do?
If anyone can show me how this should be structured it would be immensely helpful; I have kept the functions as simple as possible for this question.
Yes, promises are very nice to structure solutions for this kind of problems.
Simplified solution (more or less pseudo-code):
$http(...)
.then(function(response) {
// do something with response, for example:
var list = reponse.data.list;
// return it so that you can use it in the next 'then'.
return list;
})
.then(function(list) {
var promises = [];
angular.forEach(list, function(item) {
// perform a request for each item
var promise = $http(...).then(function(itemResponse) {
itemResponse.extraProperty = true;
return itemResponse;
});
// we make an array of promises
promises.push(promise);
});
// combine all promises into one and return it for the next then()
return $q.all(promises);
})
.then(function(itemsList) {
// itemsList is now an array of all parsed item responses.
console.log(itemsList);
});
(Hopefully this is right, I did not tested it.)
As you can see, you can return values in a callback to pass it to the next then(), or you can pass a promise, and this will result in calling the next callback when it resolves. $q.all() is used to combine multiple promises into one and resolve if all are resolved.
Edit: I realised that you can optionally leave out these three lines:
return list;
})
.then(function(list) {
But it is nice syntax though, because the separation of tasks is more visible.
Check code below, it could contains syntax error, the important is the structure. Step3 contains multiple(6) $http requests, it waits until the last request response to return a unique response object (array) containing response for each $http requets.
//Step 1
var Step1 = function () {
$http.get('api/controller').success(function (resp) {
var object1 = resp;
Step2(object1);
Step3(object1).then(function (resp) {
//resp.data is an array containing the response of each $http request
Step4(resp);
Step5();
});
});
}
//Step2
var Step2 = function(obj){
//do whatever with the object
}
//Step3
var Step3 = function (object1) {
var call = $q.defer();
var get1 = $http.get(object1[0].url);
var get2 = $http.get(object[1].url2);
//...
var get6 = $http.get(object[5].url6);
$q.all([get1, get2,..get6]).then(function (resp) {
call.resolve(resp);
});
return call.promise;
}
//Step4
var Step4 = function (resp) {
for (var i=0; i<resp.data.lenght;i++){
DoWhatEver(resp.data[i]);
};
}
//Step5
var Step5 = function () {
alert("Finished");
}
Step1(); //Call Step1 function
Don't know why you have difficulty implementing this, but maybe $q.all() is what you're missing:
var config1={method:'GET',url:'/api/...'};
$http(config1).success(function(resultsFrom1){
functionForResultsOf1(resultsFrom1);
})
var functionForResultsOf1 = function(resultsOf1){
//remove something from the result, assuming this is a synchronous operation
resultsOf1.splice()...;
var promises=makePromises(*pass whatever you want*);
$q.all(promises).then(function(aggregateOfAllCallsToServer){
angular.forEach(aggregateOfAllCallsToServer,function(data){
//do something to data from each call to the server
})
console.log("finished");
})
}
var makePromises = function(serverUrls){
var promises = [];
angular.forEach(serverUrls, function(url) {
var promise=$http({
url : '/api/'+url,
method: 'GET',
})
promises.push(promise);
});
return $q.all(promises);
}

Promises.all does not hit catch() block when one promise fails?

I'm trying to run a function when two compilation steps are complete, but the success callback keeps getting called even one fails. Here's the code:
function compile(tplStr) {
return new Promise(function(resolve,reject) {
// compile template here
var tpl = new function(){};
resolve(tpl);
});
}
function parse(json) {
return new Promise(function(resolve,reject) {
try {
var obj = JSON.parse(json);
resolve(obj);
} catch(err) {
console.log('JSON parse failed');
reject(err);
}
});
}
var i = 0;
function bothReady() {
$('#c').text(++i);
}
function oneFailed(err) {
console.log('oneFailed hit');
$('#c').text(err.message);
}
var compileProm = compile($('#a').val());
var parseProm = parse($('#b').val());
Promise.all([compileProm,parseProm]).then(bothReady).catch(oneFailed);
$('#a').on('input', function() {
Promise.all([compile($('#a').val()),parseProm]).then(bothReady).catch(oneFailed);
});
$('#b').on('input', function() {
Promise.all(compileProm,parse($('#b').val())).then(bothReady).catch(oneFailed);
});
code pen
When I create a syntax error in the JSON portion it logs "JSON parse failed" but does not log "oneFailed hit" like I'd expect. Why not? Shouldn't the .catch block be ran if any of the promises are rejected?
Your code doesn't work correctly when something is typed inside of #b because instead of passing an iterable to Promise.All 2 parameters are passed instead.
The result is that while both promises run, only the result of the first one is taken into account by the continuation of all.
The code read
Promise.all(compileProm,parse($('#b').val())).then(bothReady).catch(oneFailed);
Instead of
Promise.all([compileProm,parse($('#b').val())]).then(bothReady).catch(oneFailed);
PS: The 2 other calls are correct it explain why the problem seem to happen only when editing the JSON.

