Hopefully someone can point me to the right direction. I read up on waiting for functions to complete before continuing and I resolved myself to using await/async but I am just stuck now.
I tried to get the Async/Await process to work, tried to inject the await in various locations, with adjusting the functions to be async, but i can not get the PSA_Resultbody to return to the original request. Any pointers would be appreciated.
Thank you,
CE
PSA_Resultbody = ProcessPSAAPI(xmlpackage, PSA_Action);
console.log("3 - Returned data:" + PSA_Resultbody);
calls the below:
async function ProcessPSAAPI(xmlpackage, PSA_Action) { //psa action is part of the options
var options = {...};
var req = https.request(options, function (res) {
var chunks = [];
res.on("data", function (chunk) {
chunks.push(chunk);
});
res.on("end", function (chunk) {
var body = Buffer.concat(chunks);
console.log('0 - Start '+body.toString());
if(res.statusCode != 200) {
PSA_Resultcode = "Error: " +res.statusCode +" - "+ res.statusMessage;
} else {
PSA_Resultcode = "Success: " +res.statusCode +" - "+ res.statusMessage;
PSA_Resultbody = ParseResults(body.toString()); //parse the results for later use --SCRIPT NEEDS TO WAIT FOR RESULTBODY TO COMPLETE
console.log("1 -PSA_Resultbody as part of RES = "+PSA_Resultbody);
}
});
res.on("error", function (error) {
console.error(error);
PSA_Resultcode = res.statusCode +" - "+ res.statusMessage;
});
});
console.log('2 -RESULT BODY BEFORE SENDING BACK TO INITIATING FUNCTION: '+PSA_Resultbody);
req.write(xmlpackage);
req.end();
return PSA_Resultbody;
Based on the above, my console log order is: 3,2,0,1 in stead of 0,1,2,3.
0 and 1 will have the correct data, so the API Call does work, but 2 will be "undefined" and should have the same data that is in 1.
There's no way to await an event emitter, so using async in this case isn't going to be useful. You cannot "return" from inside an event either.
The solution here is to return a new custom promise and to use resolve() inside of the "end" event of your emitter.
It will look something like this:
function ProcessPSAAPI(xmlpackage, PSA_Action) {
return new Promise( (resolve, reject) => {
// other code
res.on("end", function (chunk) {
// other code
resolve(PSA_Resultbody);
});
res.on("error", function (error) {
// other code
reject(error);
});
});
}
Here's a quick tutorial on creating your own promises, which I've written to simplify comprehension of the subject (official docs are somewhat dry and complex imho).
I did not change your code. I just put the appropriate promise structure in to get you started. This should really be a lesson in promises. async await is a shorthand promise structure. A Promise is one way you wait on code. It can be thought of as an array of callbacks that will be executed when the Promise is resolved.
A simple promise works like this:
const myPromise = new Promise(function(resolve, reject) {
/* Your logic goes in here. It can be anything.
* But the important part to remember is that when you have success, resolve it.
* When you have a failure, reject it.
*/
someCallBackPattern(function(error, data) {
if(error) {
reject(error);
} else {
resolve(data);
}
});
});
// To get the data out you use 'then', and 'catch'. then has two arguments.
myPromise.then(function(data) {
// The first argument is the result from resolve.
}, function(err) {
// The second argument is the result from reject.
}).catch((err) => {
// you can also get to the error from the catch callback
});
This is kinda messy and complex. So there is async await.
async function() {
try {
const result = await myFunctionThatReturnsAPromise();
// result is the resolved data
} catch (err) {
// err is the rejected Error
}
}
function myFunctionThatReturnsAPromise() {
return new Promise((resolve, reject) => {
// your code
})
}
And thats how it works.
async function someFunction () { // You can not wait on results unless you are in an await function
PSA_Resultbody = await ProcessPSAAPI(xmlpackage, PSA_Action); // await on your results.
console.log("3 - Returned data:" + PSA_Resultbody);
}
function ProcessPSAAPI(xmlpackage, PSA_Action) { // This does not need to be async. Unless you are awaiting in it.
return new Promise((resolve, reject) => { // async await is a shorthand promise structure. Although you do not need to use promises. It really helps to get the structure correct.
