here's my function...
exports.createSomeFunc = async (param1, callback) => {
for (var i = 0; i < 10; i++) {
await anotherFunc(param2, function (err, result, request, response) {
console.log(result + i);
});
}
})
For some reason I get "SyntaxError: await is only valid in async function" pointing at the Await line. What am I missing?
Someone asked what am I trying to do? I am trying to have the for loop wait at the function until it finishes before continuing to the next iteration. Here's another example:
exports.anotherFunc = (parm, callback) => {
for (var i = 0; i < 5; i++) {
asyncForEach();
console.log(i);
}
}
async function asyncForEach () {
console.log("in");
await setTimeout(function () {
console.log("out");
}, 100);
}
This body of code returns the following:
0
in
1
in
2
in
3
in
4
out
out
out
out
out
so again the For-loop continues. How do I make the FOR loop wait for result of:
1
in
out
2
in
out
...
You are not waiting for setTimeout to complete. If you want to wait for each setTimeout call to complete in each iteration of for loop. You should use promise inside setTimeout and resolve only when callback is executed.
const myFun = async(parm, callback) => {
for (var i = 0; i < 5; i++) {
console.log(i);
await asyncForEach();
}
}
function asyncForEach() {
return new Promise((resolve) => {
console.log("in");
setTimeout(function() {
console.log("out");
return resolve();
}, 100);
})
}
myFun();
the await is used for promise.
const anotherFunc = async (oarams, callback) => {
for (var i = 0; i < 5; i++) {
await asyncProcess();
console.log(i)
}
}
function asyncProcess() {
return new Promise(async (resolve) => {
console.log("in");
await delay(100)
resolve(true)
})
}
const delay = ms => new Promise((resolve) =>setTimeout(() => {
console.log("out")
resolve(true)
}, ms));
anotherFunc();
Related
I have a loop which calls a method that does stuff asynchronously. This loop can call the method many times. After this loop, I have another loop that needs to be executed only when all the asynchronous stuff is done.
So this illustrates what I want:
for (i = 0; i < 5; i++) {
doSomeAsyncStuff();
}
for (i = 0; i < 5; i++) {
doSomeStuffOnlyWhenTheAsyncStuffIsFinish();
}
I'm not very familiar with promises, so could anyone help me to achieve this?
This is how my doSomeAsyncStuff() behaves:
function doSomeAsyncStuff() {
var editor = generateCKEditor();
editor.on('instanceReady', function(evt) {
doSomeStuff();
// There should be the resolve() of the promises I think.
})
}
Maybe I have to do something like this:
function doSomeAsyncStuff() {
var editor = generateCKEditor();
return new Promise(function(resolve,refuse) {
editor.on('instanceReady', function(evt) {
doSomeStuff();
resolve(true);
});
});
}
But I'm not sure of the syntax.
You can use Promise.all (spec, MDN) for that: It accepts a bunch of individual promises and gives you back a single promise that is resolved when all of the ones you gave it are resolved, or rejected when any of them is rejected.
So if you make doSomeAsyncStuff return a promise, then:
const promises = [];
// ^^^^^−−−−−−−−−−−−−−−−−−−−−−−−−−− use `const` or `let`, not `var`
for (let i = 0; i < 5; i++) {
// ^^^−−−−−−−−−−−−−−−−−−−−−−−− added missing declaration
promises.push(doSomeAsyncStuff());
}
Promise.all(promises)
.then(() => {
for (let i = 0; i < 5; i++) {
// ^^^−−−−−−−−−−−−−−−− added missing declaration
doSomeStuffOnlyWhenTheAsyncStuffIsFinish();
}
})
.catch((e) => {
// handle errors here
});
MDN has an article on promises here. I also cover promsies in detail in Chapter 8 of my book JavaScript: The New Toys, links in my profile if you're interested.
