Correct way to write loops for promise

Question

How to correctly construct a loop to make sure the following promise call and the chained logger log res  runs synchronously through iteration   bluebird   db getUser email  then function res    logger log res          this is a promise   I tried the following way  method from http   blog victorquinn com javascript-promise-while-loop    var Promise   require  bluebird     var promiseWhile   function condition  action        var resolver   Promise defer         var loop   function             if   condition    return resolver resolve            return Promise cast action                 then loop               catch resolver reject               process nextTick loop        return resolver promise       var count   0  promiseWhile function         return count  lt  10     function         return new Promise function resolve  reject            db getUser email             then function res                   logger log res                  count                  resolve                              then function         console log  all done           Although it seems to work  but I don t think it guarantees the order of calling logger log res      Any suggestions

User · Answer

First take array of promises promise array  and after resolve these promise array using Promise all promisearray    var arry   raju   ram   abdul   kruthika     var promiseArry     for var i 0 i lt arry length i        promiseArry push dbFechFun arry i        Promise all promiseArry     then  result    gt        console log result           catch  error    gt         console log error          function dbFetchFun name         we need to return a  promise   return db find  name name       any db operation we can write hear

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Here s another method  ES6 w std Promise    Uses lodash underscore type exit criteria  return     false   Note that you could easily add an exitIf   method in options to run in doOne     const whilePromise    fnReturningPromise options         gt            loop until fnReturningPromise       false        options delay - setTimeout ms  set to 0 for 1 tick to make non-blocking      return new Promise  resolve reject    gt            const doOne        gt                fnReturningPromise                then     args    gt                    if  args length  amp  amp  args 0      false                        resolve    args                     else                       iterate                                                        const iterate        gt                if  options delay     undefined                    setTimeout doOne options delay                 else                   doOne                                    Promise resolve            then iterate           catch reject

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I d make something like this   var request      while count lt 10      request push db getUser email  then function res    return res          count       Promise all request  then  dataAll   gt     for  var i   0  i  lt  dataAll length  i             logger log dataAll i                 in this way  dataAll is an ordered array of all element to log  And log operation will perform when all promises are done

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Given   asyncFn function array of items   Required   promise chaining  then   s in series  in order  native es6   Solution  let asyncFn    item    gt      return new Promise  resolve  reject    gt        setTimeout       gt   console log item   resolve true    1000              asyncFn  a       then     gt  return async  b         then     gt  return async  c         then     gt  return async  d      let a     a   b   c   d    a reduce  previous  current  index  array    gt      return previous                                       initiates the promise chain    then     gt  return asyncFn array index            adds  then   promise for each item    Promise resolve

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How about this one using BlueBird   function fetchUserDetails arr        return Promise each arr  function email            return db getUser email  done function res                logger log res

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There is a new way to solve this and it s by using async await   async function myFunction       while    my condition           const res   await db getUser email       logger log res          myFunction   then      gt         do other stuff         https   developer mozilla org en-US docs Web JavaScript Reference Statements async function https   ponyfoo com articles understanding-javascript-async-await

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I don t think it guarantees the order of calling logger log res      Actually  it does  That statement is executed before the resolve call      Any suggestions    Lots  The most important is your use of the create-promise-manually antipattern - just do only  promiseWhile      function         return db getUser email                then function res                      logger log res                     count                             Second  that while function could be simplified a lot   var promiseWhile   Promise method function condition  action        if   condition    return      return action   then promiseWhile bind null  condition  action          Third  I would not use a while loop  with a closure variable  but a for loop   var promiseFor   Promise method function condition  action  value        if   condition value   return value      return action value  then promiseFor bind null  condition  action         promiseFor function count        return count  lt  10     function count        return db getUser email                then function res                      logger log res                     return   count                      0  then console log bind console   all done

