Make ForEach Finish Before Each Loop - javascript

This piece of code waits for 2s and then console.log's each element straight away. How can I change this forEach to wait for every timeout to finish before looping over to the next one? So what I want is for 2s to go by, then it logs "1", then another 2s goes by, then it logs "2" etc. I looked at promises (half implemented here) but I don't understand how it works.
MyArray = [1, 2, 3];
MyArray.forEach(async element => {
await setTimeout(() => {
console.log(element);
}, 2000);
});

Async does not work with setTimeout, you can return a promise that resolves later and when that resolves return the next value and so on. Reduce can do this for you:
const MyArray = [1, 2, 3];
const later = (howLong, element) =>
new Promise(
resolve =>
setTimeout(() => {
console.log(element);
resolve(element)
}, howLong)
);
MyArray.reduce(
(all, element) =>
all.then(
() => later(2000, element)
),
Promise.resolve()
).then(()=>console.log("done"));
The following is in parallel:
const MyArray = [1, 2, 3];
Promise.all(
MyArray.map(element =>
new Promise(
resolve=>
setTimeout(() => {
console.log(element);
resolve(element)
}, 2000)
)
)
).then(
results=>console.log("results",results)
)

You may also use a simple recursive asynchronous sequencer as follows;
var sequenceAsync = ([d,...ds]) => d !== void 0 && asyncTask(d).then(v => (console.log(v), sequenceAsync(ds))),
asyncTask = n => new Promise(v => setTimeout(v, 1000, n)),
datas = [1,2,3];
sequenceAsync(datas);
void 0 is a perfect undefined value to check for undefined.

You are looking to run the promises in series, where each promise will resolve after 2 seconds and then next one will run
MyArray.reduce(function(chain, element) {
return chain.then(function() {
return new Promise(function(resolve, reject) {
setTimeout(() => {
console.log(element);
resolve(element)
}, 2000);
});
})
}, Promise.resolve([]))

Try this
function resolveAfter2Seconds(i) {
return new Promise(resolve => {
setTimeout(() => {
resolve(i);
}, 2000);
});
}
async function asyncCall(MyArray) {
for(var i=0; i<MyArray.length; i++){
var result = await resolveAfter2Seconds(MyArray[i]);
console.log(result);
}
}
var MyArray = [1, 2, 3];
asyncCall(MyArray);

Related

how to do Sequential API Calls in javascript? [duplicate]

