async for loop in node js

Question

I am new to this node js   I am little bit confused about this callback  In my app inside a for loop i am calling a asynchronous function call i think my problem is that before i am getting response of async call my for loop get looped   My code   async forEach Object keys config   function key  next            search config key  query  function err  result                console log  fffffffffff    util inspect result  --------  gt  gt  gt Getting undefined               if  err  return next err                  var json   JSON stringify                    result   result                             results key                       result   result                           console log  rrrrrrrr   util inspect results key                next       lt ---- critical piece   This is how the forEach knows to continue to the next loop   Must be called inside search s callback so that it doesn t loop prematurely                                           function err            console log  iterating done              res writeHead 200             content-type    application json              res end JSON stringify results                    Search function code   var matches           var qrySubString   query substring 0  4       client query  select   from xxxxxxxxx where level4 ILIKE       query         function err  row1  fields            for  var id in row1 rows                                var match  name                              if  query    row1 rows id  level4                    match   true                  name   row1 rows id  level4                            else                   match   false                  name   query                            matches push                    id   id                   name   row1 rows id  level4                   score   100                   match   match                   type                           id     people presidents                        name    US President                                                                 callback matches            I want to execute for loop after successful execution of 1 search function I think i have to use async for loop Please guide me to solve this  Thanks in advance

User · Answer

You ve correctly diagnosed your problem  so good job   Once you call into your search code  the for loop just keeps right on going   I m a big fan of https   github com caolan async  and it serves me well   Basically with it you d end up with something like   var async   require  async   async eachSeries Object keys config   function  key  next      search config key  query  function err  result        lt ----- I added an err here     if  err  return next err       lt ---- don t keep going if there was an error      var json   JSON stringify          result   result             results key             result   result           next          lt ---- critical piece   This is how the forEach knows to continue to                        the next loop   Must be called inside search s callback so that                        it doesn t loop prematurely            function err      console log  iterating done           I hope that helps

User · Answer

I ve reduced your code sample to the following lines to make it easier to understand the explanation of the concept   var results       var config   JSON parse queries   for  var key in config        var query   config key  query      search query  function result            results push result             res writeHead         res end results     The problem with the previous code is that the search function is asynchronous  so when the loop has ended  none of the callback functions have been called  Consequently  the list of results is empty   To fix the problem  you have to put the code after the loop in the callback function       search query  function result            results push result              Put res writeHead        and res end results  here           However  since the callback function is called multiple times  once for every iteration   you need to somehow know that all callbacks have been called  To do that  you need to count the number of callbacks  and check whether the number is equal to the number of asynchronous function calls   To get a list of all keys  use Object keys  Then  to iterate through this list  I use  forEach  you can also use for  var i   0  key   keys i   i  lt  keys length    i          but that could give problems  see JavaScript closure inside loops     simple practical example    Here s a complete example   var results       var config   JSON parse queries   var onComplete   function         res writeHead             res end results      var keys   Object keys config   var tasksToGo   keys length  if  tasksToGo     0       onComplete      else          There is at least one element  so the callback will be called      keys forEach function key            var query   config key  query          search query  function result                results push result               if  --tasksToGo     0                       No tasks left  good to go                 onComplete                                          Note  The asynchronous code in the previous example are executed in parallel  If the functions need to be called in a specific order  then you can use recursion to get the desired effect   var results       var config   JSON parse queries   var keys   Object keys config    function next index        if  index     keys length       No items left         res writeHead                 res end results           return            var key   keys index       var query   config key  query      search query  function result            results push result           next index   1              0     What I ve shown are the concepts  you could use one of the many  third-party  NodeJS modules in your implementation  such as async

User · Answer

I like to use the recursive pattern for this scenario  For example  something like this      If config is an array of queries var config   JSON parse queries querrryArray          Array of results var results   processQueries config    function processQueries queries        var searchQuery       if  queries length    0               All queries complete         res writeHead 200    content-type    application json             res end JSON stringify  results  results             return             searchQuery   queries pop         search searchQuery  function result            results push JSON stringify  result  result             processQueries                             processQueries is a recursive function that will pull a query element out of an array of queries to process  Then the callback function calls processQueries again when the query is complete  The processQueries knows to end when there are no queries left   It is easiest to do this using arrays  but it could be modified to work with object key values I imagine

User · Answer

Node js introduced async await in 7 6 so this makes Javascript more beautiful   var results       var config   JSON parse queries   for  var key in config      var query   config key  query    results push await search query      res writeHead         res end results     For this to work search fucntion has to return a promise or it has to be async function  If it is not returning a Promise you can help it to return a Promise  function asyncSearch query      return new Promise  resolve  reject    gt       search query  result   gt       resolve result                  Then replace this line await search query   by await asyncSearch query

[javascript] async for loop in node.js

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