Call An Asynchronous Javascript Function Synchronously

Question

First  this is a very specific case of doing it the wrong way on-purpose to retrofit an asynchronous call into a very synchronous codebase that is many thousands of lines long and time doesn t currently afford the ability to make the changes to  do it right   It hurts every fiber of my being  but reality and ideals often do not mesh  I know this sucks   OK  that out of the way  how do I make it so that I could   function doSomething        var data     function callBack d        data   d         myAsynchronousCall param1  callBack         block here and return data when the callback is finished   return data      The examples  or lack thereof  all use libraries and or compilers  both of which are not viable for this solution  I need a concrete example of how to make it block  e g  NOT leave the doSomething function until the callback is called  WITHOUT freezing the UI  If such a thing is possible in JS

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You can force asynchronous JavaScript in NodeJS to be synchronous with sync-rpc   It will definitely freeze your UI though  so I m still a naysayer when it comes to whether what it s possible to take the shortcut you need to take  It s not possible to suspend the One And Only Thread in JavaScript  even if NodeJS lets you block it sometimes  No callbacks  events  anything asynchronous at all will be able to process until your promise resolves  So unless you the reader have an unavoidable situation like the OP  or  in my case  are writing a glorified shell script with no callbacks  events  etc    DO NOT DO THIS   But here s how you can do this     calling-file js  var createClient   require  sync-rpc    var mySynchronousCall   createClient require resolve    my-asynchronous-call     init data     var param1    test data  var data   mySynchronousCall param1   console log data      prints  received  test data  after  init data      my-asynchronous-call js  function init initData      return function param1           Return a promise here and the resulting rpc client will be synchronous     return Promise resolve  received      param1      after      initData                module exports   init    LIMITATIONS   These are both a consequence of how sync-rpc is implemented  which is by abusing require  child process   spawnSync    This will not work in the browser  The arguments to your function must be serializable  Your arguments will pass in and out of JSON stringify  so functions and non-enumerable properties like prototype chains will be lost

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One thing people might not consider  If you control the async function  which other pieces of code depend on   AND the codepath it would take is not necessarily asynchronous  you can make it synchronous  without breaking those other pieces of code  by creating an optional parameter  Currently  async function myFunc args etcetc           you wrote this     return  stuff       async function main         var result   await myFunc  argsetcetc        console log  async result   result         Consider  function myFunc args etcetc  opts                      param opts     sync Boolean  -- whether to return a Promise or not            var  sync false    opts      if  sync   true          return  stuff       else         return new Promise  RETURN REJECT   gt                RETURN  stuff                       async code still works just like before   async function main         var result   await myFunc  argsetcetc        console log  async result    result            prints   stuff      new sync code works  if you specify sync mode   function main         var result   myFunc  argsetcetc    sync true        console log  sync result    result            prints   stuff   Of course this doesn t work if the async function relies on inherently async operations  network requests  etc    in which case the endeavor is futile  without effectively waiting idle-spinning for no reason   Also this is fairly ugly to return either a value or a Promise depending on the options passed in    quot Why would I have written an async function if it didn t use async constructs  quot  one might ask  Perhaps some modalities parameters of the function require asynchronicity and others don t  and due to code duplication you wanted a monolithic block rather than separate modular chunks of code in different functions    For example perhaps the argument is either localDatabase  which doesn t require await  or remoteDatabase  which does   Then you could runtime error if you try to do  sync true  on the remote database  Perhaps this scenario is indicative of another problem  but there you go

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There is one nice workaround at http   taskjs org   It uses generators which are new to javascript  So it s currently not implemented by most browsers  I tested it in firefox  and for me it is nice way to wrap asynchronous function   Here is example code from project GitHub  var   Deferred     task   spawn function         out innerHTML    reading    n       try           var d   yield read  read html            alert d responseText length         catch  e            e stack split   n   forEach function line    console log line              console log              out innerHTML    error      e              function read url  method        method   method     GET       var xhr   new XMLHttpRequest        var deferred   new Deferred        xhr onreadystatechange   function             if  xhr readyState     4                if  xhr status  gt   400                    var e   new Error xhr statusText                   e status   xhr status                  deferred reject e                 else                   deferred resolve                       responseText  xhr responseText                                                        xhr open method  url  true       xhr send        return deferred promise

