How to make a function wait until a callback has been called using node js

Question

I have a simplified function that looks like this   function query      myApi exec  SomeCommand   function response        return response            Basically i want it to call myApi exec  and return the response that is given in the callback lambda  However  the above code doesn t work and simply returns immediately   Just for a very hackish attempt  i tried the below which didn t work  but at least you get the idea what i m trying to achieve   function query      var r    myApi exec  SomeCommand   function response        r   response          while   r       return r      Basically  what s a good  node js event driven  way of going about this  I want my function to wait until the callback gets called  then return the value that was passed to it

User · Answer

If you don t want to use call back then you can Use  Q  module   For example    function getdb         var deferred   Q defer        MongoClient connect databaseUrl  function err  db            if  err                console log  Problem connecting database                deferred reject new Error err              else               var collection   db collection  url                deferred resolve collection                         return deferred promise      getdb   then function collection          This function will be called afte getdb   will be executed       fail function err          If Error accrued          For more information refer this  https   github com kriskowal q

User · Answer

The  good node js  event driven  way of doing this is to not wait   Like almost everything else when working with event driven systems like node  your function should accept a callback parameter that will be invoked when then computation is complete  The caller should not wait for the value to be  returned  in the normal sense  but rather send the routine that will handle the resulting value   function query  callback      myApi exec  SomeCommand   function response           other stuff here           bla bla       callback response      this will  return  your value to the original caller           So you dont use it like this   var returnValue   myFunction query     But like this   myFunction query  function returnValue         use the return value here instead of like a regular  non-evented  return value

User · Answer

Note  This answer should probably not be used in production code  It s a hack and you should know about the implications   There is the uvrun module  updated for newer Nodejs versions here  where you can execute a single loop round of the libuv main event loop  which is the Nodejs main loop    Your code would look like this   function query      var r    myApi exec  SomeCommand   function response        r   response          var uvrun   require  uvrun      while   r      uvrun runOnce      return r       You might alternative use uvrun runNoWait    That could avoid some problems with blocking  but takes 100  CPU    Note that this approach kind of invalidates the whole purpose of Nodejs  i e  to have everything async and non-blocking  Also  it could increase your callstack depth a lot  so you might end up with stack overflows  If you run such function recursively  you definitely will run into troubles   See the other answers about how to redesign your code to do it  right    This solution here is probably only useful when you do testing and esp  want to have synced and serial code

User · Answer

It s 2020 and chances are the API has already a promise-based version that works with await  However  some interfaces  especially event emitters will require this workaround     doesn t wait let value  someEventEmitter once  e    gt    value   e value          waits let value   await new Promise  resolve    gt      someEventEmitter once  event    e    gt    resolve e value            In this particular case it would be  let response   await new Promise  resolve    gt      myAPI exec  SomeCommand    response    gt    resolve response            Await has been in new Node js releases for the past 3 years  since v7 6

User · Answer

supposing you have a function   var fetchPage page  callback               request uri  function  error  response  body                         if  something good              callback true  page 1             else             callback false                                         you can make use of callbacks like this    fetchPage 1  x   function next  page    if  next        console log      CALLBACK -- gt   fetchPage      page       fetchPage page  x

User · Answer

Using async and await it is lot more easy   router post   login  async  req  res  next    gt    i   await queries checkUser req body   console log  i    JSON stringify i           User Available Check async function checkUser request    try       let response   await sql query  select   from login where email             request email        return response 0          catch  err        console log err

User · Answer

check this  https   github com luciotato waitfor-ES6  your code with wait for   requires generators  --harmony flag   function   query      var r   yield wait for  myApi exec   SomeCommand      return r

User · Answer

One way to achieve this is to wrap the API call into a promise and then use await to wait for the result     let s say this is the API function with two callbacks     one for success and the other for error function apiFunction query  successCallback  errorCallback        if  query     quot bad query quot             errorCallback  quot problem with the query quot              successCallback  quot Your query was  lt  quot    query    quot  gt  quot          myFunction wraps the above API call into a Promise    and handles the callbacks with resolve and reject function apiFunctionWrapper query        return new Promise  resolve  reject    gt            apiFunction query  successResponse    gt                resolve successResponse               errorResponse    gt                reject errorResponse                             now you can use await to get the result from the wrapped api function    and you can use standard try-catch to handle the errors async function businessLogic         try           const result   await apiFunctionWrapper  quot query all users quot            console log result                       the next line will fail         const result2   await apiFunctionWrapper  quot bad query quot          catch error            console error  quot ERROR  quot    error               call the main function businessLogic     Output  Your query was  lt query all users gt  ERROR problem with the query

User · Answer

Since node 4 8 0 you are able to use the feature of ES6 called generator  You may follow this article for deeper concepts  But basically you can use generators and promises to get this job done  I m using bluebird to promisify and manage the generator   Your code should be fine like the example below   const Promise   require  bluebird     function  getResponse query      const r   yield new Promise resolve   gt  myApi exec  SomeCommand   resolve     return r     Promise coroutine getResponse       then response   gt  console log response

User · Answer

If you want it very simple and easy  no fancy libraries  to wait for callback functions to be executed in node  before executing some other code  is like this     initialize a global var to control the callback state var callbackCount   0    call the function that has a callback someObj executeCallback function          callbackCount        runOtherCode        someObj2 executeCallback function          callbackCount        runOtherCode           call function that has to wait continueExec     function continueExec           here is the trick  wait until var callbackCount is set number of callback functions     if  callbackCount  lt  2            setTimeout continueExec  1000           return              Finally  do what you need     doSomeThing

User · Answer

That defeats the purpose of non-blocking IO -- you re blocking it when it doesn t need blocking     You should nest your callbacks instead of forcing node js to wait  or call another callback inside the callback where you need the result of r   Chances are  if you need to force blocking  you re thinking about your architecture wrong

[javascript] How to make a function wait until a callback has been called using node.js

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