As the title suggests. How do I do this?
I want to call whenAllDone() after the forEach-loop has gone through each element and done some asynchronous processing.
[1, 2, 3].forEach(
function(item, index, array, done) {
asyncFunction(item, function itemDone() {
console.log(item + " done");
done();
});
}, function allDone() {
console.log("All done");
whenAllDone();
}
);
Possible to get it to work like this? When the second argument to forEach is a callback function which runs once it went through all iterations?
Expected output:
3 done
1 done
2 done
All done!
This question is related to
javascript
node.js
asynchronous
callback
Array.forEach does not provide this nicety (oh if it would) but there are several ways to accomplish what you want:
function callback () { console.log('all done'); }
var itemsProcessed = 0;
[1, 2, 3].forEach((item, index, array) => {
asyncFunction(item, () => {
itemsProcessed++;
if(itemsProcessed === array.length) {
callback();
}
});
});
(thanks to @vanuan and others) This approach guarantees that all items are processed before invoking the "done" callback. You need to use a counter that gets updated in the callback. Depending on the value of the index parameter does not provide the same guarantee, because the order of return of the asynchronous operations is not guaranteed.
(a promise library can be used for older browsers):
Process all requests guaranteeing synchronous execution (e.g. 1 then 2 then 3)
function asyncFunction (item, cb) {
setTimeout(() => {
console.log('done with', item);
cb();
}, 100);
}
let requests = [1, 2, 3].reduce((promiseChain, item) => {
return promiseChain.then(() => new Promise((resolve) => {
asyncFunction(item, resolve);
}));
}, Promise.resolve());
requests.then(() => console.log('done'))
Process all async requests without "synchronous" execution (2 may finish faster than 1)
let requests = [1,2,3].map((item) => {
return new Promise((resolve) => {
asyncFunction(item, resolve);
});
})
Promise.all(requests).then(() => console.log('done'));
There are other asynchronous libraries, async being the most popular, that provide mechanisms to express what you want.
EditThe body of the question has been edited to remove the previously synchronous example code, so i've updated my answer to clarify. The original example used synchronous like code to model asynchronous behaviour, so the following applied:
array.forEach is synchronous and so is res.write, so you can simply put your callback after your call to foreach:
posts.foreach(function(v, i) {
res.write(v + ". index " + i);
});
res.end();
My solution:
//Object forEachDone
Object.defineProperty(Array.prototype, "forEachDone", {
enumerable: false,
value: function(task, cb){
var counter = 0;
this.forEach(function(item, index, array){
task(item, index, array);
if(array.length === ++counter){
if(cb) cb();
}
});
}
});
//Array forEachDone
Object.defineProperty(Object.prototype, "forEachDone", {
enumerable: false,
value: function(task, cb){
var obj = this;
var counter = 0;
Object.keys(obj).forEach(function(key, index, array){
task(obj[key], key, obj);
if(array.length === ++counter){
if(cb) cb();
}
});
}
});
Example:
var arr = ['a', 'b', 'c'];
arr.forEachDone(function(item){
console.log(item);
}, function(){
console.log('done');
});
// out: a b c done
This is the solution for Node.js which is asynchronous.
using the async npm package.
(JavaScript) Synchronizing forEach Loop with callbacks inside
It's odd how many incorrect answers has been given to asynchronous case! It can be simply shown that checking index does not provide expected behavior:
// INCORRECT
var list = [4000, 2000];
list.forEach(function(l, index) {
console.log(l + ' started ...');
setTimeout(function() {
console.log(index + ': ' + l);
}, l);
});
output:
4000 started
2000 started
1: 2000
0: 4000
If we check for index === array.length - 1, callback will be called upon completion of first iteration, whilst first element is still pending!