Here's an example:
function doSomethingAsync(value) {
return new Promise((resolve) => {
setTimeout(() => {
console.log("Resolving " + value);
resolve(value);
}, Math.floor(Math.random() * 1000));
});
}
function test() {
const promises = [];
for (let i = 0; i < 5; ++i) {
promises.push(doSomethingAsync(i));
}
Promise.all(promises)
.then((results) => {
console.log("All done", results);
})
.catch((e) => {
// Handle errors here
});
}
test();
Sample output (because of the Math.random, what finishes first may vary):
Resolving 3
Resolving 2
Resolving 1
Resolving 4
Resolving 0
All done [0,1,2,3,4]
A reusable function works nicely for this pattern:
function awaitAll(count, asyncFn) {
const promises = [];
for (i = 0; i < count; ++i) {
promises.push(asyncFn());
}
return Promise.all(promises);
}
OP example:
awaitAll(5, doSomeAsyncStuff)
.then(results => console.log('doSomeStuffOnlyWhenTheAsyncStuffIsFinished', results))
.catch(e => console.error(e));
A related pattern, is iterating over an array and performing an async operation on each item:
function awaitAll(list, asyncFn) {
const promises = [];
list.forEach(x => {
promises.push(asyncFn(x));
});
return Promise.all(promises);
}
Example:
const books = [{ id: 1, name: 'foo' }, { id: 2, name: 'bar' }];
function doSomeAsyncStuffWith(book) {
return Promise.resolve(book.name);
}
awaitAll(books, doSomeAsyncStuffWith)
.then(results => console.log('doSomeStuffOnlyWhenTheAsyncStuffIsFinished', results))
.catch(e => console.error(e));
/*** Worst way ***/
for(i=0;i<10000;i++){
let data = await axios.get(
"https://yourwebsite.com/get_my_data/"
)
//do the statements and operations
//that are dependant on data
}
//Your final statements and operations
//That will be performed when the loop ends
//=> this approach will perform very slow as all the api call
// will happen in series
/*** One of the Best way ***/
const yourAsyncFunction = async (anyParams) => {
let data = await axios.get(
"https://yourwebsite.com/get_my_data/"
)
//all you statements and operations here
//that are dependant on data
}
var promises = []
for(i=0;i<10000;i++){
promises.push(yourAsyncFunction(i))
}
await Promise.all(promises)
//Your final statement / operations
//that will run once the loop ends
//=> this approach will perform very fast as all the api call
// will happen in parallal
const doSomeAsyncStuff = async (funcs) => {
const allPromises = funcs.map(func => func());
return await Promise.all(allPromises);
}
doSomeAsyncStuff([
() => new Promise(resolve => setTimeout(() => resolve(), 100)),
() => new Promise(resolve => setTimeout(() => resolve(), 100)),
() => new Promise(resolve => setTimeout(() => resolve(), 100)),
() => new Promise(resolve => setTimeout(() => resolve(), 100)),
() => new Promise(resolve => setTimeout(() => resolve(), 100)),
]);
Here is code that I wrote for myself in order to understand the answers stated here. I have mongoose queries in a for loop, so I put here the asyncFunction to take its place. Hope it helps anyone. You can run this script in node or any of many Javascript runtimes.
let asyncFunction = function(value, callback)
{
setTimeout(function(){console.log(value); callback();}, 1000);
}
// a sample function run without promises
asyncFunction(10,
function()
{
console.log("I'm back 10");
}
);
//here we use promises
let promisesArray = [];
let p = new Promise(function(resolve)
{
asyncFunction(20,
function()
{
console.log("I'm back 20");
resolve(20);
}
);
});
promisesArray.push(p);
for(let i = 30; i < 80; i += 10)
{
let p = new Promise(function(resolve)
{
asyncFunction(i,
function()
{
console.log("I'm back " + i);
resolve(i);
}
);
});
promisesArray.push(p);
}
// We use Promise.all to execute code after all promises are done.
Promise.all(promisesArray).then(
function()
{
console.log("all promises resolved!");
}
)
Here's an elegant solution for you if you want to do the same thing multiple times:
await Promise.all(new Array(10).fill(0).map(() => asyncFn()));
This creates an array with 10 items, fills it with zeros and then maps it to an array of promises.
My code:
console.log('Start')
getMember((member) => { console.log(member) })
console.log('End')
function getMember(next) {
for (let index = 0; index < 1000000000; index++) {
// Simulation setTimeout()
}
next('In getMember')
}
Return:
Start
In getMember
End
I thought I had the following result (see below) but it doesn't, can you explain me?
Return:
Start
End
In getMember
Thanks,
Yves
console.log('Start')
function resolveAfter2Seconds() {
return new Promise(resolve => {
// setTimeout(() => { resolve('resolved') }, 2000);
for (let index = 0; index < 1000000000; index++) { }
resolve('resolved !');
})
};
async function asyncCall() {
console.log('calling');
const result = await resolveAfter2Seconds();
console.log(result);
}
asyncCall();
console.log('End');
Return:
Start
calling
End
resolved !
I have the impression that promises handle callbacks better !
I want to add a timeout such that if any of these "tasks" takes longer than 5 minutes, it should stop that function and resolve the promise. I've been struggling a bit, any help is appreciated. Thanks!
if (require.main === module) {
(async () => {
const tasks = [];
for (let i = 1; i <= NB_PARALLEL; i++) {
tasks.push(buildReferenceSpaceCollection(json));
}
const results = await Promise.all(tasks);
console.log(results);
process.exit(0);
})().catch(console.error);
}
You could define a wait-function that rejects after the given amount of time and then use Promise.race on that wait-function and your Promise.all. Now, if your promises inside Promise.all take longer than the wait, Promise.race will reject, otherwise the resolved values will be assigned to results.
