Let's say I have a set of Promises that are making network requests, of which one will fail:
// http://does-not-exist will throw a TypeError
var arr = [ fetch('index.html'), fetch('http://does-not-exist') ]
Promise.all(arr)
.then(res => console.log('success', res))
.catch(err => console.log('error', err)) // This is executed
Let's say I want to wait until all of these have finished, regardless of if one has failed. There might be a network error for a resource that I can live without, but which if I can get, I want before I proceed. I want to handle network failures gracefully.
Since Promise.all doesn't leave any room for this, what is the recommended pattern for handling this, without using a promises library?
Update, you probably want to use the built-in native Promise.allSettled:
Promise.allSettled([promise]).then(([result]) => {
//reach here regardless
// {status: "fulfilled", value: 33}
});
As a fun fact, this answer below was prior art in adding that method to the language :]
Sure, you just need a reflect:
const reflect = p => p.then(v => ({v, status: "fulfilled" }),
e => ({e, status: "rejected" }));
reflect(promise).then((v) => {
console.log(v.status);
});
Or with ES5:
function reflect(promise){
return promise.then(function(v){ return {v:v, status: "fulfilled" }},
function(e){ return {e:e, status: "rejected" }});
}
reflect(promise).then(function(v){
console.log(v.status);
});
Or in your example:
var arr = [ fetch('index.html'), fetch('http://does-not-exist') ]
Promise.all(arr.map(reflect)).then(function(results){
var success = results.filter(x => x.status === "fulfilled");
});
Similar answer, but more idiomatic for ES6 perhaps:
const a = Promise.resolve(1);
const b = Promise.reject(new Error(2));
const c = Promise.resolve(3);
Promise.all([a, b, c].map(p => p.catch(e => e)))
.then(results => console.log(results)) // 1,Error: 2,3
.catch(e => console.log(e));
const console = { log: msg => div.innerHTML += msg + "<br>"};
<div id="div"></div>
Depending on the type(s) of values returned, errors can often be distinguished easily enough (e.g. use undefined for "don't care", typeof for plain non-object values, result.message, result.toString().startsWith("Error:") etc.)
Benjamin's answer offers a great abstraction for solving this issue, but I was hoping for a less abstracted solution. The explicit way to to resolve this issue is to simply call .catch on the internal promises, and return the error from their callback.
let a = new Promise((res, rej) => res('Resolved!')),
b = new Promise((res, rej) => rej('Rejected!')),
c = a.catch(e => { console.log('"a" failed.'); return e; }),
d = b.catch(e => { console.log('"b" failed.'); return e; });
Promise.all([c, d])
.then(result => console.log('Then', result)) // Then ["Resolved!", "Rejected!"]
.catch(err => console.log('Catch', err));
Promise.all([a.catch(e => e), b.catch(e => e)])
.then(result => console.log('Then', result)) // Then ["Resolved!", "Rejected!"]
.catch(err => console.log('Catch', err));
Taking this one step further, you could write a generic catch handler that looks like this:
const catchHandler = error => ({ payload: error, resolved: false });
then you can do
> Promise.all([a, b].map(promise => promise.catch(catchHandler))
.then(results => console.log(results))
.catch(() => console.log('Promise.all failed'))
< [ 'Resolved!', { payload: Promise, resolved: false } ]
The problem with this is that the caught values will have a different interface than the non-caught values, so to clean this up you might do something like:
const successHandler = result => ({ payload: result, resolved: true });
So now you can do this:
> Promise.all([a, b].map(result => result.then(successHandler).catch(catchHandler))
.then(results => console.log(results.filter(result => result.resolved))
.catch(() => console.log('Promise.all failed'))
< [ 'Resolved!' ]
Then to keep it DRY, you get to Benjamin's answer:
const reflect = promise => promise
.then(successHandler)
.catch(catchHander)
where it now looks like
> Promise.all([a, b].map(result => result.then(successHandler).catch(catchHandler))
.then(results => console.log(results.filter(result => result.resolved))
.catch(() => console.log('Promise.all failed'))
< [ 'Resolved!' ]
The benefits of the second solution are that its abstracted and DRY. The downside is you have more code, and you have to remember to reflect all your promises to make things consistent.