To solve this problem without using external libraries such as async, I think your best bet is to save length of list and decrement if after each iteration. Since there's just one thread we're sure there no chance of race condition.
var list = [4000, 2000];
var counter = list.length;
list.forEach(function(l, index) {
console.log(l + ' started ...');
setTimeout(function() {
console.log(index + ': ' + l);
counter -= 1;
if ( counter === 0)
// call your callback here
}, l);
});
With ES2018 you can use async iterators:
const asyncFunction = a => fetch(a);
const itemDone = a => console.log(a);
async function example() {
const arrayOfFetchPromises = [1, 2, 3].map(asyncFunction);
for await (const item of arrayOfFetchPromises) {
itemDone(item);
}
console.log('All done');
}
var i=0;
const waitFor = (ms) =>
{
new Promise((r) =>
{
setTimeout(function () {
console.log('timeout completed: ',ms,' : ',i);
i++;
if(i==data.length){
console.log('Done')
}
}, ms);
})
}
var data=[1000, 200, 500];
data.forEach((num) => {
waitFor(num)
})
You shouldn't need a callback for iterating through a list. Just add the end() call after the loop.
posts.forEach(function(v, i){
res.write(v + ". Index " + i);
});
res.end();
Hope this will fix your problem, i usually work with this when i need to execute forEach with asynchronous tasks inside.
foo = [a,b,c,d];
waiting = foo.length;
foo.forEach(function(entry){
doAsynchronousFunction(entry,finish) //call finish after each entry
}
function finish(){
waiting--;
if (waiting==0) {
//do your Job intended to be done after forEach is completed
}
}
with
function doAsynchronousFunction(entry,callback){
//asynchronousjob with entry
callback();
}
I try Easy Way to resolve it, share it with you :
let counter = 0;
arr.forEach(async (item, index) => {
await request.query(item, (err, recordset) => {
if (err) console.log(err);
//do Somthings
counter++;
if(counter == tableCmd.length){
sql.close();
callback();
}
});
request is Function of mssql Library in Node js. This can replace each function or Code u want.
GoodLuck
If you encounter asynchronous functions, and you want to make sure that before executing the code it finishes its task, we can always use the callback capability.
For example:
var ctr = 0;
posts.forEach(function(element, index, array){
asynchronous(function(data){
ctr++;
if (ctr === array.length) {
functionAfterForEach();
}
})
});
Note: functionAfterForEach is the function to be executed after foreach tasks are finished. asynchronous is the asynchronous function executed inside foreach.
How about setInterval, to check for complete iteration count, brings guarantee. not sure if it won't overload the scope though but I use it and seems to be the one
_.forEach(actual_JSON, function (key, value) {
// run any action and push with each iteration
array.push(response.id)
});
setInterval(function(){
if(array.length > 300) {
callback()
}
}, 100);
My solution without Promise (this ensures that every action is ended before the next one begins):
Array.prototype.forEachAsync = function (callback, end) {_x000D_
var self = this;_x000D_
_x000D_
function task(index) {_x000D_
var x = self[index];_x000D_
if (index >= self.length) {_x000D_
end()_x000D_
}_x000D_
else {_x000D_
callback(self[index], index, self, function () {_x000D_
task(index + 1);_x000D_
});_x000D_
}_x000D_
}_x000D_
_x000D_
task(0);_x000D_
};_x000D_
_x000D_
_x000D_
var i = 0;_x000D_
var myArray = Array.apply(null, Array(10)).map(function(item) { return i++; });_x000D_
console.log(JSON.stringify(myArray));_x000D_
myArray.forEachAsync(function(item, index, arr, next){_x000D_
setTimeout(function(){_x000D_
$(".toto").append("<div>item index " + item + " done</div>");_x000D_
console.log("action " + item + " done");_x000D_
next();_x000D_
}, 300);_x000D_
}, function(){_x000D_
$(".toto").append("<div>ALL ACTIONS ARE DONE</div>");_x000D_
console.log("ALL ACTIONS ARE DONE");_x000D_
});<script src="https://ajax.googleapis.com/ajax/libs/jquery/2.1.1/jquery.min.js"></script>_x000D_
<div class="toto">_x000D_
_x000D_
</div> var counter = 0;
var listArray = [0, 1, 2, 3, 4];
function callBack() {
if (listArray.length === counter) {
console.log('All Done')
}
};
listArray.forEach(function(element){
console.log(element);
counter = counter + 1;
callBack();
});
Source: Stackoverflow.com