var options = {...};
var req = https.request(options, function (res) {
var chunks = [];
res.on("data", function (chunk) {
chunks.push(chunk);
});
res.on("end", function (chunk) {
var body = Buffer.concat(chunks);
console.log('0 - Start '+body.toString());
if(res.statusCode != 200) {
PSA_Resultcode = "Error: " +res.statusCode +" - "+ res.statusMessage;
reject(new Error(PSA_Resultcode)); // Reject you errors
} else {
PSA_Resultcode = "Success: " +res.statusCode +" - "+ res.statusMessage;
PSA_Resultbody = ParseResults(body.toString()); //parse the results for later use --SCRIPT NEEDS TO WAIT FOR RESULTBODY TO COMPLETE
console.log("1 -PSA_Resultbody as part of RES = "+PSA_Resultbody);
resolve(PSA_Resultbody); // Resolve your result
}
});
res.on("error", function (error) {
console.error(error);
PSA_Resultcode = res.statusCode +" - "+ res.statusMessage;
reject(new Error(PSA_Resultcode)); // Reject you errors
});
});
console.log('2 -RESULT BODY BEFORE SENDING BACK TO INITIATING FUNCTION: '+PSA_Resultbody);
req.write(xmlpackage);
req.end();
})
}
Related
I have been trying to create an api like this where I tried different things like using array methods like map/filter/reduce where either I get pending promises or result returned before execution of api call.
So my doubt is -->
How do I get total number of drawn matches of all pages ( so I need to add data.total from all pages).
How to better understand this behaviour.
async function getNumDraws(year) {
const goals = [...Array(11).keys()];
let result = 0;
console.log(`before loop ${new Date()}`);
for(let goal of goals){
console.log(`in loop before await ${new Date()}`);
await require('https').get(`https://jsonmock.hackerrank.com/api/football_matches?year=${year}&team1goals=${goal}&team2goals=${goal}`,res=>{
let data="";
res.on('data', (chunk) => {
data += chunk;
});
// The whole res has been received. Print out the result.
res.on('end', () => {
data=JSON.parse(data);
console.log(result,data.total)
result= result + data.total;
});
})
console.log(`in loop after await ${new Date()}`);
}
console.log(`after loop ${new Date()}`);
return result;
}
console.log(getNumDraws(2011));
https.get is a callbacked function so await won't work. You should promisify it first like they did in this other SO question;
const https = require("https"); // only require this once
const getJSONAsync = url => new Promise((resolve, reject) => { // define a function getJSONAsync which returns a Promise that wraps the https.get call, getJSONAsync is awaitable
let req = https.get(url, res => { // make the https.get request
if(res.statusCode < 200 || res.statusCode >= 300) { // if the status code is an error one
return reject(new Error('statusCode=' + res.statusCode)); // reject and skip the rest
}
let body = []; // otherwise accumulate..
res.on('data', chunk => body.push(chunk)); // ..the data
res.on('end', () => { // on end
try {
body = JSON.parse(Buffer.concat(body).toString()); // try to JSON.parse the data
} catch(e) {
reject(e); // reject if an error occurs
}
resolve(body); // resolve the parsed json object otherwise
});
});
req.on("error", error => reject(error)); // reject if the request fails too (if something went wrong before the request is sent for example)
});
async function getNumDraws(year) {
let result = 0;
for(let goal = 0; goal < 11; goal++) {
let data = await getJSONAsync(`https://jsonmock.hackerrank.com/api/football_matches?year=${year}&team1goals=${goal}&team2goals=${goal}`);
result += data.total;
}
return result;
}
Note: getJSONAsync is not specific to getNumDraws, you can use it somewhere else if you need it, and since it returns a Promise you can either await it like getNumDraws does or use it with then/catch blocks like so:
getJSONAsync("url")
.then(data => {
// data is the parsed json returned by the request
})
.catch(error => {
// the error message if something fails
})
I spawn a python child process in javascript (for coding a Bot with Microsoft Botframework) like this:
async function searchForRelevantDoc (context) {
var msg = context.activity.text;
var spawn = require('child_process').spawn,
py = spawn('python', ['../path/to/file.py', msg]),
output = '';
py.stdin.setEncoding = 'utf-8';
py.stdout.on('data',
(data) => {
output += data.toString();
console.log('output was generated: ' + output);
});
// Handle error output
py.stderr.on('data', (data) => {
// As said before, convert the Uint8Array to a readable string.
console.log('error:' + data);
});
py.stdout.on('end', async function(code){
console.log('output: ' + output);
console.log(`Exit code is: ${code}`);
// "return" is probably wrong, what should be done instead?
return output;
}
});
}
I want output to be returned as the value of the function searchForRelevanceDoc(). How can this be done? I am not able to use
await context.sendActivity(output)
instead of the return statement. Error message:
TypeError: Cannot perform 'get' on a proxy that has been revoked
Function searchForRelevanceDoc is called like so:
//in bot.js
const pysearch = require('../bboti/python-search');
class MyBot {
// constructor...
async onTurn(context) {
// ...
var search_engine_answer = pysearch.searchForRelevantDoc(context);
context.sendActivity(search_engine_answer)
// ...