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You can also convert it into callbacks   function thirdPartyFoo callback          callback  Hello World           function foo           var fooVariable     thirdPartyFoo function data        fooVariable   data           return fooVariable     var temp   foo      console log temp

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Take a look at JQuery Promises   http   api jquery com promise   http   api jquery com jQuery when   http   api jquery com deferred promise   Refactor the code         var dfd   new jQuery Deferred          function callBack data           dfd notify data                 do the async call      myAsynchronousCall param1  callBack        function doSomething data            do stuff with data                 when dfd  then doSomething

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The idea that you hope to achieve can be made possible if you tweak the requirement a little bit   The below code is possible if your runtime supports the ES6 specification    More about async functions  async function myAsynchronousCall param1           logic for myAsynchronous call     return d     function doSomething        var data   await myAsynchronousCall param1      blocks  here until the async call is finished   return data

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What you want is actually possible now  If you can run the asynchronous code in a service worker  and the synchronous code in a web worker  then you can have the web worker send a synchronous XHR to the service worker  and while the service worker does the async things  the web worker s thread will wait  This is not a great approach  but it could work

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Async functions  a feature in ES2017  make async code look sync by using promises  a particular form of async code  and the await keyword  Also notice in the code examples below the keyword async in front of the function keyword that signifies an async await function  The await keyword won t work without being in a function pre-fixed with the async keyword  Since currently there is no exception to this that means no top level awaits will work  top level awaits meaning an await outside of any function   Though there is a proposal for top-level await   ES2017 was ratified  i e  finalized  as the standard for JavaScript on June 27th  2017  Async await may already work in your browser  but if not you can still use the functionality using a javascript transpiler like babel or traceur  Chrome 55 has full support of async functions  So if you have a newer browser you may be able to try out the code below   See kangax s es2017 compatibility table for browser compatibility   Here s an example async await function called doAsync which takes three one second pauses and prints the time difference after each pause from the start time    x000D   x000D  function timeoutPromise  time    x000D    return new Promise function  resolve    x000D      setTimeout function      x000D        resolve Date now     x000D         time  x000D       x000D    x000D   x000D  function doSomethingAsync      x000D    return timeoutPromise 1000   x000D    x000D   x000D  async function doAsync      x000D    var start   Date now    time  x000D    console log 0   x000D    time   await doSomethingAsync    x000D    console log time - start   x000D    time   await doSomethingAsync    x000D    console log time - start   x000D    time   await doSomethingAsync    x000D    console log time - start   x000D    x000D   x000D  doAsync    x000D   x000D   x000D    When the await keyword is placed before a promise value  in this case the promise value is the value returned by the function doSomethingAsync  the await keyword will pause execution of the function call  but it won t pause any other functions and it will continue executing other code until the promise resolves  After the promise resolves it will unwrap the value of the promise and you can think of the await and promise expression as now being replaced by that unwrapped value    So  since await just pauses waits for then unwraps a value before executing the rest of the line you can use it in for loops and inside function calls like in the below example which collects time differences awaited in an array and prints out the array     x000D   x000D  function timeoutPromise  time    x000D    return new Promise function  resolve    x000D      setTimeout function      x000D        resolve Date now     x000D         time  x000D       x000D    x000D   x000D  function doSomethingAsync      x000D    return timeoutPromise 1000   x000D    x000D   x000D     this calls each promise returning function one after the other x000D  async function doAsync      x000D    var response       x000D    var start   Date now    x000D       each index is a promise returning function x000D    var promiseFuncs   doSomethingAsync  doSomethingAsync  doSomethingAsync   x000D    for var i   0  i  lt  promiseFuncs length    i    x000D      var promiseFunc   promiseFuncs i   x000D      response push await promiseFunc   - start   x000D      console log response   x000D      x000D       do something with response which is an array of values that were from resolved promises  x000D    return response x000D    x000D   x000D  doAsync   then function  response    x000D    console log response  x000D     x000D   x000D   x000D    The async function itself returns a promise so you can use that as a promise with chaining like I do above or within another async await function   The function above would wait for each response before sending another request if you would like to send the requests concurrently you can use Promise all    x000D   x000D     no change x000D  function timeoutPromise  time    x000D    return new Promise function  resolve    x000D      setTimeout function      x000D        resolve Date now     x000D         time  x000D       x000D    x000D   x000D     no change x000D  function doSomethingAsync      x000D    return timeoutPromise 1000   x000D    x000D   x000D     this function calls the async promise returning functions all at around the same time x000D  async function doAsync      x000D    var start   Date now    x000D       we are now using promise all to await all promises to settle x000D    var responses   await Promise all  doSomethingAsync    doSomethingAsync    doSomethingAsync      x000D    return responses map x  gt x-start   x000D    x000D   x000D     no change x000D  doAsync   then function  response    x000D    console log response  x000D     x000D   x000D   x000D    If the promise possibly rejects you can wrap it in a try catch or skip the try catch and let the error propagate to the async await functions catch call  You should be careful not to leave promise errors unhandled especially in Node js  Below are some examples that show off how errors work    x000D   x000D  function timeoutReject  time    x000D    return new Promise function  resolve  reject    x000D      setTimeout function      x000D        reject new Error  OOPS well you got an error at TIMESTAMP      Date now      x000D         time  x000D       x000D    x000D   x000D  function doErrorAsync      x000D    return timeoutReject 1000   x000D    x000D   x000D  var log       args   gt console log    args   x000D  var logErr       args   gt console error    args   x000D   x000D  async function unpropogatedError      x000D       promise is not awaited or returned so it does not propogate the error x000D    doErrorAsync    x000D    return  finished unpropogatedError successfully   x000D    x000D   x000D  unpropogatedError   then log  catch logErr  x000D   x000D  async function handledError      x000D    var start   Date now    x000D    try   x000D      console log  await doErrorAsync    - start   x000D      console log  past error    x000D      catch  e    x000D      console log  in catch we handled the error    x000D      x000D     x000D    return  finished handledError successfully   x000D    x000D   x000D  handledError   then log  catch logErr  x000D   x000D     example of how error propogates to chained catch method x000D  async function propogatedError      x000D    var start   Date now    x000D    var time   await doErrorAsync   - start  x000D    console log time - start   x000D    return  finished propogatedError successfully   x000D    x000D   x000D     this is what prints propogatedError s error  x000D  propogatedError   then log  catch logErr  x000D   x000D   x000D    If you go here you can see the finished proposals for upcoming ECMAScript versions   An alternative to this that can be used with just ES2015  ES6  is to use a special function which wraps a generator function  Generator functions have a yield keyword which may be used to replicate the await keyword with a surrounding function  The yield keyword and generator function are a lot more general purpose and can do many more things then just what the async await function does  If you want a generator function wrapper that can be used to replicate async await I would check out co js  By the way co s function much like async await functions return a promise  Honestly though at this point browser compatibility is about the same for both generator functions and async functions so if you just want the async await functionality you should use Async functions without co js   Browser support is actually pretty good now for Async functions  as of 2017  in all major current browsers  Chrome  Safari  and Edge  except IE