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Bergi s suggested function is really nice   var promiseWhile   Promise method function condition  action          if   condition    return      return action   then promiseWhile bind null  condition  action          Still I want to make a tiny addition  which makes sense  when using promises    var promiseWhile   Promise method function condition  action  lastValue      if   condition    return lastValue    return action   then promiseWhile bind null  condition  action          This way the while loop can be embedded into a promise chain and resolves with lastValue  also if the action   is never run   See example   var count   10  util promiseWhile    function condition         return count  gt  0         function action         return new Promise function resolve  reject          count   count - 1        resolve count                count

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Using the standard promise object  and having the promise return the results   function promiseMap  data  f      const reducer    promise  x    gt      promise then acc   gt  f x  then y   gt  acc push y   amp  amp  acc     return data reduce reducer  Promise resolve         var emails       function getUser email      return db getUser email     promiseMap emails  getUser  then emails   gt      console log emails

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Here s how I do it with the standard Promise object      Given async function sayHi function sayHi       return new Promise  resolve    gt        setTimeout      gt          console log  Hi          resolve           3000               And an array of async functions to loop through const asyncArray    sayHi  sayHi  sayHi       We create the start of a promise chain let chain   Promise resolve        And append each function in the array to the promise chain for  const func of asyncArray      chain   chain then func         Output     Hi    Hi  After 3 seconds     Hi  After 3 more seconds

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Use async and await  es6    function taskAsync paramets    return new Promise  reslove reject   gt      your logic after reslove respoce  or reject error        async function fName    let arry   list of items      for var i 0 i lt arry length i        let result await taskAsync  parameters

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function promiseLoop promiseFunc  paramsGetter  conditionChecker  eachFunc  delay        function callNext             return promiseFunc apply null  paramsGetter                 then eachFunc             function loop promise  fn            if  delay                return new Promise function resolve                    setTimeout function                         resolve                       delay                                   then function                         return promise                          then fn                           then function condition                                if   condition                                    return true                                                            return loop callNext    fn                                                                   return promise              then fn               then function condition                    if   condition                        return true                                    return loop callNext    fn                            return loop callNext    conditionChecker       function makeRequest param        return new Promise function resolve  reject            var req   https request function res                var data                   res on  data   function  chunk                    data    chunk                              res on  end   function                      resolve data                                       req on  error   function e                reject e                       req write param           req end              function getSomething         var param   0       var limit   10       var results            function paramGetter             return  param             function conditionChecker             return param  lt   limit            function callback result            results push result           param               return promiseLoop makeRequest  paramGetter  conditionChecker  callback           then function                 return results                 getSomething   then function res        console log  results   res      catch function err        console log  some error along the way   err

User · Answer

If you really want a general promiseWhen   function for this and other purposes  then by all means do so  using Bergi s simplifications  However  because of the way promises work  passing callbacks in this way is generally unnecessary and forces you to jump through complex little hoops    As far as I can tell you re trying     to asynchronously fetch a series of user details for a collection of email addresses  at least  that s the only scenario that makes sense   to do so by building a  then   chain via recursion  to maintain the original order when handling the returned results    Defined thus  the problem is actually the one discussed under  The Collection Kerfuffle  in Promise Anti-patterns  which offers two simple solutions     parallel asynchronous calls using Array prototype map    serial asynchronous calls using Array prototype reduce      The parallel approach will  straightforwardly  give the issue that you are trying to avoid - that the order of the responses is uncertain  The serial approach will build the required  then   chain - flat - no recursion    function fetchUserDetails arr        return arr reduce function promise  email            return promise then function                 return db getUser email  done function res                    logger log res                                      Promise resolve         Call as follows      Compose here  by whatever means  an array of email addresses  var arrayOfEmailAddys           fetchUserDetails arrayOfEmailAddys  then function         console log  all done          As you can see  there s no need for the ugly outer var count or it s associated condition function  The limit  of 10 in the question  is determined entirely by the length of the array arrayOfEmailAddys

[javascript] Correct way to write loops for promise.

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