}
}
The easiest way to return/resolve output from searchForRelevantDoc, using async/await is to use events.once which was added in Node 11.13.0, and wait for close event of py
const { once } = require('events'); // Added in Node 11.13.0
async function searchForRelevantDoc (context) {
var msg = context.activity.text;
var spawn = require('child_process').spawn,
py = spawn('python', ['../path/to/file.py', msg]),
output = '';
py.stdin.setEncoding = 'utf-8';
py.stdout.on('data', (data) => {
output += data.toString();
console.log('output was generated: ' + output);
});
// Handle error output
py.stderr.on('data', (data) => {
// As said before, convert the Uint8Array to a readable string.
console.log('error:' + data);
});
py.stdout.on('end', async function(code){
console.log('output: ' + output);
console.log(`Exit code is: ${code}`);
});
await once(py, 'close')
return output;
}
If you're using an older version of Node, you can wrap it in a new Promise
async function searchForRelevantDoc (context) {
// ...
// You can also check for `code` and reject if a non zero code is returned
await new Promise(resolve => py.on('close', resolve));
return output;
}
Then when calling searchForRelevantDoc you need to use await or .then
class MyBot {
// constructor...
async onTurn(context) {
// ...
var search_engine_answer = await pysearch.searchForRelevantDoc(context);
context.sendActivity(search_engine_answer)
// ...
}
}
#Marcos Casagrande's answer is more beautiful, I just wanted to add the promise solution as well.
You can just return a new Promise and wrap the stdout.on inside the promise.
async function searchForRelevantDoc (context) {
...
return new Promise((res, rej) => {
py.stdout.on('end', async function(code){
console.log('output: ' + output);
console.log(`Exit code is: ${code}`);
// "return" is probably wrong, what should be done instead?
res(output);
})
});
}
Promises hot-load so whenever you declare a promise the function will start to run. So basically the event will be attached.
Make your function
return new Promise((resovle, reject) => ...)
and resolve needed value
I'm trying to get my head around promises, I think I can see how they work in the way that you can say do Step 1, Step 2 and then Step 3 for example.
I have created this download function using node-fetch (which uses native Promises)
## FileDownload.js
const fetch = require('node-fetch');
const fs = require('fs');
module.exports = function(url, target) {
fetch(url)
.then(function(res) {
var dest = fs.createWriteStream(target);
res.body.pipe(dest);
}).then(function(){
console.log(`File saved at ${target}`)
}).catch(function(err){
console.log(err)
});
}
So this all executes in order and I can see how that works.
I have another method that then converts a CSV file to JSON (again using a promise)
## CSVToJson.js
const csvjson = require('csvjson');
const fs = require('fs');
const write_file = require('../helpers/WriteToFile');
function csvToJson(csv_file, json_path) {
return new Promise(function(resolve, reject) {
fs.readFile(csv_file, function(err, data){
if (err)
reject(err);
else
var data = data.toString();
var options = {
delimiter : ',',
quote : '"'
};
const json_data = csvjson.toObject(data, options);
write_file(json_path, json_data)
resolve(data);
});
});
}
module.exports = {
csvToJson: csvToJson
}
When I call these functions one after another the second function fails as the first has not completed.
Do I need to wrap these two function calls inside another promise, even though on their own they each have promises implemented?
Please advise if I am totally misunderstanding this
When I call these functions one after another the second function fails as the first has not completed.
There are two issues with the first:
It doesn't wait for the file to be written; all it does is set up the pipe, without waiting for the process to complete
It doesn't provide any way for the caller to know when the process is complete
To deal with the first issue, you have to wait for the finish event on the destination stream (which pipe returns). To deal with the second, you need to return a promise that won't be fulfilled until that happens. Something along these lines (see ** comments):
module.exports = function(url, target) {
// ** Return the end of the chain
return fetch(url)
.then(function(res) {
// ** Unfortunately, `pipe` is not Promise-enabled, so we have to resort
// to creating a promise here
return new Promise((resolve, reject) => {
var dest = fs.createWriteStream(target);
res.body.pipe(dest)
.on('finish', () => resolve()) // ** Resolve on success
.on('error', reject); // ** Reject on error
});
}).then(result => {
console.log(`File saved at ${target}`);
return result;
});
// ** Don't `catch` here, let the caller handle it
}
Then you can use then and catch on the result to proceeed to the next step:
theFunctionAbove("/some/url", "some-target")
.then(() = {
// It worked, do the next thing
})
.catch(err => {
// It failed
});
(Or async/await.)