I would characterize my solution as explicit and KISS, but indeed less robust. The interface doesn't guarantee that you know exactly whether the promise succeeded or failed.
For example you might have this:
const a = Promise.resolve(new Error('Not beaking, just bad'));
const b = Promise.reject(new Error('This actually didnt work'));
This won't get caught by a.catch, so
> Promise.all([a, b].map(promise => promise.catch(e => e))
.then(results => console.log(results))
< [ Error, Error ]
There's no way to tell which one was fatal and which was wasn't. If that's important then you're going to want to enforce and interface that tracks whether it was successful or not (which reflect does).
If you just want to handle errors gracefully, then you can just treat errors as undefined values:
> Promise.all([a.catch(() => undefined), b.catch(() => undefined)])
.then((results) => console.log('Known values: ', results.filter(x => typeof x !== 'undefined')))
< [ 'Resolved!' ]
In my case, I don't need to know the error or how it failed--I just care whether I have the value or not. I'll let the function that generates the promise worry about logging the specific error.
const apiMethod = () => fetch()
.catch(error => {
console.log(error.message);
throw error;
});
That way, the rest of the application can ignore its error if it wants, and treat it as an undefined value if it wants.
I want my high level functions to fail safely and not worry about the details on why its dependencies failed, and I also prefer KISS to DRY when I have to make that tradeoff--which is ultimately why I opted to not use reflect.
There is a finished proposal for a function which can accomplish this natively, in vanilla Javascript: Promise.allSettled, which has made it to stage 4, is officialized in ES2020, and is implemented in all modern environments. It is very similar to the reflect function in this other answer. Here's an example, from the proposal page. Before, you would have had to do:
function reflect(promise) {
return promise.then(
(v) => {
return { status: 'fulfilled', value: v };
},
(error) => {
return { status: 'rejected', reason: error };
}
);
}
const promises = [ fetch('index.html'), fetch('https://does-not-exist/') ];
const results = await Promise.all(promises.map(reflect));
const successfulPromises = results.filter(p => p.status === 'fulfilled');
Using Promise.allSettled instead, the above will be equivalent to:
const promises = [ fetch('index.html'), fetch('https://does-not-exist/') ];
const results = await Promise.allSettled(promises);
const successfulPromises = results.filter(p => p.status === 'fulfilled');
Those using modern environments will be able to use this method without any libraries. In those, the following snippet should run without problems:
Promise.allSettled([
Promise.resolve('a'),
Promise.reject('b')
])
.then(console.log);
Output:
[
{
"status": "fulfilled",
"value": "a"
},
{
"status": "rejected",
"reason": "b"
}
]
For older browsers, there is a spec-compliant polyfill here.
I really like Benjamin's answer, and how he basically turns all promises into always-resolving-but-sometimes-with-error-as-a-result ones. :)
Here's my attempt at your request just in case you were looking for alternatives. This method simply treats errors as valid results, and is coded similar to Promise.all otherwise:
Promise.settle = function(promises) {
var results = [];
var done = promises.length;
return new Promise(function(resolve) {
function tryResolve(i, v) {
results[i] = v;
done = done - 1;
if (done == 0)
resolve(results);
}
for (var i=0; i<promises.length; i++)
promises[i].then(tryResolve.bind(null, i), tryResolve.bind(null, i));
if (done == 0)
resolve(results);
});
}
var err;
Promise.all([
promiseOne().catch(function(error) { err = error;}),
promiseTwo().catch(function(error) { err = error;})
]).then(function() {
if (err) {
throw err;
}
});
The Promise.all will swallow any rejected promise and store the error in a variable, so it will return when all of the promises have resolved. Then you can re-throw the error out, or do whatever. In this way, I guess you would get out the last rejection instead of the first one.
I had the same problem and have solved it in the following way:
const fetch = (url) => {
return node-fetch(url)
.then(result => result.json())
.catch((e) => {
return new Promise((resolve) => setTimeout(() => resolve(fetch(url)), timeout));
});
};
tasks = [fetch(url1), fetch(url2) ....];
Promise.all(tasks).then(......)
In that case Promise.all will wait for every Promise will come into resolved or rejected state.