function wait(ms) {
return new Promise((_, reject) => {
setTimeout(() => {
reject(new Error("wait time exceeded"));
}, ms);
})
}
(async () => {
const tasks = [];
for (let i = 1; i <= NB_PARALLEL; i++) {
tasks.push(buildReferenceSpaceCollection(json));
}
// 5 mins in ms
const wait5MinPromise = wait(5*60*1000);
const results = await Promise.race([wait5MinPromise, Promise.all(tasks)]);
console.log(results);
process.exit(0);
})().catch(console.error)
;
Just for fun (Live demo):
import { CPromise } from "c-promise2";
const generateYourAsyncTaskHereToAddInQueue= async (value)=> {
console.log(`Inner task started [${value}]`);
await CPromise.delay(1000);
return value;
}
(async () => {
const results = await CPromise.all(function* () {
for (let i = 0; i < 10; i++) {
yield generateYourAsyncTaskHereToAddInQueue(`Task result ${i}`);
}
}, {concurrency: 3}).timeout(5000);
console.log(`results:`, results);
return results;
})().catch(console.error);
How can I wait until the function a is complete, but is not working as I expected.
Here is my code:
var start = Date.now();
function a() {
setTimeout(function(){ console.log(Date.now() - start); }, 500);
for(var i=0; i<100; i++) {
console.log(i);
//
}
}
const main = async () => {
await a();
console.log("Done");
};
main().catch(console.error);
You have to return promise when you call await. The await operator is used to wait for a Promise. It can only be used inside an async function. Check here for more details:
async function
var start = Date.now();
function a() {
return new Promise(function(resolve, reject) {
setTimeout(function(){ console.log(Date.now() - start); resolve()}, 500);
for(var i=0; i<100; i++) {
console.log(i);
//
}
})
}
const main = async () => {
await a();
console.log("Done");
};
main().catch(console.error);
var start = Date.now();
function a() {
return new Promise((resolve, reject)=> {
setTimeout(function(){ console.log(Date.now() - start); resolve() }, 500);
for(var i=0; i<100; i++) {
console.log(i);
//
}
});
}
const main = async () => {
await a();
console.log("Done");
};
main().catch(console.error);
you can use q module for make promise also:
var q = require('q')
var start = Date.now();
function a() {
let defer = q.defer()
setTimeout(function(){ console.log(Date.now() - start); defer.resolve();}, 500)
for(var i=0; i<100; i++) {
console.log(i);
}
return defer.promise;
}
const main = async () => {
await a();
console.log("Done");
};
main().catch(console.error);
I'm new to node, playing with promise loops
on completion of my loop (10 iterations) I want to know that the function has completed, how do I do this?
here is my code
var promise1 = new Promise((resolve, reject) => {
for (let i = 0, p = Promise.resolve() ; i < 10; i++) {
p = p.then(_ =>
new Promise(resolve => {
setTimeout(function () {
console.log(i);
resolve();
}, 50);
}
));
}
});
//start
promise1.then(function (result) {
// Resolve
console.log("done");
}, function (result) {
// Reject
console.error(result);
});
promise1 never completes because you forgot to call resolve() or reject() for it.
Please check the code snippet below. I added comments before the lines I added or changed.
// Use different names for the `resolve` callbacks to be clear which is which
var promise1 = new Promise((res) => {
// Move the declaration of `p` outside the loop to let it be visible after the loop
let p = Promise.resolve();
for (let i = 0; i < 10; i++) {
p = p.then(_ =>
new Promise(resolve => {
setTimeout(function () {
console.log(i);
resolve();
}, 50);
}
));
}
// Chain the completion of the outer promise when `p` resolves
p.then(() => res());
});
//start
promise1.then(function (result) {
// Resolve
console.log("done");
}, function (result) {
// Reject
console.error(result);
});
If you want to resolve multiple promises at a time you can use Promise.all.
What you can do is put every promise into array and resolve by using Promise.all function like below.
var promises = [];
for (let i = 0, i < 10; i++) {
promises.push(new Promise.resolve(i));
}
var results = Promise.all(promises);
console.log(results);
function abc() {
return new Promise(function (resolve, reject) {
var events = [];
for(var i=0;i<5;i++) {
var event_arr = {};
var event_city = {};
event_arr.event_city = {"city":"m-"+i};
events.push(event_arr);
}
if(events) {
resolve(events)
}else {
reject("NOp");
}
}).then((events)=>{
var p = Promise.resolve()
var events2 = [];
var i=0;
events.forEach(function(element) {
p = p.then(() => {
return new Promise(function (resolve, reject) {
var address_info = {};
var event_arr = {};
var event_city = element.event_city;
event_arr.event_cityy = event_city;
setTimeout(function () {
var address_info = {"address":"B-"+i};
console.log(i);
i++;
event_arr.address_info = address_info;
events2.push(event_arr);
// console.log(events2)
//resolve(events2);
if(events2) {
resolve(events2);
}else {
reject("NOp2");
}
}, 5000)
}).then((events2)=>{
return events2;
});
})
});
return p.then((events2)=>{
return events2;
});
});
}
var v = abc();
v.then(function(events2) {
console.log(events2)
console.log("-------X------------")
})