Side note: I haven't code-reviewed it, but a serious issue in csvToJson jumped out, a minor issue as well, and #Bergi has highlighted a second one:
It's missing { and } around the else logic
The minor issue is that you have var data = data.toString(); but data was a parameter of that function, so the var is misleading (but harmless)
It doesn't properly handle errors in the part of the code in the else part of the readFile callback
We can fix both by doing a resolve in the else and performing the rest of the logic in a then handler:
function csvToJson(csv_file, json_path) {
return new Promise(function(resolve, reject) {
fs.readFile(csv_file, function(err, data){
if (err)
reject(err);
else
resolve(data);
});
})
.then(data => {
data = data.toString();
var options = {
delimiter : ',',
quote : '"'
};
const json_data = csvjson.toObject(data, options);
write_file(json_path, json_data);
return data;
});
}
async lsEntered(){
if(this.service.wd == '')
{
await basic((this.service.wd));
}
else
{
await basic(('/'+this.service.wd));
}
this.files = await JSON.parse(localStorage.getItem('FILENAMES'));
var filesList = document.getElementById(this.trackLine.toString());
var li;
for (var i = 0; i < this.files.length; i++) {
li = document.createElement('li');
li.appendChild(document.createTextNode(this.files[i].name));
filesList.appendChild(li);
}
localStorage.clear();
}
I want to wait until basic is finished and JSON.parse finishes before displaying the values in the DOM. I'm getting the values of the previous call every time which is tell me the async is no working. To be fair I don't have tons of TS experience.
Edit: This is basic I was hoping not to have to deal with it as it's a javascript function and fragily integrated into this app.
var basic = function (path) {
var ACCESS_TOKEN = '';
var dbx = new Dropbox({ accessToken: ACCESS_TOKEN });
dbx.filesListFolder({ path: path })
.then(function (response) {
localStorage.setItem('FILENAMES',JSON.stringify(response.entries));
console.log(response);
})
.catch(function (error) {
console.error(error);
});
return false;
}
let myPromise = new Promise((resolve, reject) => {
// Work you want to execute
resolve("I am done");
});
myPromise.then((successMessage) => {
// successMessage is whatever we passed in the resolve(...) function above.
console.log("Yay! " + successMessage);
});
You can only await promises1 and basic is not returning a promise. You need to do
return dbx.filesListfolder(...)...
Also consider what Bergi said in their comment.
1: Actually, you can await any value, but there is no point in awaiting something that is not a promise. By not returning the promise from basic, lsEntered won't wait for the local storage to be set.
Inside a promise, I need to call and process an indeterminate number of asynch API responses after individually calling them either inside another promise, or after said promise, but before another so the order of execution is respected.
var promiseA = function() {
return new Promise(function(resolve, reject) {
// 1. Establish objects needed from one API endpoint
// 2. Call API endpoint for each object and parse
// 3. Only then continue to next promise
}
}
var finalPromise = function() {
return new Promise(function(resolve, reject) {
//
}
}
promiseA()
.then(finalPromise)
So inside promiseA, I find out how many objects I'll need to poll individually from an API. Each request is of course asynchronous. I need to make these calls and process the response before the final promise is called.
I am struggling to determine a pattern for this with promises, where I can dynamically create these promises and only allow the final promise to execute after the indeterminate and asynchronous have executed and processed. I've worked with other languages where this is possible, but I'm struggling to see it here with Promises.
Any help is appreciated.
I have changed the answer to incorporate the comments below. Since, you mentioned ES6 promises I shall stick to that. There are two basic types of callbacks that we might care about.
DOM load or other one time event callbacks (window.onload and so on)
Async method callback (AJAX call, setTimout and so on)
Since,
1.DOM load or other one time event
var p = new Promise(function(res, rej) {
window.onload = res();
};
2.Plain callback: these are callbacks that don't conform to a convention. e.g. setTimeout
var p = new Promise(function(res, rej){
setTimeout(function() {
//your business/view logic
success? res():rej(); //if successful resolve else reject
}, 2000);
});
In each of the above case the promise (var p) can be wrapped to be returned by a function.
var myAsyncMethod = function () {
var p = new ... // as mentioned in 1 or 2
return p;
}
Then the usage:
myAsyncMethod()
.then(function(){/* success-handler */})
.catch(function(/* failure-handler */));
Specific to your question you may have many such methods:
function baseAJAXCall (url) {
new Promise(functoin(rej, res) {
$.get(url, function(err, data){
if(err) {
rej();
}
else {
resolve(data);
}
});
}
};
function callAPIEndpoint(url) {
return baseAJAXCall(url);
}
function finalPromiseHandler () {
//your final business/view logic
}
//USAGE
callAPIEndpoint('/my-first-call')
.then(function(data){
var promiseArray = data.map(function(item){
return baseAJAXCall(item.url);
});
return Promise.all(promiseArray);
})
.then(finalPromiseHandler)
.catch(function(){
console.log('.error-message.');
});
Ref:
How do I convert an existing callback API to promises?.
http://www.datchley.name/es6-promises/
Links from comments below.