var myArray = [1, 2, 3, 4, 5, 6]
function myPromise(num){
return new Promise(res => {
window.setTimeout(()=>{
res( console.log("done: " + num) )
},2000)
})
}
myPromise(myArray[0])
.then(x => myPromise(myArray[1]))
.then(x => myPromise(myArray[2]))
.then(x => myPromise(myArray[3]))
.then(x => myPromise(myArray[4]))
.then(x => myPromise(myArray[5]))
Right now, if I execute the statement above, it will run sequentially. In my actual use case the array is dynamically populated and I need to execute the myPromise() function for each member in myArray.
How can I make a "pauseable loop" that will loop for each item in the array, execute myPromise and wait for the promise to be resolved before continuing to the next iteration?
You can make the repeated application of .then into a fold pretty neatly if you’re okay with creating as many promises as array elements as is the case in the question:
myArray.reduce(
(p, x) =>
p.then(() => myPromise(x)),
Promise.resolve()
)
but given support, an async function is a better choice. It’s nicely readable and has O(1) instead of O(n) memory overhead.
const forEachSeries = async (iterable, action) => {
for (const x of iterable) {
await action(x)
}
}
forEachSeries(myArray, myPromise)
If you want to collect the return values as an array, that’s:
const mapSeries = async (iterable, fn) => {
const results = []
for (const x of iterable) {
results.push(await fn(x))
}
return results
}
or, without async function support,
const mapSeries = (iterable, fn) => {
const iterator = iterable[Symbol.iterator]()
const results = []
const go = () => {
const {value, done} = iterator.next()
if (done) {
return results
}
return fn(value).then(mapped => {
results.push(mapped)
return go()
})
}
return Promise.resolve().then(go)
}
Runnable snippet:
const myArray = [1, 2, 3, 4, 5, 6]
const sleep = ms =>
new Promise(res => {
setTimeout(res, ms)
})
const myPromise = num =>
sleep(500).then(() => {
console.log('done: ' + num)
})
const forEachSeries = async (iterable, action) => {
for (const x of iterable) {
await action(x)
}
}
forEachSeries(myArray, myPromise)
.then(() => {
console.log('all done!')
})
Don't create an array of promises. Create an array of functions returning a promise.
const f = x => new Promise(resolve => setTimeout(() => resolve(console.log(x)), 2000))
(async () => {
for (let job of [1, 2, 3, 4, 5, 6].map(x => () => f(x)))
await job()
})()
Promises start running immediately after creation. Therefore, sequential execution is ensured by constructing the next promise only after finishing the current one.
I know I am very late, and my answer is similar to what others have posted. But I thought I could post a clearer answer which may help any beginner.
Instead of using promises directly, we can use promise factory. Since the promise starts executing as soon as they are created using promise factory we delay the creation of promise.
In this example I create 5 which resolve after a second. I use a promiseCreator to create promises. Now the array promises uses promiseCreator to create 5 instances of promises. But array promiseFactories wraps promiseCreator in a function so promise is not invoked immediately. It is invoked when used.
Function executeSequentially executes all promiseLike sequentially.
When promise array is passed result is promise array itself executes parallely (actually they executed as soon as they are created, not when this line is called).
When promiseFactory array is passed result is new Promise is created when earlier promise has completed their execution.
const promiseCreator = (i, time, text) => {
return new Promise(resolve => setTimeout(
() => resolve(console.log(`${i} ${text}`)),
time)
);
}
const promises = [
promiseCreator(1, 1000, "parallel"),
promiseCreator(2, 1000, "parallel"),
promiseCreator(3, 1000, "parallel"),
promiseCreator(4, 1000, "parallel"),
promiseCreator(5, 1000, "parallel"),
]
const promiseFactories = [
() => promiseCreator(1, 1000, "sequential"),
() => promiseCreator(2, 1000, "sequential"),
() => promiseCreator(3, 1000, "sequential"),
() => promiseCreator(4, 1000, "sequential"),
() => promiseCreator(5, 1000, "sequential"),
]
function executeSequentially(promiseLikeArray) {
var result = Promise.resolve();
promiseLikeArray.forEach(function (promiseLike) {
result = result.then(promiseLike);
});
return result;
}
executeSequentially(promises)
executeSequentially(promiseFactories)
Also you can do it via recursive approach - executeSequentially calls itself:
function createPromise(x) {
return new Promise(res => {
setTimeout(() => {
console.log(x)
res(x);
}, x * 1000)
