Node.js/Javascript - nested promises and for loop with when.js - javascript

I am currently struggling with the control flow of promise (promise newbie!).
I make a call to Redis which returns an array object. I then iterate through each of the results and call back to Redis to get a value and wish to populate these in to a final object out.
The out object is never populated, im guessing as the forEach has not completed:
(note the Redis client lib returns a when.js based promise as default)
var out = {};
REDISCLIENT.keys("apikey:*")
.then(function (replies) {
replies.forEach(function (key, index) {
REDISCLIENT.get(key)
.then(function (value) {
out[key] = value;
})
.done(console.log(out));
});
})
.catch(console.log)
.done(console.log(out));
How can I guarantee that the forEach loop is complete?
I have read many similar posts (I know this is a duplicate) however have not been able to understand why the inner done() method does not contain the fully complete out obj.
Im guessing I need to wrap the forEach itself in a promise? Appreciate any direction.
Update 1: huge thanks to #David_Aurelio. I now need to populate out with the key and values. Here is my attempt:
GLOBAL.REDISCLIENT.keys("apikey:*")
.then(function (replies) {
return when.all(replies.map(function (key, index) {
return GLOBAL.REDISCLIENT.get(key)
.then(function (val) {
out[key] = val;
console.log(key, val);
});
}));
})
.catch(console.log)
.done(function (out) {console.log(out); });
The inner console.log prints the correct key/values
key1 val1
key2 val2
The final done now prints:
[ undefined, undefined ]

It's important to understand that flow control and the data conveyed by a promise chain, are determined by :
the composition of the chain, and any inner chain(s)
promise aggregators, such as when.all()
return statements
Here's how to achieve what you want with out as an inner member.
REDISCLIENT.keys("apikey:*")
.then(function (replies) {
var out = {}: //<<<<< initiate `out` as an inner member
return when.all(replies.map(function (key, index) { //<<<<< here's David Aurelio's when.all(replies.map(...))
return REDISCLIENT.get(key).then(function (value) { //<<<<< `return` here causes `.map()` to build an array of promises.
out[key] = value;
});
})).then(function() { //<<<< here's an additional `.then()` chained to `when.all(...)`
return out; //<<<<< `return` here makes the populated `out` available to the `.done()` callback below.
});
})
.catch(console.log)
.done(function (out_) {
console.log(out_);
});
The ugly outer member has disappeared!
In the .done() callback, I have changed the member name to out_ in order to emphasize that it is passed as a consequence of that return out, which happens only when all [geddit] the promises returned by REDISCLIENT.get() calls have successfully settled.

The forEach loop completes, but the Promises you create inside don’t. when.js implements the Promises/A+ spec, which guarantees asynchronous resolution of promise callbacks. That means, that the callback passed to then() is guaranteed to be invoked after the current call stack has finished to execute.
You need to return a promise from your then-callback in order to connect the inner promises to the outer promise. More specifically, you need a promise over all inner promises.
The easiest way to do that is to use when.all:
REDISCLIENT.keys("apikey:*")
.then(function (replies) {
return when.all(replies.map(function (key, index) {
return REDISCLIENT.get(key);
}));
})
.catch(console.log)
.done(function (out) {console.log(out); });
In your original code, you also don't register a callback to done, but call console.log(out) immediately, before the first promise has even resolved.

Related

How to execute promises in series?