---OLD ANSWER: PLEASE OVERLOOK---
I am familiar with this library : https://github.com/kriskowal/q. And, you can do this using using the q.all and q.allSettled constructs. May be that is what you are looking for.
Normally, the pattern is to create a function that returns a promise.
function someAsyncFuncName1(url) {
var def = q.defer();
//async function
$.get(url, function(err, data){ //suppose
if(err){
def.reject();
}
else {
def.resolve(data); //pass the data to the .then() handler.
}
});
return def.promise;
}
function someAsyncFuncName2() {
var def = q.defer();
//async function
setTimeout(function(){ //suppose
//do something
if(good) {
def.resolve();
} else {
def.reject();
}
}, 1000); //arbitrary timeout of 1 second
return def.promise;
}
USAGE:
q.all([someAsyncFuncName1('/api-1'), someAsyncFuncName2()])
.then(function() {
//final handler
});
On a similar line of thought one can use q.allSettled() if you want to wait for all promises to return.
Hope this helps.
---EOF OLD ANSWER---
First of all, if async functions used in PromiseA don't return promises, you need to promisify them. You can do that with Promise constructor, but it's much better to use libraries, such as bluebird with their promisify methods.
Let's imagine, that we have two functions getUserIdsAsync and getUserAsync. The first on returns a list of user ids, getUserAsync returns an user data by userId. And you need to get a list of users by their ids. The code of PromiseA could look so:
var promiseA = function() {
return getUserIdsAsync()
.then(userIds => {
let ops = users.map(uid => getUserAsync(uid));
return Promise.all(ops);
});
}
The following snippet shows a solution without using any external library like bluebird. It follows the code snippet in your question (which seems to be more complicate than needed).
You have to collect all api promisses in an array. Then you can call Promise.all() to get a Promise for the end of all api promisses. Then you can do some final stuff, like parsing the result of each promise and continue afterwards.
function getRandomInt(min, max) {
return Math.floor(Math.random() * (max - min + 1)) + min;
}
var apiEndpoint = function (name) {
return new Promise( (resolve, reject) => {
setTimeout(() => resolve('API ' + name + ' job done'), 1000);
});
}
var promiseA = function() {
return new Promise( (resolve, reject) => {
const promisses = [];
for (var i=1; i < getRandomInt(3,6); i++) {
// 1. Establish objects needed from one API endpoint
promisses.push(apiEndpoint('This is number ' + i));
}
Promise.all(promisses).then( results => {
// do final stuff
for (const s of results) {
// 2. Call API endpoint for each object and parse
console.log(s);
}
// continue ...
// 3. Only then continue to next promise
resolve('now it is finished');
}).catch( err => reject(err) );
});
}
var finalPromise = function() {
return new Promise( (resolve, reject) => {
console.log('finalPromise');
resolve();
});
}
promiseA()
.then( () => finalPromise())
.catch(err => console.log(err) );
Please hold in mind that this solution is not easy to read. Using external libraries or reducing promisses can improve readability. Maybe you should take a look to the async/await pattern to get a much more better (readable) solution.
Here is a solution with async/await:
function getRandomInt(min, max) {
return Math.floor(Math.random() * (max - min + 1)) + min;
}
const apiEndpoint = function (name) {
return new Promise( (resolve, reject) => {
setTimeout(() => resolve('API ' + name + ' job done'), 1000);
});
}
async function promiseParallel () {
const promisses = [];
for (let i = 1; i < getRandomInt(3,6); i++) {
promisses.push(apiEndpoint('This is number ' + i));
}
for (const p of promisses) {
const x = await p;
console.log(x);
}
return ('everything is done');
}
promiseParallel().then( result => {
console.log(result);
}).catch( err => console.log(err) );
If you want call the promisses sequentially you can replace with:
async function promiseSequ () {
for (let i = 1; i < getRandomInt(3,6); i++) {
const x = await apiEndpoint('This is number ' + i);
console.log(x);
}
return ('everything is done');
}