})
}
function executeSequentially(array) {
return createPromise(array.shift())
.then(x => array.length == 0 ? x : executeSequentially(array));
}
console.time('executeSequentially');
executeSequentially([1, 2, 3]).then(x => {
console.log('last value: ' + x);
console.timeEnd('executeSequentially');
});
Sequential:
you can use async await features to run promises sequentially . here's a snippet
async function chainPromiseCalls(asyncFunctions=[],respectiveParams=[]){
for(let i=0;i<asyncFunctions.length;i++){
const eachResult = await asyncFunctions[i](...respectiveParams[i]);
// do what you want to do with each result
}
return ;
}
Parallel:
for parallel you can just call each async function once in a loop , but if you do want to get their combined result , you can use Promise.all
function parallelPromiseCalls(asyncFunctions=[],respectiveParams=[]){
return Promise.all(asyncFunctions.map((func,index)=>func(...respectiveParams[index])))
.then(resultsList=>{
resultsList.forEach((result,index)=>{
// do what you want to do with each result in the list
})
return ;
})
}
note : I am considering respective parameters as an list of lists since multiple parameters should be passed to any one of the function , else if you have to pass only a single parameter to each then you can remove the spread operator.
You could use Array.reduce.
//type: [number]
var myArray = [1, 2, 3, 4, 5, 6] //doesn't really matter
//type: number -> Promise<number>
function myPromise(num){
return new Promise((resolve) => {
window.setTimeout(()=>{
resolve(console.log("done: " + num) )
},2000)
})
}
//Array.reduce has type: [a] ~> ((b, a) -> b), b) -> b
//So it can have type:
//[number] ~> ((Promise<number>, number) -> Promise<number>), Promise<number>) -> Promise<number>
//Therefore we need to give reduce a function that takes a Promise
//resolving to a number and a number which makes a new promise.
//This is the function we want:
function sequencePromises(promise, number) {
return new Promise((resolve) => {
resolve(promise.then(_ => myPromise(number)));
});
}
myArray.reduce(sequencePromises, Promise.resolve());
Of course, this simplistic approach won't work if you have a promise which can error, or if you need previous results, so you might want to make sequencePromises more generic:
function genericSequencePromises(promiseFunction) {
return (promise, parameter) => {
return new Promise((resolve, reject) =>
return promiseFunction(resolve,
reject,
promise,
parameter));
}
}
Then you can do whatever you want as long as you return a Promise.
Finally, you might benefit from this little helper:
function promiseSeries(array, reducer) {
return array.reduce(reducer, Promise.resolve());
}
Bringing it all together:
let sequencePromises = genericSequencePromises((resolve, reject, promise, num) => {
resolve(promise.then(_ => console.log(`done: ${num}`)));
}
promiseSeries(myArray, sequencePromises);
This way, you can not only handle the case in your question, but much more complex cases.
Here's a concise way using Array.reduce and async/await. Taking the eg array and myPromise function from OP:
var myArray = [1, 2, 3, 4, 5, 6];
function myPromise(num) {
return new Promise((res) => {
setTimeout(() => {
res(console.log("done: " + num));
}, 2000);
});
}
if your myPromise fn does not return result, then
myArray.reduce(async (a, b) => {
await a;
await myPromise(b);
}, null);
this can be easily modified to accumulate result. Eg, if your myPromise were to return a result, you could accumulate them in an array, in the order of myArray elements:
const resultArray = myArray.reduce(
async (a, b) => [...(await a), await myPromise(b)],
[]
);
or an object
const resultObject = myArray.reduce(async (a, b) => ({
async (a, b) => ({
...(await a),
[b]: await myPromise(b),
}),
{}
);
I would use babel and do it this way:
let args = [1, 2, 3];
const myPromise = async x => console.log('arg:',x);
const test = async () => {
for (let task of args.map(myPromise))
await task;
}
test().then(console.log('Done'));
<script src="https://unpkg.com/babel-standalone#6.24.0/babel.min.js"></script>
You can iterate over the array of elements and pass the params like this:
const arr = [1, 2, 3, 4, 5, 6]
const MyPromiseFunction = num => new Promise(
(resolve, reject) =>
// Your logic...
setTimeout(() => num <= 4
? resolve('Success!')
: reject('Rejected!'), 1000 * num)
)
const logMessage = (num, msg) =>
console.log(`For number ${num} promise result: ${msg}`)
arr.map(
async (num) => await MyPromiseFunction(num)
.then(message => logMessage(num, message))
.catch(reason => logMessage(num, reason))
)

