Callback after all asynchronous forEach callbacks are completed

Question

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

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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  gt  300             callback              100

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Array forEach does not provide this nicety  oh if it would  but there are several ways to accomplish what you want   Using a simple counter  function callback      console log  all done       var itemsProcessed   0    1  2  3  forEach  item  index  array    gt      asyncFunction item       gt        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   Using ES6 Promises   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      gt        console log  done with   item       cb         100      let requests    1  2  3  reduce  promiseChain  item    gt        return promiseChain then      gt  new Promise  resolve    gt          asyncFunction item  resolve               Promise resolve      requests then      gt  console log  done     Process all async requests without  synchronous  execution  2 may finish faster than 1   let requests    1 2 3  map  item    gt        return new Promise  resolve    gt          asyncFunction item  resolve               Promise all requests  then      gt  console log  done        Using an async library  There are other asynchronous libraries  async being the most popular  that provide mechanisms to express what you want   Edit    The 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

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var i 0  const waitFor    ms    gt        new Promise  r    gt          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    gt      waitFor num

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I try Easy Way to resolve it  share it with you    let counter   0              arr forEach async  item  index    gt                    await request query item   err  recordset    gt                        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

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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

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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

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This is the solution for Node js which is asynchronous    using the async npm package    JavaScript  Synchronizing forEach Loop with callbacks inside

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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

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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

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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

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My solution without Promise  this ensures that every action is ended before the next one begins     x000D   x000D  Array prototype forEachAsync   function  callback  end    x000D          var self   this  x000D       x000D          function task index    x000D              var x   self index   x000D              if  index  gt   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   lt div gt item index     item     done lt  div gt     x000D          console log  action     item     done    x000D          next    x000D           300   x000D         function    x000D              toto   append   lt div gt ALL ACTIONS ARE DONE lt  div gt     x000D          console log  ALL ACTIONS ARE DONE    x000D          x000D   lt script src  https   ajax googleapis com ajax libs jquery 2 1 1 jquery min js  gt  lt  script gt  x000D   lt div class  toto  gt  x000D   x000D   lt  div gt  x000D   x000D   x000D

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With ES2018 you can use async iterators   const asyncFunction   a   gt  fetch a   const itemDone   a   gt  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

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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

[javascript] Callback after all asynchronous forEach callbacks are completed

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