Returning a value from callback function in Node.js [duplicate]

This question already has answers here:
How do I return the response from an asynchronous call?
(41 answers)
Closed 5 years ago.
I am facing small trouble in returning a value from callback function in Node.js, I will try to explain my situation as easy as possible. Consider I have a snippet, which takes URL and hits that url and gives the output:
urllib.request(urlToCall, { wd: 'nodejs' }, function (err, data, response) {
var statusCode = response.statusCode;
finalData = getResponseJson(statusCode, data.toString());
});
I tried to wrap it inside a function and return a value like this:
function doCall(urlToCall) {
urllib.request(urlToCall, { wd: 'nodejs' }, function (err, data, response) {
var statusCode = response.statusCode;
finalData = getResponseJson(statusCode, data.toString());
return finalData;
});
}
Because in my Node.js code, I have a lot of if-else statement where value of urlToCall will be decided, like this:
if(//somecondition) {
urlToCall = //Url1;
} else if(//someother condition) {
urlToCall = //Url2;
} else {
urlToCall = //Url3;
}
The thing is all of the statements inside a urllib.request will remain same, except value of urlToCall. So definitely I need to put those common code inside a function. I tried the same but in doCall will always return me undefined. I tried like this:
response = doCall(urlToCall);
console.log(response) //Prints undefined
But if I print value inside doCall() it prints perfectly, but it will always return undefined. As per my research I came to know that we cannot return values from callback functions! (is it true)? If yes, can anyone advice me how to handle this situation, as I want to prevent duplicate code in every if-else blocks.
Its undefined because, console.log(response) runs before doCall(urlToCall); is finished. You have to pass in a callback function aswell, that runs when your request is done.
First, your function. Pass it a callback:
function doCall(urlToCall, callback) {
urllib.request(urlToCall, { wd: 'nodejs' }, function (err, data, response) {
var statusCode = response.statusCode;
finalData = getResponseJson(statusCode, data.toString());
return callback(finalData);
});
}
Now:
var urlToCall = "http://myUrlToCall";
doCall(urlToCall, function(response){
// Here you have access to your variable
console.log(response);
})
#Rodrigo, posted a good resource in the comments. Read about callbacks in node and how they work. Remember, it is asynchronous code.
I am facing small trouble in returning a value from callback function in Node.js
This is not a "small trouble", it is actually impossible to "return" a value in the traditional sense from an asynchronous function.
Since you cannot "return the value" you must call the function that will need the value once you have it. #display_name already answered your question, but I just wanted to point out that the return in doCall is not returning the value in the traditional way. You could write doCall as follow:
function doCall(urlToCall, callback) {
urllib.request(urlToCall, { wd: 'nodejs' }, function (err, data, response) {
var statusCode = response.statusCode;
finalData = getResponseJson(statusCode, data.toString());
// call the function that needs the value
callback(finalData);
// we are done
return;
});
}
Line callback(finalData); is what calls the function that needs the value that you got from the async function. But be aware that the return statement is used to indicate that the function ends here, but it does not mean that the value is returned to the caller (the caller already moved on.)
Example code for node.js - async function to sync function:
var deasync = require('deasync');
function syncFunc()
{
var ret = null;
asyncFunc(function(err, result){
ret = {err : err, result : result}
});
while((ret == null))
{
deasync.runLoopOnce();
}
return (ret.err || ret.result);
}
If what you want is to get your code working without modifying too much. You can try this solution which gets rid of callbacks and keeps the same code workflow:
Given that you are using Node.js, you can use co and co-request to achieve the same goal without callback concerns.
Basically, you can do something like this:
function doCall(urlToCall) {
return co(function *(){
var response = yield urllib.request(urlToCall, { wd: 'nodejs' }); // This is co-request.
var statusCode = response.statusCode;
finalData = getResponseJson(statusCode, data.toString());
return finalData;
});
}
Then,
var response = yield doCall(urlToCall); // "yield" garuantees the callback finished.
console.log(response) // The response will not be undefined anymore.
By doing this, we wait until the callback function finishes, then get the value from it. Somehow, it solves your problem.

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