var promiseReturningFuncs = [];
for(var i = 0; i < 5; i++){
promiseReturningFuncs.push(askQuestion);
}
var programmers = [];
Promise.reduce(promiseReturningFuncs, function(resp, x) {
console.log(typeof resp);
if(typeof resp != "function") {
programmers.push(resp);
}
return x();
})
.then(function(resp) {
programmers.push(resp);
console.log(programmers);
});
My goal: execute the askQuestion function in series and resolve an array of objects created by that function. (this function must execute in series so that it can respond to user input)
So imagine that the askQuestion function returns a promise that resolves a object I want to add to an array.
This is my messy way of doing it.
I am looking to find a cleaner way of doing it, ideally, i wouldn't even need to push to an array, I would just have a final .then, where the response is an array.
Since you appear to be using the Bluebird promise library, you have a number of built-in options for sequencing your promise returning functions. You can use Promise.reduce(), Promise.map() with a concurrency value of 1, Promise.mapSeries or Promise.each(). If the iterator function returns a promise, all of these will wait for the next iteration until that promise resolves. Which to use depends more upon the mechanics of how your data is structured and what result you want (neither of which you actually show or describe).
Let's suppose you have an array of promise returning functions and you want to call them one at a time, waiting for the one to resolve before calling the next one. If you want all the results, then I'd suggest Promise.mapSeries():
let arrayOfPromiseReturningFunctions = [...];
// call all the promise returning functions in the array, one at a time
// wait for one to resolve before calling the next
Promise.mapSeries(arrayOfPromiseReturningFunctions, function(fn) {
return fn();
}).then(function(results) {
// results is an array of resolved results from all the promises
}).catch(function(err) {
// process error here
});
Promise.reduce() could also be used, but it would accumulate a single result, passing it from one to the next and end with one final result (like Array.prototype.reduce() does).
Promise.map() is a more general version of Promise.mapSeries() that lets you control the concurrency number (the number of async operations in flight at the same time).
Promise.each() will also sequence your functions, but does not accumulate a result. It assumes you either don't have a result or you are accumulating the result out-of-band or via side effects. I tend to not like to use Promise.each() because I don't like side effect programming.
You could solve this in pure JS using ES6 (ES2015) features:
function processArray(arr, fn) {
return arr.reduce(
(p, v) => p.then((a) => fn(v).then(r => a.concat([r]))),
Promise.resolve([])
);
}
It applies the function given to the array in series and resolves to an array of the results.
Usage:
const numbers = [0, 4, 20, 100];
const multiplyBy3 = (x) => new Promise(res => res(x * 3));
// Prints [ 0, 12, 60, 300 ]
processArray(numbers, multiplyBy3).then(console.log);
You'll want to double check browser compatibility but this works on reasonably current Chrome (v59), NodeJS (v8.1.2) and probably most others.
You can use recursion so that you can move to the next iteration in a then block.
function promiseToExecuteAllInOrder(promiseReturningFunctions /* array of functions */) {
var resolvedValues = [];
return new Promise(function(resolve, reject) {
function executeNextFunction() {
var nextFunction = promiseReturningFunctions.pop();
if(nextFunction) {
nextFunction().then(function(result) {
resolvedValues.push(result);
executeNextFunction();
});
} else {
resolve(resolvedValues);
}
}
executeNextFunction();
}
}
Executing one after another using a recursive function( in a non promise way):
(function iterate(i,result,callback){
if( i>5 ) callback(result);askQuestion().then(res=>iterate(i+1,result.concat([res]),callback);
})(0,[],console.log);
For shure this can be wrapped in a promise:
function askFive(){
return new Promise(function(callback){
(function iterate(i,result){
if( i>5 ) callback(result);askQuestion().then(res=>iterate(i+1,result.concat([res]),callback);
})(0,[],console.log);
});
}
askFive().then(console.log);
Or:
function afteranother(i,promise){
return new Promise(function(resolve){
if(!i) return resolve([]);
afteranother(i-1,promise).then(val=>promise().then(val2=>resolve(val.concat([val2])));
});
}
afteranother(5,askQuestion).then(console.log);