And having this solution we are "stopping catch execution" in a non-blocking way. In fact, we're not stopping anything, we just returning back the Promise in a pending state which returns another Promise when it's resolved after the timeout.
This should be consistent with how Q does it:
if(!Promise.allSettled) {
Promise.allSettled = function (promises) {
return Promise.all(promises.map(p => Promise.resolve(p).then(v => ({
state: 'fulfilled',
value: v,
}), r => ({
state: 'rejected',
reason: r,
}))));
};
}
Instead of rejecting, resolve it with a object.
You could do something like this when you are implementing promise
const promise = arg => {
return new Promise((resolve, reject) => {
setTimeout(() => {
try{
if(arg != 2)
return resolve({success: true, data: arg});
else
throw new Error(arg)
}catch(e){
return resolve({success: false, error: e, data: arg})
}
}, 1000);
})
}
Promise.all([1,2,3,4,5].map(e => promise(e))).then(d => console.log(d))
Benjamin Gruenbaum answer is of course great,. But I can also see were Nathan Hagen point of view with the level of abstraction seem vague. Having short object properties like e & v don't help either, but of course that could be changed.
In Javascript there is standard Error object, called Error,. Ideally you always throw an instance / descendant of this. The advantage is that you can do instanceof Error, and you know something is an error.
So using this idea, here is my take on the problem.
Basically catch the error, if the error is not of type Error, wrap the error inside an Error object. The resulting array will have either resolved values, or Error objects you can check on.
The instanceof inside the catch, is in case you use some external library that maybe did reject("error"), instead of reject(new Error("error")).
Of course you could have promises were you resolve an error, but in that case it would most likely make sense to treat as an error anyway, like the last example shows.
Another advantage of doing it this, array destructing is kept simple.
const [value1, value2] = PromiseAllCatch(promises);
if (!(value1 instanceof Error)) console.log(value1);
Instead of
const [{v: value1, e: error1}, {v: value2, e: error2}] = Promise.all(reflect..
if (!error1) { console.log(value1); }
You could argue that the !error1 check is simpler than an instanceof, but your also having to destruct both v & e.
function PromiseAllCatch(promises) {
return Promise.all(promises.map(async m => {
try {
return await m;
} catch(e) {
if (e instanceof Error) return e;
return new Error(e);
}
}));
}
async function test() {
const ret = await PromiseAllCatch([
(async () => "this is fine")(),
(async () => {throw new Error("oops")})(),
(async () => "this is ok")(),
(async () => {throw "Still an error";})(),
(async () => new Error("resolved Error"))(),
]);
console.log(ret);
console.log(ret.map(r =>
r instanceof Error ? "error" : "ok"
).join(" : "));
}
test();
I think the following offers a slightly different approach... compare fn_fast_fail() with fn_slow_fail()... though the latter doesn't fail as such... you can check if one or both of a and b is an instance of Error and throw that Error if you want it to reach the catch block (e.g. if (b instanceof Error) { throw b; }) . See the jsfiddle.
var p1 = new Promise((resolve, reject) => {
setTimeout(() => resolve('p1_delayed_resolvement'), 2000);
});
var p2 = new Promise((resolve, reject) => {
reject(new Error('p2_immediate_rejection'));
});
var fn_fast_fail = async function () {
try {
var [a, b] = await Promise.all([p1, p2]);
console.log(a); // "p1_delayed_resolvement"
console.log(b); // "Error: p2_immediate_rejection"
} catch (err) {
console.log('ERROR:', err);
}
}
var fn_slow_fail = async function () {
try {
var [a, b] = await Promise.all([
p1.catch(error => { return error }),
p2.catch(error => { return error })
]);
console.log(a); // "p1_delayed_resolvement"
console.log(b); // "Error: p2_immediate_rejection"
} catch (err) {
// we don't reach here unless you throw the error from the `try` block
console.log('ERROR:', err);
}
}
fn_fast_fail(); // fails immediately
fn_slow_fail(); // waits for delayed promise to resolve
I just wanted a polyfill that exactly replicated ES2020 behaviour since I'm locked into node versions a lot earlier than 12.9 (when Promise.allSettled appeared), unfortunately. So for what it's worth, this is my version:
const settle = (promise) => (promise instanceof Promise) ?
promise.then(val => ({ value: val, status: "fulfilled" }),
err => ({ reason: err, status: "rejected" })) :
{ value: promise, status: 'fulfilled' };
const allSettled = async (parr) => Promise.all(parr.map(settle));
This handles a mixed array of promise and non-promise values, as does the ES version. It hands back the same array of { status, value/reason } objects as the native version.
