How to call async functions in loops - javascript

Hey There i came across an issue recently while performing the Async db calls inside for loops in javascript.

ex:

for (let i=0 ; i<10 ; i++) {
   db.User.find({},function(err,res){
            console.log(i);
   });
}

Consider above code snippet. Here if you see db is a database call and it takes obviously time to execute query and hit call back but our for loop wont wait till it hit the call back. The for loop will be executed even before you receive the result from call back.

So you will not get actual result .

How I solved it

I used promises to solve it
q is a  package from NOde js and in angular js (Now Angular 2 moved to Observables)

Steps:

1. Frame all the async function calls you want to make in for loop.
2. Pass all this async calls in Q.all() function. (Q.all will get all the promise objects from the async functions you pass and club in the form of array.)
3. Now using this array you can map and reduce the results.


Ex:

asyncFunc1()
 {
setTimeout(function() {
return 'Hello' }, 3000); // consider this returns after 3 s of execution
}

asyncFunc2()
{
setTimeout(function() {
return 'Hello' }, 5000); // consider this returns after 5 s of execution
}

asyncFunc3()
{
setTimeout(function() {
return 'Hello' }, 2000); // consider this returns after 2 s of execution
}


var asyncCalls = [];
asyncCalls[0] = asyncFunc1();
asyncCalls[1] = asyncFunc2();
asyncCalls[2] = asyncFunc3();


now pass this array of function calls to Q.all like below


var Q = require('q');

Q.all(asynCalls,function(res) {

console.log(res)  // this result is nothing but array of collecting objects from asynccalls. now you can

//map and reduce the results according to you .


})

How to Copy files from MAC to pen-drive- NTFS

Install NTFS-3G from Homebrew by opening a Terminal and entering the following command.

brew install ntfs-3g

After installing NTFS-3G you can manually mount NTFS volumes in read-write mode by executing the following commands in Terminal. Replace /dev/disk1s1 with the actual NTFS partition you want to mount. You can find the partition name using diskutil list.

sudo mkdir /Volumes/NTFS
sudo /usr/local/bin/ntfs-3g /dev/disk1s1 /Volumes/NTFS -olocal -oallow_other

if you face issue with mounting . please unmount the disk .


sudo umount /dev/disk2s1

then do the mount.