How can I run these js function in row (node.js)? [duplicate]

var myArray = [1, 2, 3, 4, 5, 6]
function myPromise(num){
return new Promise(res => {
window.setTimeout(()=>{
res( console.log("done: " + num) )
},2000)
})
}
myPromise(myArray[0])
.then(x => myPromise(myArray[1]))
.then(x => myPromise(myArray[2]))
.then(x => myPromise(myArray[3]))
.then(x => myPromise(myArray[4]))
.then(x => myPromise(myArray[5]))
Right now, if I execute the statement above, it will run sequentially. In my actual use case the array is dynamically populated and I need to execute the myPromise() function for each member in myArray.
How can I make a "pauseable loop" that will loop for each item in the array, execute myPromise and wait for the promise to be resolved before continuing to the next iteration?
You can make the repeated application of .then into a fold pretty neatly if you’re okay with creating as many promises as array elements as is the case in the question:
myArray.reduce(
(p, x) =>
p.then(() => myPromise(x)),
Promise.resolve()
)
but given support, an async function is a better choice. It’s nicely readable and has O(1) instead of O(n) memory overhead.
const forEachSeries = async (iterable, action) => {
for (const x of iterable) {
await action(x)
}
}
forEachSeries(myArray, myPromise)
If you want to collect the return values as an array, that’s:
const mapSeries = async (iterable, fn) => {
const results = []
for (const x of iterable) {
results.push(await fn(x))
}
return results
}
or, without async function support,
const mapSeries = (iterable, fn) => {
const iterator = iterable[Symbol.iterator]()
const results = []
const go = () => {
const {value, done} = iterator.next()
if (done) {
return results
}
return fn(value).then(mapped => {
results.push(mapped)
return go()
})
}
return Promise.resolve().then(go)
}
Runnable snippet:
const myArray = [1, 2, 3, 4, 5, 6]
const sleep = ms =>
new Promise(res => {
setTimeout(res, ms)
})
const myPromise = num =>
sleep(500).then(() => {
console.log('done: ' + num)
})
const forEachSeries = async (iterable, action) => {
for (const x of iterable) {
await action(x)
}
}
forEachSeries(myArray, myPromise)
.then(() => {
console.log('all done!')
})
Don't create an array of promises. Create an array of functions returning a promise.
const f = x => new Promise(resolve => setTimeout(() => resolve(console.log(x)), 2000))
(async () => {
for (let job of [1, 2, 3, 4, 5, 6].map(x => () => f(x)))
await job()
})()
Promises start running immediately after creation. Therefore, sequential execution is ensured by constructing the next promise only after finishing the current one.
I know I am very late, and my answer is similar to what others have posted. But I thought I could post a clearer answer which may help any beginner.
Instead of using promises directly, we can use promise factory. Since the promise starts executing as soon as they are created using promise factory we delay the creation of promise.
In this example I create 5 which resolve after a second. I use a promiseCreator to create promises. Now the array promises uses promiseCreator to create 5 instances of promises. But array promiseFactories wraps promiseCreator in a function so promise is not invoked immediately. It is invoked when used.
Function executeSequentially executes all promiseLike sequentially.
When promise array is passed result is promise array itself executes parallely (actually they executed as soon as they are created, not when this line is called).
When promiseFactory array is passed result is new Promise is created when earlier promise has completed their execution.
const promiseCreator = (i, time, text) => {
return new Promise(resolve => setTimeout(
() => resolve(console.log(`${i} ${text}`)),
time)
);
}
const promises = [
promiseCreator(1, 1000, "parallel"),
promiseCreator(2, 1000, "parallel"),
promiseCreator(3, 1000, "parallel"),
promiseCreator(4, 1000, "parallel"),
promiseCreator(5, 1000, "parallel"),
]
const promiseFactories = [
() => promiseCreator(1, 1000, "sequential"),
() => promiseCreator(2, 1000, "sequential"),
() => promiseCreator(3, 1000, "sequential"),
() => promiseCreator(4, 1000, "sequential"),
() => promiseCreator(5, 1000, "sequential"),
]
function executeSequentially(promiseLikeArray) {
var result = Promise.resolve();
promiseLikeArray.forEach(function (promiseLike) {
result = result.then(promiseLike);
});
return result;
}
executeSequentially(promises)
executeSequentially(promiseFactories)
Also you can do it via recursive approach - executeSequentially calls itself:
function createPromise(x) {
return new Promise(res => {
setTimeout(() => {