Here's my custom settledPromiseAll()
const settledPromiseAll = function(promisesArray) {
var savedError;
const saveFirstError = function(error) {
if (!savedError) savedError = error;
};
const handleErrors = function(value) {
return Promise.resolve(value).catch(saveFirstError);
};
const allSettled = Promise.all(promisesArray.map(handleErrors));
return allSettled.then(function(resolvedPromises) {
if (savedError) throw savedError;
return resolvedPromises;
});
};
Compared to Promise.all
If all promises are resolved, it performs exactly as the standard one.
If one of more promises are rejected, it returns the first one rejected much the same as the standard one but unlike it waits for all promises to resolve/reject.
For the brave we could change Promise.all():
(function() {
var stdAll = Promise.all;
Promise.all = function(values, wait) {
if(!wait)
return stdAll.call(Promise, values);
return settledPromiseAll(values);
}
})();
CAREFUL. In general we never change built-ins, as it might break other unrelated JS libraries or clash with future changes to JS standards.
My settledPromiseall is backward compatible with Promise.all and extends its functionality.
People who are developing standards -- why not include this to a new Promise standard?
I recently built a library that allows what you need. it executes promises in parallel, and if one fails, the process continues, at the end it returns an array with all the results, including errors.
https://www.npmjs.com/package/promise-ax
I hope and it is helpful for someone.
const { createPromise } = require('promise-ax');
const promiseAx = createPromise();
const promise1 = Promise.resolve(4);
const promise2 = new Promise((resolve, reject) => setTimeout(reject, 100, new Error("error")));
const promise3 = Promise.reject("error");
const promise4 = promiseAx.resolve(8);
const promise5 = promiseAx.reject("errorAx");
const asyncOperation = (time) => {
return new Promise((resolve, reject) => {
if (time < 0) {
reject("reject");
}
setTimeout(() => {
resolve(time);
}, time);
});
};
const promisesToMake = [promise1, promise2, promise3, promise4, promise5, asyncOperation(100)];
promiseAx.allSettled(promisesToMake).then((results) => results.forEach((result) => console.log(result)));
// Salida esperada:
// 4
// Error: error
// error
// 8
// errorAx
// 100
I would do:
var err = [fetch('index.html').then((success) => { return Promise.resolve(success); }).catch((e) => { return Promise.resolve(e); }),
fetch('http://does-not-exist').then((success) => { return Promise.resolve(success); }).catch((e) => { return Promise.resolve(e); })];
Promise.all(err)
.then(function (res) { console.log('success', res) })
.catch(function (err) { console.log('error', err) }) //never executed
I've been using following codes since ES5.
Promise.wait = function(promiseQueue){
if( !Array.isArray(promiseQueue) ){
return Promise.reject('Given parameter is not an array!');
}
if( promiseQueue.length === 0 ){
return Promise.resolve([]);
}
return new Promise((resolve, reject) =>{
let _pQueue=[], _rQueue=[], _readyCount=false;
promiseQueue.forEach((_promise, idx) =>{
// Create a status info object
_rQueue.push({rejected:false, seq:idx, result:null});
_pQueue.push(Promise.resolve(_promise));
});
_pQueue.forEach((_promise, idx)=>{
let item = _rQueue[idx];
_promise.then(
(result)=>{
item.resolved = true;
item.result = result;
},
(error)=>{
item.resolved = false;
item.result = error;
}
).then(()=>{
_readyCount++;
if ( _rQueue.length === _readyCount ) {
let result = true;
_rQueue.forEach((item)=>{result=result&&item.resolved;});
(result?resolve:reject)(_rQueue);
}
});
});
});
};
The usage signature is just like Promise.all. The major difference is that Promise.wait will wait for all the promises to finish their jobs.
