Simplest way to wait some asynchronous tasks complete in Javascript

Question

I want to drop some mongodb collections  but that s an asynchronous task  The code will be   var mongoose   require  mongoose     mongoose connect  mongo   localhost xxx     var conn   mongoose connection     aaa   bbb   ccc   forEach function name       conn collection name  drop function err            console log  dropped                console log  all dropped      The console displays   all dropped dropped dropped dropped   What is the simplest way to make sure all dropped will be printed after all collections has been dropped  Any 3rd-party can be used to simplify the code

User · Answer

All answers are quite old  Since the beginning of 2013 Mongoose started to support promises gradually for all queries  so that would be the recommended way of structuring several async calls in the required order going forward I guess

User · Answer

The way to do it is to pass the tasks a callback that updates a shared counter  When the shared counter reaches zero you know that all tasks have finished so you can continue with your normal flow   var ntasks left to go   4   var callback   function        ntasks left to go -  1      if ntasks left to go  lt   0            console log  All tasks have completed  Do your stuff             task1 callback   task2 callback   task3 callback   task4 callback     Of course  there are many ways to make this kind of code more generic or reusable and any of the many async programing libraries out there should have at least one function to do this kind of thing

User · Answer

If you are using Babel or such transpilers and using async await you could do    function onDrop        console log  dropped       async function dropAll  collections        const drops   collections map col   gt  conn collection col  drop onDrop        await drops     console log  all dropped

User · Answer

Use Promises   var mongoose   require  mongoose     mongoose connect  your MongoDB connection string    var conn   mongoose connection   var promises     aaa    bbb    ccc   map function name      return new Promise function resolve  reject        var collection   conn collection name       collection drop function err          if  err    return reject err           console log  dropped     name         resolve                       Promise all promises   then function     console log  all dropped         catch console error     This drops each collection  printing    dropped    after each one  and then prints    all dropped    when complete  If an error occurs  it is displayed to stderr     Previous answer  this pre-dates Node   s native support for Promises    Use Q promises or Bluebird promises   With Q   var Q   require  q    var mongoose   require  mongoose     mongoose connect  your MongoDB connection string    var conn   mongoose connection   var promises     aaa   bbb   ccc   map function name       var collection   conn collection name       return Q ninvoke collection   drop          then function     console log  dropped     name            Q all promises   then function     console log  all dropped        fail console error     With Bluebird   var Promise   require  bluebird    var mongoose   Promise promisifyAll require  mongoose      mongoose connect  your MongoDB connection string    var conn   mongoose connection   var promises     aaa    bbb    ccc   map function name      return conn collection name  dropAsync   then function         console log  dropped     name              Promise all promises   then function     console log  all dropped        error console error

User · Answer

Expanding upon  freakish answer  async also offers a each method  which seems especially suited for your case   var async   require  async     async each   aaa   bbb   ccc    function name  callback        conn collection name  drop  callback       function err        if  err     return console log err         console log  all dropped          IMHO  this makes the code both more efficient and more legible  I ve taken the liberty of removing the console log  dropped   - if you want it  use this instead   var async   require  async     async each   aaa   bbb   ccc    function name  callback           if you really want the console log   dropped            replace the  callback  here with an anonymous function     conn collection name  drop  function err            if  err     return callback err             console log  dropped            callback              function err        if  err     return console log err         console log  all dropped

User · Answer

I see you are using mongoose so you are talking about server-side JavaScript  In that case I advice looking at async module and use async parallel       You will find this module really helpful - it was developed to solve the problem you are struggling with  Your code may look like this  var async   require  async     var calls          aaa   bbb   ccc   forEach function name       calls push function callback            conn collection name  drop function err                if  err                  return callback err               console log  dropped                callback null  name                           async parallel calls  function err  result           this code will run after all calls finished the job or        when any of the calls passes an error        if  err          return console log err       console log result

User · Answer

With deferred  another promise deferred implementation  you can do      Setup  pdrop   promise version of  drop  method var deferred   require  deferred    mongoose Collection prototype pdrop       deferred promisify mongoose Collection prototype drop       Drop collections  deferred map   aaa   bbb   ccc    function name       return conn collection name  pdrop   function            console log  dropped               end function          console log  all dropped       null

User · Answer

I do this without external libaries   var yourArray     aaa   bbb   ccc    var counter        yourArray forEach function name       conn collection name  drop function err            counter push true           console log  dropped            if counter length     yourArray length               console log  all dropped

[javascript] Simplest way to wait some asynchronous tasks complete, in Javascript?

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