How to wrap async function calls into a sync function in Node js or Javascript

Question

Suppose you maintain a library that exposes a function getData  Your users call it to get actual data  var output   getData    Under the hood data is saved in a file so you implemented getData using Node js built-in fs readFileSync  It s obvious both getData and fs readFileSync are sync functions  One day you were told to switch the underlying data source to a repo such as MongoDB which can only be accessed asynchronously  You were also told to avoid pissing off your users  getData API cannot be changed to return merely a promise or demand a callback parameter  How do you meet both requirements   Asynchronous function using callback promise is the DNA of JavasSript and Node js  Any non-trivial JS app is probably permeated with this coding style  But this practice can easily lead to so called callback pyramid of doom  Even worse  if any code in any caller in the call chain depends on the result of the async function  those code has to be wrapped in callback function as well  imposing a coding style constraint on caller  From time to time I find the need to encapsulate an async function  often provided in a 3rd party library  into a sync function in order to avoid massive global re-factoring  Searching for a solution on this subject usually ended up with Node Fibers or npm packages derived from it  But Fibers just cannot solve the problem I am facing  Even the example provided by Fibers  author illustrated the deficiency       Fiber function         console log  wait        new Date       sleep 1000       console log  ok        new Date      run    console log  back in main      Actual output   wait    Fri Jan 21 2011 22 42 04 GMT 0900  JST  back in main ok    Fri Jan 21 2011 22 42 05 GMT 0900  JST    If function Fiber really turns async function sleep into sync  the output should be   wait    Fri Jan 21 2011 22 42 04 GMT 0900  JST  ok    Fri Jan 21 2011 22 42 05 GMT 0900  JST  back in main   I have created another simple example in JSFiddle and looking for code to yield expected output  I ll accept a solution that only works in Node js so you are free to require any npm package despite not working in JSFiddle

User · Accepted Answer

deasync turns async function into sync  implemented with a blocking mechanism by calling Node js event loop at JavaScript layer  As a result  deasync only blocks subsequent code from running without blocking entire thread  nor incuring busy wait  With this module  here is the answer to the jsFiddle challenge   function AnticipatedSyncFunction      var ret    setTimeout function          ret    hello       3000     while ret     undefined        require  deasync   runLoopOnce          return ret          var output   AnticipatedSyncFunction      expected  output hello  after waiting for 3 sec  console log  output   output     actual  output hello  after waiting for 3 sec     disclaimer  I am the co-author of deasync  The module was created after posting this question and found no workable proposal

User · Answer

You ve got to use promises   const asyncOperation        gt        return new Promise  resolve  reject    gt            setTimeout     gt  resolve  hi     3000            const asyncFunction   async      gt        return await asyncOperation       const topDog        gt        asyncFunction   then  res    gt            console log res               I like arrow function definitions more  But any string of the form               could also be written as  function            So topDog is not async despite calling an async function     EDIT  I realize a lot of the times you need to wrap an async function inside a sync function is inside a controller  For those situations  here s a party trick   const getDemSweetDataz    req  res    gt         async      gt            try              res status 200  json                  await asyncOperation                                    catch e               res status 500  json serviceResponse     or whatever                      So we defined and immediately called this async function      Utilizing this with callbacks  you can do a wrap that doesn t use promises   const asyncOperation        gt        return new Promise  resolve  reject    gt            setTimeout     gt  resolve  hi     3000            const asyncFunction   async  callback    gt        let res   await asyncOperation        callback res      const topDog        gt        let callback    res    gt            console log res                async      gt            await asyncFunction callback               By applying this trick to an EventEmitter  you can get the same results  Define the EventEmitter s listener where I ve defined the callback  and emit the event where I called the callback

User · Answer

I can t find a scenario that cannot be solved using node-fibers  The example you provided using node-fibers behaves as expected  The key is to run all the relevant code inside a fiber  so you don t have to start a new fiber in random positions   Lets see an example  Say you use some framework  which is the entry point of your application  you cannot modify this framework   This framework loads nodejs modules as plugins  and calls some methods on the plugins  Lets say this framework only accepts synchronous functions  and does not use fibers by itself   There is a library that you want to use in one of your plugins  but this library is async  and you don t want to modify it either   The main thread cannot be yielded when no fiber is running  but you still can create plugins using fibers  Just create a wrapper entry that starts the whole framework inside a fiber  so you can yield the execution from the plugins   Downside  If the framework uses setTimeout or Promises internally  then it will escape the fiber context  This can be worked around by mocking setTimeout  Promise then  and all event handlers   So this is how you can yield a fiber until a Promise is resolved  This code takes an async  Promise returning  function and resumes the fiber when the promise is resolved   framework-entry js  console log require    my-plugin   run       async-lib js  exports getValueAsync        gt      return new Promise resolve   gt        setTimeout      gt          resolve  Async Value           100              my-plugin js  const Fiber   require  fibers     function fiberWaitFor promiseOrValue      var fiber   Fiber current  error  value    Promise resolve promiseOrValue  then v   gt        error   false      value   v      fiber run         e   gt        error   true      value   e      fiber run            Fiber yield      if  error        throw value      else       return value         const asyncLib   require    async-lib     exports run        gt      return fiberWaitFor asyncLib getValueAsync          my-entry js  require  fibers        gt      require    framework-entry       run      When you run node framework-entry js it will throw an error  Error  yield   called with no fiber running  If you run node my-entry js it works as expected