I know that this question has a lot of answers, and I'm sure must (if not all) are correct.
However it was very hard for me to understand the logic/flow of these answers.
So I looked at the Original Implementation on Promise.all(), and I tried to imitate that logic - with the exception of not stopping the execution if one Promise failed.
public promiseExecuteAll(promisesList: Promise<any>[] = []): Promise<{ data: any, isSuccess: boolean }[]>
{
let promise: Promise<{ data: any, isSuccess: boolean }[]>;
if (promisesList.length)
{
const result: { data: any, isSuccess: boolean }[] = [];
let count: number = 0;
promise = new Promise<{ data: any, isSuccess: boolean }[]>((resolve, reject) =>
{
promisesList.forEach((currentPromise: Promise<any>, index: number) =>
{
currentPromise.then(
(data) => // Success
{
result[index] = { data, isSuccess: true };
if (promisesList.length <= ++count) { resolve(result); }
},
(data) => // Error
{
result[index] = { data, isSuccess: false };
if (promisesList.length <= ++count) { resolve(result); }
});
});
});
}
else
{
promise = Promise.resolve([]);
}
return promise;
}
Explanation:
- Loop over the input promisesList and execute each Promise.
- No matter if the Promise resolved or rejected: save the Promise's result in a result array according to the index. Save also the resolve/reject status (isSuccess).
- Once all Promises completed, return one Promise with the result of all others.
Example of use:
const p1 = Promise.resolve("OK");
const p2 = Promise.reject(new Error(":-("));
const p3 = Promise.resolve(1000);
promiseExecuteAll([p1, p2, p3]).then((data) => {
data.forEach(value => console.log(`${ value.isSuccess ? 'Resolve' : 'Reject' } >> ${ value.data }`));
});
/* Output:
Resolve >> OK
Reject >> :-(
Resolve >> 1000
*/
You can execute your logic sequentially via synchronous executor nsynjs. It will pause on each promise, wait for resolution/rejection, and either assign resolve's result to data property, or throw an exception (for handling that you will need try/catch block). Here is an example:
function synchronousCode() {
function myFetch(url) {
try {
return window.fetch(url).data;
}
catch (e) {
return {status: 'failed:'+e};
};
};
var arr=[
myFetch("https://ajax.googleapis.com/ajax/libs/jquery/2.0.0/jquery.min.js"),
myFetch("https://ajax.googleapis.com/ajax/libs/jquery/2.0.0/NONEXISTANT.js"),
myFetch("https://ajax.NONEXISTANT123.com/ajax/libs/jquery/2.0.0/NONEXISTANT.js")
];
console.log('array is ready:',arr[0].status,arr[1].status,arr[2].status);
};
nsynjs.run(synchronousCode,{},function(){
console.log('done');
});
<script src="https://rawgit.com/amaksr/nsynjs/master/nsynjs.js"></script>
Promise.all with using modern async/await approach
const promise1 = //...
const promise2 = //...
const data = await Promise.all([promise1, promise2])
const dataFromPromise1 = data[0]
const dataFromPromise2 = data[1]
I don't know which promise library you are using, but most have something like allSettled.
Edit: Ok since you want to use plain ES6 without external libraries, there is no such method.
In other words: You have to loop over your promises manually and resolve a new combined promise as soon as all promises are settled.
I have a few async methods that I need to wait for completion before I return from the request. I'm using Promises, but I keep getting the error:
Each then() should return a value or throw // promise/always-return
Why is this happpening? This is my code:
router.get('/account', function(req, res) {
var id = req.user.uid
var myProfile = {}
var profilePromise = new Promise(function(resolve, reject) {
var userRef = firebase.db.collection('users').doc(id)
userRef.get()
.then(doc => { // Error occurs on this line
if (doc.exists) {
var profile = doc.data()
profile.id = doc.id
myProfile = profile
resolve()
} else {
reject(Error("Profile doesn't exist"))
}
})
.catch(error => {
reject(error)
})
})
// More promises further on, which I wait for
})
Add at the end of the then()
return null
That's it.