console.log(x)
res(x);
}, x * 1000)
})
}
function executeSequentially(array) {
return createPromise(array.shift())
.then(x => array.length == 0 ? x : executeSequentially(array));
}
console.time('executeSequentially');
executeSequentially([1, 2, 3]).then(x => {
console.log('last value: ' + x);
console.timeEnd('executeSequentially');
});
Sequential:
you can use async await features to run promises sequentially . here's a snippet
async function chainPromiseCalls(asyncFunctions=[],respectiveParams=[]){
for(let i=0;i<asyncFunctions.length;i++){
const eachResult = await asyncFunctions[i](...respectiveParams[i]);
// do what you want to do with each result
}
return ;
}
Parallel:
for parallel you can just call each async function once in a loop , but if you do want to get their combined result , you can use Promise.all
function parallelPromiseCalls(asyncFunctions=[],respectiveParams=[]){
return Promise.all(asyncFunctions.map((func,index)=>func(...respectiveParams[index])))
.then(resultsList=>{
resultsList.forEach((result,index)=>{
// do what you want to do with each result in the list
})
return ;
})
}
note : I am considering respective parameters as an list of lists since multiple parameters should be passed to any one of the function , else if you have to pass only a single parameter to each then you can remove the spread operator.
You could use Array.reduce.
//type: [number]
var myArray = [1, 2, 3, 4, 5, 6] //doesn't really matter
//type: number -> Promise<number>
function myPromise(num){
return new Promise((resolve) => {
window.setTimeout(()=>{
resolve(console.log("done: " + num) )
},2000)
})
}
//Array.reduce has type: [a] ~> ((b, a) -> b), b) -> b
//So it can have type:
//[number] ~> ((Promise<number>, number) -> Promise<number>), Promise<number>) -> Promise<number>
//Therefore we need to give reduce a function that takes a Promise
//resolving to a number and a number which makes a new promise.
//This is the function we want:
function sequencePromises(promise, number) {
return new Promise((resolve) => {
resolve(promise.then(_ => myPromise(number)));
});
}
myArray.reduce(sequencePromises, Promise.resolve());
Of course, this simplistic approach won't work if you have a promise which can error, or if you need previous results, so you might want to make sequencePromises more generic:
function genericSequencePromises(promiseFunction) {
return (promise, parameter) => {
return new Promise((resolve, reject) =>
return promiseFunction(resolve,
reject,
promise,
parameter));
}
}
Then you can do whatever you want as long as you return a Promise.
Finally, you might benefit from this little helper:
function promiseSeries(array, reducer) {
return array.reduce(reducer, Promise.resolve());
}
Bringing it all together:
let sequencePromises = genericSequencePromises((resolve, reject, promise, num) => {
resolve(promise.then(_ => console.log(`done: ${num}`)));
}
promiseSeries(myArray, sequencePromises);
This way, you can not only handle the case in your question, but much more complex cases.
Here's a concise way using Array.reduce and async/await. Taking the eg array and myPromise function from OP:
var myArray = [1, 2, 3, 4, 5, 6];
function myPromise(num) {
return new Promise((res) => {
setTimeout(() => {
res(console.log("done: " + num));
}, 2000);
});
}
if your myPromise fn does not return result, then
myArray.reduce(async (a, b) => {
await a;
await myPromise(b);
}, null);
this can be easily modified to accumulate result. Eg, if your myPromise were to return a result, you could accumulate them in an array, in the order of myArray elements:
const resultArray = myArray.reduce(
async (a, b) => [...(await a), await myPromise(b)],
[]
);
or an object
const resultObject = myArray.reduce(async (a, b) => ({
async (a, b) => ({
...(await a),
[b]: await myPromise(b),
}),
{}
);
I would use babel and do it this way:
let args = [1, 2, 3];
const myPromise = async x => console.log('arg:',x);
const test = async () => {
for (let task of args.map(myPromise))
await task;
}
test().then(console.log('Done'));
<script src="https://unpkg.com/babel-standalone#6.24.0/babel.min.js"></script>
You can iterate over the array of elements and pass the params like this:
const arr = [1, 2, 3, 4, 5, 6]
const MyPromiseFunction = num => new Promise(
(resolve, reject) =>
// Your logic...
setTimeout(() => num <= 4
? resolve('Success!')
: reject('Rejected!'), 1000 * num)
)
const logMessage = (num, msg) =>
console.log(`For number ${num} promise result: ${msg}`)
arr.map(
async (num) => await MyPromiseFunction(num)
.then(message => logMessage(num, message))
.catch(reason => logMessage(num, reason))
)