Flattening promise chain with readable function name

I saw promise implementation in Handling multiple catches in promise chain which produce a very readable chain
return validateInput
.then(checkLoginPermission)
.then(checkDisableUser)
.then(changePassword);
However, in order to do this each function needs to return a value instead of a Promise? Since Promise can resolves to either value or a Promise so this is not a problem. My goal is to turn every function to have readable clear logic as such.
The problem occurs when trying to unwind the nested promise function
return validateInput
.then(function(resultA) {
return checkLoginPermission
.then (function(resultB) {
// Do something with resultA
})
});
Imagine the original implementation involves accessing the value from previous promise. With nested promise, it is easily achievable. But with flatten chain, I would need to break up each function like this
function validateInput = function (resultA ) {
return Promise.resolve({resultA : resultA, resultB :
}
function checkLoginPermission = function (mix ) {
let resultA = mix.resultA;
let resultB = mix.resultB
//Do something with resultA
...
}
This is worse when the last function in the chain rely on something from the very beginning. That means the value have to be passed down from the beginning of the chain even if it was not used.
So am I accidentally stepping on some kind of anti-pattern that might affect performance? How else can I achieve good readability without all these hassles?
This is actually where async and await come in. It's good when you need results across multiple asynchronous calls/promises to be in scope. If you can use that, I'd say try it.
async function foo () {
const input = await validateInput()
const hasPermission = await checkLoginPermission(input)
const result = await checkDisableUser(hasPermission)
return await changePassword(result)
}
Just pass the variables into what function as they need to be. Just showing an example there. I was also a bit unsure of how you're setting validateInput, i think you need to put await infront of the function call itself.
If you cannot use async/await, I usually go with your 2nd code snippet, or define the higher scope variables ontop:
let resultA
return validateInput
.then(function(result) {
resultA = result
return checkLoginPermission
.then (function(resultB) {
// Do something with resultA
})
});
Promises are pattern, related to functional programming, there direct passing data from one function to other is a basic (it's called compose, here examples: http://scott.sauyet.com/Javascript/Talk/Compose/2013-05-22/). So it's not anti-pattern by no means.
I can't see any problem in such pattern. You can pass any data you want to next Promises and in nested Promises grab what they need. It's preety transparent and clear:
function validateInput() {
return Promise.resolve({resultA: 1});
}
function checkLoginPermission(result) {
return new Promise(function(resolve, reject) {
// ...
// code
// ...
result.resultB = 2;
return resolve(result);
});
}
function checkDisableUser(result) {
return new Promise(function(resolve, reject) {
// grab some data from previous function
let resultB = result.resultB;
// ...
// code
// ...
result.resultC = 3;
return resolve(result);
});
}
function changePassword(result) {
return new Promise(function(resolve, reject) {
// grab some data from previous functions
let resultB = result.resultB;
let resultC = result.resultC;
// ...
// code
// ...
result.resultD = resultB * resultC;
return resolve(result);
});
}
validateInput()
.then(checkLoginPermission)
.then(checkDisableUser)
.then(changePassword);
Also you can collect data in some variable, declared before Promises and so you will not have to pass result. But it will destroy functional nature of Promises.
The inner .then(/* ... */) callbacks can return either a primitive value or a Promise that resolves to some value. If it is another promise then the next .then won't start until the inner promise is resolved. Essentially, Promises always resolve to a non-promise type. If you resolve or return another Promise, it will be automatically unwrapped.
I would like to propose a solution using ramda.js#pipeP().
The good thing about this function is that it resolves promises sequentially.
We can rewrite your example using pipeP():
import pipeP from 'ramda/src/pipeP'
pipeP([
checkLoginPermission,
checkDisableUser,
changePassword
])(initialValue)
.then(responseChangePassword => { ... })
The results of a previous promise are passed to the following one.

Trouble using forEach and mongoose's findById in tandem

I am writing a Node route which should push objects onto an array declared outside the forEach loop after the objects have a property added to them. When I console.log the array within the loop, it seems to be taking on data, but when I return it to the client-side. It is empty.
var todaysTopItemsBySaleFrequency = [];
listOfItemIdsAndSaleFrequency.forEach((item) => {
Product.findById(item.itemId).then((foundItem) => {
var fullItemData = foundItem.toJSON();
fullItemData.occurrences = item.occurrences;
todaysTopItemsBySaleFrequency.push(fullItemData);
console.log(todaysTopItemsBySaleFrequency);
});
});
return res.status(200).json(todaysTopItemsBySaleFrequency);
The console.log statement shows that the array called todaysTopItemsBySaleFrequency is being populated correctly, but why is it empty when I return it to the client?
The callback function you're passing to Product.findById(item.itemId).then(...) is not invoked immediately. Your outer forEach completes and you return before any of your callbacks are invoked.
Mongoose's findById() method returns a promise. You can use Promise.all() to wait for an array of promises to complete, and then set res.status(200).json(...). Because this happens asynchronously you should also present an asynchronous interface, for example by returning a promise yourself.
Here's a version that gathers all of the responses and returns a promise that resolves with your original return value:
var todaysTopItemsBySaleFrequency = [];
return Promise.all(listOfItemIdsAndSaleFrequency.map((item) => {
return Product.findById(item.itemId).then((foundItem) => {
var fullItemData = foundItem.toJSON();
fullItemData.occurrences = item.occurrences;
todaysTopItemsBySaleFrequency.push(fullItemData);
console.log(todaysTopItemsBySaleFrequency);
});
})).then(() => res.status(200).json(todaysTopItemsBySaleFrequency));