Each then() should return a value or throw Firebase cloud functions
Just avoid the Promise constructor antipattern! If you don't call resolve but return a value, you will have something to return. The then method should be used for chaining, not just subscribing:
outer.get('/account', function(req, res) {
var id = req.user.uid
var userRef = firebase.db.collection('users').doc(id)
var profilePromise = userRef.get().then(doc => {
if (doc.exists) {
var profile = doc.data()
profile.id = doc.id
return profile // I assume you don't want to return undefined
// ^^^^^^
} else {
throw new Error("Profile doesn't exist")
// ^^^^^
}
})
// More promises further on, which I wait for:
// profilePromise.then(myProfile => { … });
})
In your case firebase.db.collection('users').doc(id) returning promise itself, please check firebase snippet to here for node-js.
If you have multiple promises and you need to call them one by one then use Promises chaining.
Please check this article this will help you.
Use following code in your case,
router.get('/account', function(req, res) {
var id = req.user.uid;
var myProfile = {};
var userRef = firebase.db.collection('users').doc(id)
userRef.get()
.then(doc => {
if (!doc || !doc.exists) {
throw new Error("Profile doesn't exist")
}
var profile = doc.data();
profile.id = doc.id;
myProfile = profile;
return myProfile;
})
.catch(error => {
console.log('error', error);
})
})
And use Promise.all if you have multiple promises and you want's to execute them in once.
The Promise.all(iterable) method returns a single Promise that resolves when all of the promises in the iterable argument have resolved or when the iterable argument contains no promises. It rejects with the reason of the first promise that rejects.
For example:
var promise1 = new Promise((resolve, reject) => {
setTimeout(resolve, 100, 'foo1');
});
var promise2 = new Promise((resolve, reject) => {
setTimeout(resolve, 100, 'foo2');
});
var promise3 = new Promise((resolve, reject) => {
setTimeout(resolve, 100, 'foo3');
});
Promise.all([promise1, promise2, promise3])
.then(result => console.log(result))
//result [foo1, foo2, foo3]
Hopes this will help you !!
If you can't fix this issue but still want to run your code...
open : eslintrc.json file (search from project root directory)
search : 'promise/always-return'
change : Case 1: if (existing value is 2) => change to 1
Case 2: else if(existing value is "error" => change to "warn")
It will make this error into warning, but be careful with it... Also use eslint plungin in your editor to remind of good practice. Otherwise you won't get any promise/always-return related warnings.
Also make sure you find the right eslintrc.json if more than 1 appears on your search
why is this happening ? Each then() should return a value or throw So I am adding this since the why was never explained.
For anyone else wondering the why. this is not a normal JS Error. This is a direct result of using the ES-Lint promise package:
https://github.com/xjamundx/eslint-plugin-promise
https://github.com/xjamundx/eslint-plugin-promise/blob/development/rules/always-return.js
This package has a ruleset to error when it does not find a return from a promise. This can be helpful to identify promise flow errors. You could fix this by simply adding a return as thats what the linter is triggered by or editing the es-lint rule set (not recommended). Thats why I assume it works when it is not being linted causing your confusion.
router.get('/account', function(req, res) {
var id = req.user.uid
var myProfile = {}
var profilePromise = new Promise(function(resolve, reject) {
var userRef = firebase.db.collection('users').doc(id)
userRef.get()
.then(doc => { // Error occurs on this line
if (doc.exists) {
var profile = doc.data()
profile.id = doc.id
myProfile = profile
return null
} else {
reject(Error("Profile doesn't exist"))
}
})
.catch(error => {
reject(error)
})
})
// More promises further on, which I wait for
})
Here is a list of the default rulesets used by that package. Hope it helps anyone else trying to get background on why this return was needed.
{
"rules": {
"promise/always-return": "error",
"promise/no-return-wrap": "error",
"promise/param-names": "error",
"promise/catch-or-return": "error",
"promise/no-native": "off",
"promise/no-nesting": "warn",
"promise/no-promise-in-callback": "warn",
"promise/no-callback-in-promise": "warn",
"promise/avoid-new": "warn",
"promise/no-new-statics": "error",
"promise/no-return-in-finally": "warn",
"promise/valid-params": "warn"
}
Currently try to learn Node JS and getting my head around Async/Sync workflow.