How to control setTimeout with promises

var functionsArray = [
function() {
setTimeout(function() {
console.log(1);
}, 100);
},
function() {
setTimeout(function() {
console.log(2);
}, 200);
},
function() {
setTimeout(function() {
console.log(3);
}, 10);
}
],
Say I have an array of functions like above(the number of functions in it is not known). I want to write a function which takes this array as parameter and executes them in sequence. In the example above, I want it to log 1,2,3 in the sequence. Since Promise.all does not guarantee the order of execution, is it possible to achieve this without callback hell?
You can't get a promise from a function that just calls setTimeout - it needs some help, e.g.:
function after(n, f) {
return () => new Promise(resolve => {
setTimeout(() => {
resolve(f());
}, n);
});
}
with usage:
var functionsArray = [
after(100, () => console.log(1)),
after(200, () => console.log(2)),
after( 10, () => console.log(3)),
];
With that array you can then just await each function in turn:
for (let f of functionsArray) {
await f();
}
You can write an simple setTimeoutPromise function:
function timeoutPromise(time = 0){
return new Promise(resolve => setTimeout(resolve, time)
}
await timeoutPromise(10);
console.log('waited 10 sec')
timeoutPromise(20).then(() => console.log('waited 20 sec')
//make a new array as you like
Promise.all([
timeoutPromise(100).then(() => console.log(1)),
timeoutPromise(200).then(() => console.log(2)),
timeoutPromise(300).then(() => console.log(3))
])

How to use reduce in a Promise.all instead of map

How can I use reduce in the place of map when using Promise.all? My attempt results in an error UnhandledPromiseRejectionWarning: TypeError: #<Promise> is not iterable at Function.all (<anonymous>)
Eventually I would like to conditionally add innerResult to memo but I need to use reduce first.
const _ = require('lodash');
const eq = [{id:1}, {id:2}, {id:3}];
// block to replace
var biggerEq = _.map(eq, async (e) => {
const innerResult = await wait(e.id);
return innerResult;
})
// attempt at replacing above block
// var biggerEq = _.reduce(eq, async (memo, e) => {
// const innerResult = await wait(e.id);
// memo.push(innerResult)
// return memo;
// }, []);
Promise.all(biggerEq).then((result) => {
console.log(result) // outputs [ 2, 4, 6 ]
})
function wait (id) {
return new Promise((resolve) => {
setTimeout(() => {
resolve(id * 2);
}, 1000);
})
}
If you want to replace it with reduce, it's possible, but the logic will be a bit convoluted. Make the accumulator a Promise that resolves to an array that you can push to, then return it so the next iteration can use it (as a Promise):
const eq = [{id:1}, {id:2}, {id:3}];
function wait (id) {
return new Promise((resolve) => {
setTimeout(() => {
resolve(id * 2);
}, 1000);
})
}
const biggerEq = _.reduce(eq, async (arrProm, obj) => {
const [arr, innerResult] = await Promise.all([arrProm, wait(obj.id)]);
arr.push(innerResult);
return arr;
}, Promise.resolve([]));
biggerEq.then((arr) => {
console.log(arr);
});
<script src="https://cdnjs.cloudflare.com/ajax/libs/lodash.js/4.17.15/lodash.min.js"></script>
(but .map is really more appropriate when you want to transform one array into another)
I think CertainPerformance over-complicated it. You can use reduce like this with Promise.all:
const eq = [{id: 1}, {id: 2}, {id: 3}];
function wait(id) {
return new Promise((resolve) => {
setTimeout(() => {
resolve(id * 2);
}, 1000);
});
}
const biggerEq = _.reduce(eq, (arr, obj) => {
const p = wait(obj.id);
arr.push(p);
return arr;
}, []);
Promise.all(biggerEq).then((arr) => {
console.log(arr);
});
Note that the problem was with using await inside the reduce. That meant that you were pushing the results of the promises into the array, not getting an array of promises. Using that method, you already have your results in the array, so there's no need for Promise.all, but there's the big disadvantage that the promises are resolved consecutively. If that's actually what you want, you can have:
const results = _.reduce(eq, async (arr, obj) => {
const p = await wait(obj.id);
arr.push(p);
return arr;
}, []);
console.log(results);

How to execute promises sequentially, passing the parameters from an array?

