JavaScript Node js is Array forEach asynchronous

Question

I have a question regarding the native Array forEach implementation of JavaScript  Does it behave asynchronously  For example  if I call    many many elements  forEach function     lots of work to do     Will this be non-blocking

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This is a short asynchronous function to use without requiring third party libs  Array prototype each   function  iterator  callback        var iterate   function                  pointer                if  pointer  gt   this length                    callback                    return                            iterator call iterator  this pointer   iterate  pointer         bind this           pointer   -1      iterate this

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No  it is blocking  Have a look at the specification of the algorithm   However a maybe easier to understand implementation is given on MDN   if   Array prototype forEach      Array prototype forEach   function fun     thisp              use strict        if  this     void 0    this     null        throw new TypeError         var t   Object this       var len   t length  gt  gt  gt  0      if  typeof fun      function         throw new TypeError         var thisp   arguments 1       for  var i   0  i  lt  len  i                if  i in t          fun call thisp  t i   i  t                    If you have to execute a lot of code for each element  you should consider to use a different approach   function processArray items  process        var todo   items concat         setTimeout function             process todo shift             if todo length  gt  0                setTimeout arguments callee  25                    25       and then call it with   processArray  many many elements   function     lots of work to do      This would be non-blocking then  The example is taken from High Performance JavaScript   Another option might be web workers

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It is possible to code even the solution like this for example     var loop   function i  data  callback        if  i  lt  data length              TODO  SELECT   FROM stackoverflowUsers    function res                  data i  meta   res              console log i  data i  title               return loop i 1  data  errors  callback                       else          return callback data             loop 0     title    hello      title    world     function data        console log  DONE n  data         On the other hand  it is much slower than a  for    Otherwise  the excellent Async library can do this  https   caolan github io async docs html each

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Edit 2018-10-11  It looks like there is a good chance the standard described below may not go through  consider pipelineing as an alternative  does not behave exactly the same but methods could be implemented in a similar manor    This is exactly why I am excited about es7  in future you will be able to do something like the code below  some of the specs are not complete so use with caution  I will try to keep this up to date   But basically using the new    bind operator  you will be able to run a method on an object as if the object s prototype contains the method  eg  Object    Method  where normally you would call  Object   ObjectsMethod   Note to do this today  24-July-16  and have it work in all browsers you will need to transpile your code for the following functionality Import   Export  Arrow functions  Promises  Async   Await and most importantly function bind  The code below could be modfied to use only function bind if nessesary  all this functionality is neatly available today by using babel   YourCode js  where  lots of work to do  must simply return a promise  resolving it when the asynchronous work is done    import   asyncForEach   from    ArrayExtensions js    await  many many elements   asyncForEach      gt  lots of work to do     ArrayExtensions js  export function asyncForEach callback        return Promise resolve this  then async  ar    gt                for let i 0 i lt ar length i                          await callback call ar  ar i   i  ar                         export function asyncMap callback        return Promise resolve this  then async  ar    gt                const out               for let i 0 i lt ar length i                          out i    await callback call ar  ar i   i  ar                     return out

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If you need an asynchronous-friendly version of Array forEach and similar  they re available in the Node js  async  module  http   github com caolan async    as a bonus this module also works in the browser   async each openFiles  saveFile  function err          if any of the saves produced an error  err would equal that error

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Here is a small example you can run to test it    1 2 3 4 5 6 7 8 9  forEach function n       var sum   0      console log  Start for     n       for  var i   0  i  lt    10 - n    100000000  i            sum         console log  Ended for     n  sum         It will produce something like this if it takes too less much time  increase decrease the number of iterations     index  48 Start for 1  index  52 Ended for 1 900000000  index  48 Start for 2  index  52 Ended for 2 800000000  index  48 Start for 3  index  52 Ended for 3 700000000  index  48 Start for 4  index  52 Ended for 4 600000000  index  48 Start for 5  index  52 Ended for 5 500000000  index  48 Start for 6  index  52 Ended for 6 400000000  index  48 Start for 7  index  52 Ended for 7 300000000  index  48 Start for 8  index  52 Ended for 8 200000000  index  48 Start for 9  index  52 Ended for 9 100000000  index  45  Violation   load  handler took 7285ms

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Use Promise each of bluebird library   Promise each  Iterable lt any gt  Promise lt Iterable lt any gt  gt  input  function any item  int index  int length  iterator   - gt  Promise   This method iterates over an array  or a promise of an array  which contains promises  or a mix of promises and values  with the given iterator function with the signature  value  index  length  where the value is the resolved value of a respective promise in the input array  Iteration happens serially  If the iterator function returns a promise or a thenable  then the result of the promise is awaited before continuing with next iteration  If any promise in the input array is rejected  then the returned promise is rejected as well   If all of the iterations resolve successfully  Promise each resolves to the original array unmodified  However  if one iteration rejects or errors  Promise each ceases execution immediately and does not process any further iterations  The error or rejected value is returned in this case instead of the original array   This method is meant to be used for side effects   var fileNames     1 txt    2 txt    3 txt     Promise each fileNames  function fileName        return fs readFileAsync fileName  then function val              do stuff with  val  here               then function     console log  done

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There is a common pattern for doing a really heavy computation in Node that may be applicable to you     Node is single-threaded  as a deliberate design choice  see What is Node js    this means that it can only utilize a single core   Modern boxes have 8  16  or even more cores  so this could leave 90   of the machine idle   The common pattern for a REST service is to fire up one node process per core  and put these behind a local load balancer like http   nginx org    Forking a child -  For what you are trying to do  there is another common pattern  forking off a child process to do the heavy lifting   The upside is that the child process can do heavy computation in the background while your parent process is responsive to other events   The catch is that you can t   shouldn t share memory with this child process  not without a LOT of contortions and some native code   you have to pass messages   This will work beautifully if the size of your input and output data is small compared to the computation that must be performed   You can even fire up a child node js process and use the same code you were using previously   For example    var child process   require  child process    function run in child array  cb        var process   child process exec  node libfn js   function err  stdout  stderr            var output   JSON parse stdout           cb err  output               process stdin write JSON stringify array    utf8        process stdin end

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There is a package on npm for easy asynchronous for each loops   var forEachAsync   require  futures   forEachAsync      waits for one request to finish before beginning the next  forEachAsync   dogs    cats    octocats    function  next  element  index  array      getPics element  next        then after all of the elements have been handled       the final callback fires to let you know it s all done       then function          console log  All requests have finished          Also another variation forAllAsync

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Array forEach is meant for computing stuff not waiting  and there is nothing to be gained making computations asynchronous in an event loop  webworkers add multiprocessing  if you need multi-core computation   If you want to wait for multiple tasks to end  use a counter  which you can wrap in a semaphore class

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Although Array forEach is not asynchronous  you can get asynchronous  quot end result quot   Example below   x000D   x000D  function delayFunction x        return new Promise           resolve    gt  setTimeout      gt  resolve x   1000             1  2  3  forEach async x    gt        console log x       console log await delayFunction x        x000D   x000D   x000D

[javascript] JavaScript, Node.js: is Array.forEach asynchronous?

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