Try to the follow:
Step 1:
- Get data 1 with function 1
- Get data 2 with function 2
- Get data 3 with function 3
Step2:
- Work out logic with data 1,2,3
Step 3
- Do final call
I been looking at Q and Async packages but still havent really find an example.
Can some one show me how they will go about this issue in Node JS?
Thanks
Not entirely clear on your implementation, but depending on how specific your ordering needs to be you could try something like this:
var data1 = null;
var data2 = null;
var data3 = null;
async.series([
function(httpDoneCallback){
async.parallel([
function(data1Callback){
$http(...).then(function(response){
// some logic here
data1 = response;
data1Callback();
})
},
function(data2Callback){
$http(...).then(function(response){
// some logic here
data2 = response;
data2Callback();
})
},
function(data3Callback){
$http(...).then(function(response){
// some logic here
data3 = response;
data3Callback();
})
}
], function(){
//all requests dome, move onto logic
httpDoneCallback();
})
},
function(logicDoneCallback){
// do some logic, maybe more asynchronous calls with the newly acquired data
logicDoneCallback();
}
], function(){
console.log('all done');
})
Do you want function 1, 2, and 3 to trigger at the same time? If so then this should help:
var async = require('async');
async.parallel([
function(cb1) {
cb1(null, "one")
},
function(cb2){
cb2(null, "two")
},
function(cb3){
cb3(null, "three")
}
], function(err, results) {
console.log(results); // Logs ["one", "two", "three"]
finalCall();
});
To explain, every function in the array submitted as the first param to the parallel method will also receive a callback function. Activating the callback function signifies that you're done fetching your data or doing whatever you need to do in said function. All three functions will trigger at the same time, and once all three callbacks are called, the final function is called. The callback accepts two parameters: "error", and "result." If everything's successful, pass "null" as the error parameter. The results will be given to the final function as an array containing each of the results for your individual functions.
You can setup a chain of Promises to do things sequentially:
var funcA = () => {
return new Promise((resolve, reject) => {
setTimeout(() => {
resolve('some data from A')
}, 1000)
});
}
var funcB = (dataFromA) => {
return new Promise((resolve, reject) => {
setTimeout(() => {
resolve(dataFromA + ' data from B')
}, 2000)
})
}
var funcC = (dataFromB) => {
return new Promise((resolve, reject) => {
setTimeout(() => {
resolve(dataFromB + ' data from C')
}, 500)
})
}
// Doing the functions on after another
funcA().then(funcB).then(funcC).then((data) => {
console.log(data);
})
Or if you want to do them all at the same time you can use Promise.all():
var promises = [];
promises.push(new Promise((resolve, reject) => {
setTimeout(() => {
resolve('some data from A')
}, 1000)
}));
promises.push(new Promise((resolve, reject) => {
setTimeout(() => {
resolve('some data from B')
}, 1000)
}));
promises.push(new Promise((resolve, reject) => {
setTimeout(() => {
resolve('some data from C')
}, 1000)
}));
// Execute the array of promises at the same time, and wait for them all to complete
Promise.all(promises).then((data) => {
console.log(data);
})
Probably the best thing to do is use Promises like #Tyler here states. However, for conceptual understanding it is best to first understand the node callback pattern.
Because some tasks take time, we give the task a function and say 'When you are done, put the data you retrieved into this function'. These functions that we give to other functions are called callbacks. They must be constructed to accept the data, and also an error in case there is a problem while fetching the data. In Node the error is the first callback parameter and the data is the second.
fs.readFile('/file/to/read.txt', function callback(error, data) {
if (error) console.log(error);
else console.log(data);
});
In this example, once node reads the file, it will feed that data into the callback function. In the callback we must account for the case that there was a problem and handle the error.
In your question you want to do multiple async tasks and use their results. Therefore you must take this pattern and nest several of them. So, continuing this example, if there is no error you will begin another async task.
fs.readFile('/file/to/read.txt', function callback(error, data) {
if (error) console.log(error);
else {
someOtherAsyncThing(function callback(error, data2) {
if (error) console.log(error);
else {
console.log(data + data2)
}
});
}
});