var myArray = [1, 2, 3, 4, 5, 6]
function myPromise(num){
return new Promise(res => {
window.setTimeout(()=>{
res( console.log("done: " + num) )
},2000)
})
}
myPromise(myArray[0])
.then(x => myPromise(myArray[1]))
.then(x => myPromise(myArray[2]))
.then(x => myPromise(myArray[3]))
.then(x => myPromise(myArray[4]))
.then(x => myPromise(myArray[5]))
Right now, if I execute the statement above, it will run sequentially. In my actual use case the array is dynamically populated and I need to execute the myPromise() function for each member in myArray.
How can I make a "pauseable loop" that will loop for each item in the array, execute myPromise and wait for the promise to be resolved before continuing to the next iteration?
You can make the repeated application of .then into a fold pretty neatly if you’re okay with creating as many promises as array elements as is the case in the question:
myArray.reduce(
(p, x) =>
p.then(() => myPromise(x)),
Promise.resolve()
)
but given support, an async function is a better choice. It’s nicely readable and has O(1) instead of O(n) memory overhead.
const forEachSeries = async (iterable, action) => {
for (const x of iterable) {
await action(x)
}
}
forEachSeries(myArray, myPromise)
If you want to collect the return values as an array, that’s:
const mapSeries = async (iterable, fn) => {
const results = []
for (const x of iterable) {
results.push(await fn(x))
}
return results
}
or, without async function support,
const mapSeries = (iterable, fn) => {
const iterator = iterable[Symbol.iterator]()
const results = []
const go = () => {
const {value, done} = iterator.next()
if (done) {
return results
}
return fn(value).then(mapped => {
results.push(mapped)
return go()
})
}
return Promise.resolve().then(go)
}
Runnable snippet:
const myArray = [1, 2, 3, 4, 5, 6]
const sleep = ms =>
new Promise(res => {
setTimeout(res, ms)
})
const myPromise = num =>
sleep(500).then(() => {
console.log('done: ' + num)
})
const forEachSeries = async (iterable, action) => {
for (const x of iterable) {
await action(x)
}
}
forEachSeries(myArray, myPromise)
.then(() => {
console.log('all done!')
})
Don't create an array of promises. Create an array of functions returning a promise.
const f = x => new Promise(resolve => setTimeout(() => resolve(console.log(x)), 2000))
(async () => {
for (let job of [1, 2, 3, 4, 5, 6].map(x => () => f(x)))
await job()
})()
Promises start running immediately after creation. Therefore, sequential execution is ensured by constructing the next promise only after finishing the current one.
I know I am very late, and my answer is similar to what others have posted. But I thought I could post a clearer answer which may help any beginner.
Instead of using promises directly, we can use promise factory. Since the promise starts executing as soon as they are created using promise factory we delay the creation of promise.
In this example I create 5 which resolve after a second. I use a promiseCreator to create promises. Now the array promises uses promiseCreator to create 5 instances of promises. But array promiseFactories wraps promiseCreator in a function so promise is not invoked immediately. It is invoked when used.
Function executeSequentially executes all promiseLike sequentially.
When promise array is passed result is promise array itself executes parallely (actually they executed as soon as they are created, not when this line is called).
When promiseFactory array is passed result is new Promise is created when earlier promise has completed their execution.
const promiseCreator = (i, time, text) => {
return new Promise(resolve => setTimeout(
() => resolve(console.log(`${i} ${text}`)),
time)
);
}
const promises = [
promiseCreator(1, 1000, "parallel"),
promiseCreator(2, 1000, "parallel"),
promiseCreator(3, 1000, "parallel"),
promiseCreator(4, 1000, "parallel"),
promiseCreator(5, 1000, "parallel"),
]
const promiseFactories = [
() => promiseCreator(1, 1000, "sequential"),
() => promiseCreator(2, 1000, "sequential"),
() => promiseCreator(3, 1000, "sequential"),
() => promiseCreator(4, 1000, "sequential"),
() => promiseCreator(5, 1000, "sequential"),
]
function executeSequentially(promiseLikeArray) {
var result = Promise.resolve();
promiseLikeArray.forEach(function (promiseLike) {
result = result.then(promiseLike);
});
return result;
}
executeSequentially(promises)
executeSequentially(promiseFactories)
Also you can do it via recursive approach - executeSequentially calls itself:
function createPromise(x) {
return new Promise(res => {
setTimeout(() => {
console.log(x)
res(x);
}, x * 1000)
})
}