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don t tell me about how I should just do it  the right way  or whatever    OK  but you should really do it the right way    or whatever       I need a concrete example of how to make it block     WITHOUT freezing the UI  If such a thing is possible in JS     No  it is impossible to block the running JavaScript without blocking the UI   Given the lack of information  it s tough to offer a solution  but one option may be to have the calling function do some polling to check a global variable  then have the callback set data to the global   function doSomething               callback sets the received data to a global var   function callBack d          window data   d               start the async   myAsynchronousCall param1  callBack            start the function doSomething          make sure the global is clear window data   null       start polling at an interval until the data is found at the global var intvl   setInterval function         if  window data             clearInterval intvl           console log data            100     All of this assumes that you can modify doSomething    I don t know if that s in the cards   If it can be modified  then I don t know why you wouldn t just pass a callback to doSomething   to be called from the other callback  but I better stop before I get into trouble        Oh  what the heck  You gave an example that suggests it can be done correctly  so I m going to show that solution     function doSomething  func        function callBack d        func  d           myAsynchronousCall param1  callBack       doSomething function data        console log data         Because your example includes a callback that is passed to the async call  the right way would be to pass a function to doSomething   to be invoked from the callback   Of course if that s the only thing the callback is doing  you d just pass func directly     myAsynchronousCall param1  func

[javascript] Call An Asynchronous Javascript Function Synchronously

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