How to return a promise in a function from the last promise in a chain of "then"

I'm writing a test using Selenium and JavaScript. I'm new to both, and also new to functional programming and promises. I'm trying to create a function that needs to do 3 things:
Click on an input
Clear the input
SendKeys to input
My current function does not work:
var clearAndSendKeys = function(driver, elementIdentifier, sendKeys) {
var returnValue;
driver.findElement(elementIdentifier).then(function(inputField){
inputField.click().then(function() {
inputField.clear().then(function() {
returnValue = inputField.sendKeys(sendKeys);
});
});
});
return returnValue;
}
The function would then be called as for example:
clearAndSendKeys(driver, webdriver.By.id('date_field'), '14.09.2015').then(function(){
//Do stuff
});
I expected the variable returnValue to contain the promise from sendKeys. However the function clearAndSendKeys returns the undefined variable before sendKeys is ran. I assume this is because returnValue was never defined as a promise, and so the program does not know that it needs to wait for sendKeys.
How can I make my function clearAndSendKeys return the promise from sendKeys? I'd rather avoid having to add a callback to the clearAndSendKeys function.
Edit: Removed .then({return data}) from the code as this was a typo.
You have to return each promise from the .then callback:
var clearAndSendKeys = function(driver, elementIdentifier, sendKeys) {
return driver.findElement(elementIdentifier).then(function(inputField){
return inputField.click().then(function() {
return inputField.clear().then(function() {
return inputField.sendKeys(sendKeys);
});
});
});
}
The promise returned by .then will resolve to the same value as the value returned from the callback.
See Why is my variable unaltered after I modify it inside of a function? - Asynchronous code reference for why your current code does not work. Promises are asynchronous.
First of all its probably not the best idea to nest promises, completely defeating their main purpose of eliminating callback hell. then callback can return Thenable object that allows to create nice chains of async operations.
In this case you just need to store reference to input field available as a result of the first async operation in the scope of the main function and then create chain of async operations that can be returned from this function.
var clearAndSendKeys = function(driver, elementIdentifier, sendKeys) {
var inputFieldRef;
return driver.findElement(elementIdentifier)
.then(function(inputField){
inputFieldRef = inputField;
return inputField.click();
}).then(function() {
return inputFieldRef.clear();
}).then(function() {
return inputFieldRef.sendKeys(sendKeys);
});
}

Is this a good way to generate a chain of promises for an array?

I have an array of items that I want to insert into an SQL server. I am using promises for this and in order to execute each insert sequentially I wrote the following method:
var ForeachPromise = function (array, func) {
var promise = func(array[0]);
for (var i=1; i < array.length; i++) {
promise = promise.then(function() { return func(array[i]) });
}
return promise;
}
The idea is that when func is call it will return a promise, which will then be chained to the previous promise.
...
return ForeachPromise(type.subprocessen, function(subproces) {
return newSubproces(subproces, typeId, dienstId, createData, s + 1);
});
I haven't actually tested it yet, but I assume that something like this will work. My question however is am I using promises correctly? Promises are great but easily misunderstood and I just want to be sure that I'm not making any fundamental mistakes.
Yes, that approach is fine, and works well with promises. Two minor quibbles:
you should take care for the case of an empty array. Start your chain with Promise.resolve() (a promise fulfilled with undefined), and begin your loop at index 0.
As the then callback is asynchronous, your i variable has the wrong value - the classical closure in a loop fallacy.
Using the .reduce method does help with both problems:
function foreachPromise(array, func) {
return array.reduce(function(promise, elem, i) {
return promise.then(function() { return func(elem) });
}, Promise.resolve());
}

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