function executeSequentially(array) {
return createPromise(array.shift())
.then(x => array.length == 0 ? x : executeSequentially(array));
}
console.time('executeSequentially');
executeSequentially([1, 2, 3]).then(x => {
console.log('last value: ' + x);
console.timeEnd('executeSequentially');
});
Sequential:
you can use async await features to run promises sequentially . here's a snippet
async function chainPromiseCalls(asyncFunctions=[],respectiveParams=[]){
for(let i=0;i<asyncFunctions.length;i++){
const eachResult = await asyncFunctions[i](...respectiveParams[i]);
// do what you want to do with each result
}
return ;
}
Parallel:
for parallel you can just call each async function once in a loop , but if you do want to get their combined result , you can use Promise.all
function parallelPromiseCalls(asyncFunctions=[],respectiveParams=[]){
return Promise.all(asyncFunctions.map((func,index)=>func(...respectiveParams[index])))
.then(resultsList=>{
resultsList.forEach((result,index)=>{
// do what you want to do with each result in the list
})
return ;
})
}
note : I am considering respective parameters as an list of lists since multiple parameters should be passed to any one of the function , else if you have to pass only a single parameter to each then you can remove the spread operator.
You could use Array.reduce.
//type: [number]
var myArray = [1, 2, 3, 4, 5, 6] //doesn't really matter
//type: number -> Promise<number>
function myPromise(num){
return new Promise((resolve) => {
window.setTimeout(()=>{
resolve(console.log("done: " + num) )
},2000)
})
}
//Array.reduce has type: [a] ~> ((b, a) -> b), b) -> b
//So it can have type:
//[number] ~> ((Promise<number>, number) -> Promise<number>), Promise<number>) -> Promise<number>
//Therefore we need to give reduce a function that takes a Promise
//resolving to a number and a number which makes a new promise.
//This is the function we want:
function sequencePromises(promise, number) {
return new Promise((resolve) => {
resolve(promise.then(_ => myPromise(number)));
});
}
myArray.reduce(sequencePromises, Promise.resolve());
Of course, this simplistic approach won't work if you have a promise which can error, or if you need previous results, so you might want to make sequencePromises more generic:
function genericSequencePromises(promiseFunction) {
return (promise, parameter) => {
return new Promise((resolve, reject) =>
return promiseFunction(resolve,
reject,
promise,
parameter));
}
}
Then you can do whatever you want as long as you return a Promise.
Finally, you might benefit from this little helper:
function promiseSeries(array, reducer) {
return array.reduce(reducer, Promise.resolve());
}
Bringing it all together:
let sequencePromises = genericSequencePromises((resolve, reject, promise, num) => {
resolve(promise.then(_ => console.log(`done: ${num}`)));
}
promiseSeries(myArray, sequencePromises);
This way, you can not only handle the case in your question, but much more complex cases.
Here's a concise way using Array.reduce and async/await. Taking the eg array and myPromise function from OP:
var myArray = [1, 2, 3, 4, 5, 6];
function myPromise(num) {
return new Promise((res) => {
setTimeout(() => {
res(console.log("done: " + num));
}, 2000);
});
}
if your myPromise fn does not return result, then
myArray.reduce(async (a, b) => {
await a;
await myPromise(b);
}, null);
this can be easily modified to accumulate result. Eg, if your myPromise were to return a result, you could accumulate them in an array, in the order of myArray elements:
const resultArray = myArray.reduce(
async (a, b) => [...(await a), await myPromise(b)],
[]
);
or an object
const resultObject = myArray.reduce(async (a, b) => ({
async (a, b) => ({
...(await a),
[b]: await myPromise(b),
}),
{}
);
I would use babel and do it this way:
let args = [1, 2, 3];
const myPromise = async x => console.log('arg:',x);
const test = async () => {
for (let task of args.map(myPromise))
await task;
}
test().then(console.log('Done'));
<script src="https://unpkg.com/babel-standalone#6.24.0/babel.min.js"></script>
You can iterate over the array of elements and pass the params like this:
const arr = [1, 2, 3, 4, 5, 6]
const MyPromiseFunction = num => new Promise(
(resolve, reject) =>
// Your logic...
setTimeout(() => num <= 4
? resolve('Success!')
: reject('Rejected!'), 1000 * num)
)
const logMessage = (num, msg) =>
console.log(`For number ${num} promise result: ${msg}`)
arr.map(
async (num) => await MyPromiseFunction(num)
.then(message => logMessage(num, message))
.catch(reason => logMessage(num, reason))
)

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