How to run async loops in sequence or in parallel?

Before doing asynchronous magic with loops I want to remind you how we write classical synchronous loops.

😐 Synchronous loop

Long time ago I was writing loops like this (probably you too):

for ( var i = 0 ; i < array . length ; i ++ ) { var item = array [ i ] ; }

It is good, it is fast, but it has many readability and maintenance issues. Then I used to use its better version:

array . forEach ( ( item ) => { } ) ;

JavaScript language is developing very fast. We have more features and new syntax. One of my favorite is async/await. I am using it more frequently now. And sometimes I have a situation where I need to do something with items in an array asynchronously.

🤗 Asynchronous loops

How to use await inside a loop? Let’s just write async function and await each task.

async function processArray ( array ) { array . forEach ( item => { await func ( item ) ; } ) }

This code will throw a syntax error. Why? Because we can not use await inside synchronous function. As you can see “processArray” is async function. But anonymous function that we use for forEach is synchronous.

🤔 1. Don’t wait for result

How to fix previous issue? We can define anonymous function as asynchronous as well:

async function processArray ( array ) { array . forEach ( async ( item ) => { await func ( item ) ; } ) console . log ( 'Done!' ) ; }

But forEach will not wait until all items are finished. It will just run tasks and go next. As a proof let’s write simple test:

function delay ( ) { return new Promise ( resolve => setTimeout ( resolve , 300 ) ) ; } async function delayedLog ( item ) { await delay ( ) ; console . log ( item ) ; } async function processArray ( array ) { array . forEach ( async ( item ) => { await delayedLog ( item ) ; } ) console . log ( 'Done!' ) ; } processArray ( [ 1 , 2 , 3 ] ) ;

The output will be:

Done! 1 2 3

It can be ok if you don’t need to wait for the results. But in almost all cases this is not a good logic.

👊 2. Process array in sequence

To wait the result we should return back to old-school “for loop”, but this time we can use modern version with for..of construction (thanks to Iteration Protocol) for better readability:

async function processArray ( array ) { for ( const item of array ) { await delayedLog ( item ) ; } console . log ( 'Done!' ) ; }

This will give us expected output:

1 2 3 Done!

The code will handle each item one by one in series. But we can run it in parallel!

💪 3. Process array in parallel

We can slightly change the code to run async operations in parallel:

async function processArray ( array ) { const promises = array . map ( delayedLog ) ; await Promise . all ( promises ) ; console . log ( 'Done!' ) ; }

This code will run many delayLog tasks in parallel. But be careful with very large array (too many tasks in parallel can be too heavy for CPU or memory).

Also don’t confuse “parallel” here with real threads and paralleling. That code will not guarantee real parallelism of execution. It depends on your item function ( delayedLog in this demo). Network requests, webworkers and some other tasks can be executed in parallel.

That is all. Thanks for reading!