User · Answer

You shouldn t be looking at what happens around the call that creates the fiber but rather at what happens inside the fiber  Once you are inside the fiber you can program in sync style  For example    function f1         console log  wait        new Date       sleep 1000       console log  ok        new Date         function f2         f1        f1       Fiber function         f2       run      Inside the fiber you call f1  f2 and sleep as if they were sync   In a typical web application  you will create the Fiber in your HTTP request dispatcher  Once you ve done that you can write all your request handling logic in sync style  even if it calls async functions  fs  databases  etc

User · Answer

Nowadays this generator pattern can be a solution in many situations    Here an example of sequential console prompts in nodejs using async readline question function   var main    function             just import and initialize  readline  in nodejs   var r   require  readline     var rl   r createInterface  input  process stdin  output  process stdout          magic here  the callback is the iterator next   var answerA   yield rl question  do you want this     r  gt main next r             and again  in a sync fashion   var answerB   yield rl question  are you sure     r  gt main next r                 readline boilerplate   rl close      console log answerA  answerB             lt -- executed  iterator created from generator main next          kick off the iterator                      runs until the first  yield   including rightmost code                    and waits until another main next   happens

User · Answer

There is a npm sync module also  which is used for synchronize the process of executing the query   When you want to run parallel queries in synchronous way then node restrict to do that because it never wait for response  and sync module is much perfect for that kind of solution   Sample code     require sync module   var Sync   require  sync        app get     function req res next         story find   exec function err data           var sync function data   find user sync null   name   sanjeev               res send  story data user sync function data                                    sync function defined here              function find user req json  callback            process nextTick function                   users find req json function  err data                                if   err                        callback null  data                     else                       callback null  err                                                         reference link  https   www npmjs com package sync

User · Answer

Making Node js code sync is essential in few aspects such as database  But actual advantage of Node js lies in async code  As it is single thread non-blocking   we can sync it using important functionality Fiber   Use await   and defer    we call all methods using await    then replace the callback functions with defer     Normal Async code This uses CallBack functions   function add  var a  var b  function err res          console log res         function sub  var res2  var b  function err res1              console log res             function div  var res2  var b  function err res3              console log res3             Sync the above code using Fiber    await   and defer    fiber function         var obj1   await function add var a  var b defer           var obj2   await function sub var obj1  var b  defer           var obj3   await function sub var obj2  var b  defer             I hope this will help  Thank You

User · Answer

Javascript is a single threaded language  you don t want to block your whole server  Async code eliminates  race conditions by making dependencies explicit   Learn to love asynchronous code   Have a look at promises for asynchronous code without creating a pyramid of callback hell  I recommend the promiseQ library for node js  httpGet url parse  http   example org     then function  res        console log res statusCode       maybe 302     return httpGet url parse res headers  location         then function  res        console log res statusCode       maybe 200       http   howtonode org promises  EDIT  this is by far my most controversial answer  node now has yield keyword  which allows you to treat async code as if it were sychronous  http   blog alexmaccaw com how-yield-will-transform-node

User · Answer

If function Fiber really turns async function sleep into sync   Yes  Inside the fiber  the function waits before logging ok  Fibers do not make async functions synchronous  but allow to write synchronous-looking code that uses async functions and then will run asynchronously inside a Fiber      From time to time I find the need to encapsulate an async function into a sync function in order to avoid massive global re-factoring    You cannot  It is impossible to make asynchronous code synchronous  You will need to anticipate that in your global code  and write it in async style from the beginning  Whether you wrap the global code in a fiber  use promises  promise generators  or simple callbacks depends on your preferences      My objective is to minimize impact on the caller when data acquisition method is changed from sync to async   Both promises and fibers can do that

User · Answer

I struggled with this at first with node js and async js is the best library I have found to help you deal with this   If you want to write synchronous code with node  approach is this way   var async   require  async     console log  in main     doABunchOfThings function       console log  back in main         function doABunchOfThings fnCallback      async series       function callback          console log  step 1          callback               function callback          setTimeout callback  1000              function callback          console log  step 2          callback               function callback          setTimeout callback  2000              function callback          console log  step 3          callback                function err  results        console log  done with things        fnCallback              this program will ALWAYS produce the following     in main step 1 step 2 step 3 done with things back in main

[javascript] How to wrap async function calls into a sync function in Node.js or Javascript?

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