Node JS Promise all and forEach

Question

I have an array like structure that exposes async methods  The async method calls return array structures that in turn expose more async methods  I am creating another JSON object to store values obtained from this structure and so I need to be careful about keeping track of references in callbacks   I have coded a brute force solution  but I would like to learn a more idiomatic or clean solution    The pattern should be repeatable for n levels of nesting   I need to use promise all or some similar technique to determine when to resolve the enclosing routine  Not every element will necessarily involve making an async call  So in a nested promise all I can t simply make assignments to my JSON array elements based on index  Nevertheless  I do need to use something like promise all in the nested forEach to ensure that all property assignments have been made prior to resolving the enclosing routine  I am using the bluebird promise lib but this is not a requirement   Here is some partial code -   var jsonItems        items forEach function item      var jsonItem         jsonItem name   item name    item getThings   then function things        or Promise all allItemGetThingCalls  function things        things forEach function thing  index          jsonItems index  thingName   thing name        if thing type      file             thing getFile   then function file     or promise all             jsonItems index  filesize   file getSize

User · Answer

I had through the same situation  I solved using two Promise All     I think was really good solution  so I published it on npm  https   www npmjs com package promise-foreach  I think your code will be something like this  var promiseForeach   require  promise-foreach   var jsonItems       promiseForeach each jsonItems       function  jsonItems           return new Promise function resolve  reject               if jsonItems type      file                    jsonItems getFile   then function file     or promise all                      resolve file getSize                                                            function  result  current            return               type  current type              size  jsonItems result 0                       function  err  newList            if  err                console error err              return                    console log  new jsonItems      newList

User · Answer

Just to add to the solution presented  in my case I wanted to fetch multiple data from Firebase for a list of products  Here is how I did it  useEffect      gt      const fn   p   gt  firebase firestore   doc  products   p id    get      const actions   data occasion products map fn     const results   Promise all actions     results then data   gt        const newProducts           data forEach p   gt          newProducts push   id  p id     p data                   setProducts newProducts             data

User · Answer

It s pretty straightforward with some simple rules    Whenever you create a promise in a then  return it - any promise you don t return will not be waited for outside  Whenever you create multiple promises   all them - that way it waits for all the promises and no error from any of them are silenced  Whenever you nest thens  you can typically return in the middle - then chains are usually at most 1 level deep  Whenever you perform IO  it should be with a promise - either it should be in a promise or it should use a promise to signal its completion    And some tips    Mapping is better done with  map than with for push - if you re mapping values with a function  map lets you concisely express the notion of applying actions one by one and aggregating the results  Concurrency is better than sequential execution if it s free - it s better to execute things concurrently and wait for them Promise all than to execute things one after the other - each waiting before the next    Ok  so let s get started   var items    1  2  3  4  5   var fn   function asyncMultiplyBy2 v      sample async action     return new Promise resolve   gt  setTimeout      gt  resolve v   2   100          map over forEach since it returns  var actions   items map fn      run the function over all items     we now have a promises array and we want to wait for it  var results   Promise all actions      pass array of promises  results then data   gt     or just  then console log      console log data      2  4  6  8  10         we can nest this of course  as I said   then  chains   var res2   Promise all  1  2  3  4  5  map fn   then      data   gt  Promise all data map fn     then function data          the next  then  is executed after the promise has returned from the previous         then  fulfilled  in this case it s an aggregate promise because of         the   all       return Promise all data map fn       then function data          just for good measure     return Promise all data map fn            now to get the results   res2 then function data       console log data       16  32  48  64  80

User · Answer

Here s a simple example using reduce   It runs serially  maintains insertion order  and does not require Bluebird               param items An array of items      param fn A function that accepts an item from the array and returns a promise      returns  Promise      function forEachPromise items  fn        return items reduce function  promise  item            return promise then function                  return fn item                      Promise resolve         And use it like this   var items     a    b    c     function logItem item        return new Promise  resolve  reject    gt            process nextTick      gt                console log item               resolve                          forEachPromise items  logItem  then      gt        console log  done          We have found it useful to send an optional context into loop   The context is optional and shared by all iterations   function forEachPromise items  fn  context        return items reduce function  promise  item            return promise then function                  return fn item  context                      Promise resolve         Your promise function would look like this   function logItem item  context        return new Promise  resolve  reject    gt            process nextTick      gt                console log item               context itemCount                resolve

[javascript] Node JS Promise.all and forEach

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