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.

Answer 1

"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);

Answer 2

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:

function timeoutPromise (time) {
  return new Promise(function (resolve) {
    setTimeout(function () {
      resolve(Date.now());
    }, time)
  })
}

function doSomethingAsync () {
  return timeoutPromise(1000);
}

async function doAsync () {
  var start = Date.now(), time;
  console.log(0);
  time = await doSomethingAsync();
  console.log(time - start);
  time = await doSomethingAsync();
  console.log(time - start);
  time = await doSomethingAsync();
  console.log(time - start);
}

doAsync();

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.

function timeoutPromise (time) {
  return new Promise(function (resolve) {
    setTimeout(function () {
      resolve(Date.now());
    }, time)
  })
}

function doSomethingAsync () {
  return timeoutPromise(1000);
}

// this calls each promise returning function one after the other
async function doAsync () {
  var response = [];
  var start = Date.now();
  // each index is a promise returning function
  var promiseFuncs= [doSomethingAsync, doSomethingAsync, doSomethingAsync];
  for(var i = 0; i < promiseFuncs.length; ++i) {
    var promiseFunc = promiseFuncs[i];
    response.push(await promiseFunc() - start);
    console.log(response);
  }
  // do something with response which is an array of values that were from resolved promises.
  return response
}

doAsync().then(function (response) {
  console.log(response)
})

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.

// no change
function timeoutPromise (time) {
  return new Promise(function (resolve) {
    setTimeout(function () {
      resolve(Date.now());
    }, time)
  })
}

// no change
function doSomethingAsync () {
  return timeoutPromise(1000);
}

// this function calls the async promise returning functions all at around the same time
async function doAsync () {
  var start = Date.now();
  // we are now using promise all to await all promises to settle
  var responses = await Promise.all([doSomethingAsync(), doSomethingAsync(), doSomethingAsync()]);
  return responses.map(x=>x-start);
}

// no change
doAsync().then(function (response) {
  console.log(response)
})

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.

function timeoutReject (time) {
  return new Promise(function (resolve, reject) {
    setTimeout(function () {
      reject(new Error("OOPS well you got an error at TIMESTAMP: " + Date.now()));
    }, time)
  })
}

function doErrorAsync () {
  return timeoutReject(1000);
}

var log = (...args)=>console.log(...args);
var logErr = (...args)=>console.error(...args);

async function unpropogatedError () {
  // promise is not awaited or returned so it does not propogate the error
  doErrorAsync();
  return "finished unpropogatedError successfully";
}

unpropogatedError().then(log).catch(logErr)

async function handledError () {
  var start = Date.now();
  try {
    console.log((await doErrorAsync()) - start);
    console.log("past error");
  } catch (e) {
    console.log("in catch we handled the error");
  }
  
  return "finished handledError successfully";
}

handledError().then(log).catch(logErr)

// example of how error propogates to chained catch method
async function propogatedError () {
  var start = Date.now();
  var time = await doErrorAsync() - start;
  console.log(time - start);
  return "finished propogatedError successfully";
}

// this is what prints propogatedError's error.
propogatedError().then(log).catch(logErr)

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. (I recommend just using async/await it's pretty widely supported in most environments that the above strikethrough is supported in.)

Browser support is actually pretty good now for Async functions (as of 2017) in all major current browsers (Chrome, Safari, and Edge) except IE.

Answer 3

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);

Answer 4

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:

1. This will not work in the browser.
2. 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.

Answer 5

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 >= 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;
}

Answer 6

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.

Answer 7

let result;
async_function().then(r => result = r);
while (result === undefined) // Wait result from async_function
    require('deasync').sleep(100);

Answer 8

In Node.js it's possible to write synchronous code which actually invokes asynchronous operations. node-fibers allows this. It's a 3rd party native extension provided as an npm module. It implements fibers/coroutines, so when a specific fiber is blocked waiting for asynchronous operation, the whole program events loop doesn't block - another fiber (if exists) continues its job.

With fibers your code would look like this:

var Fiber = require('fibers');

function doSomething() {
  var fiber = Fiber.current;

  function callBack(data) {
    fiber.run(data);
  }

  myAsynchronousCall(param1, callBack);

  // execution blocks here
  var data = Fiber.yield();
  return data;
}

// The whole program must be wrapped with Fiber
Fiber(function main() {

  var data = doSomething();
  console.log(data);

}).run();

Note, that you should avoid it and use async/await instead. See below a note from the project readme https://github.com/laverdet/node-fibers:

NOTE OF OBSOLESCENCE -- The author of this project recommends you avoid its use if possible. The original version of this module targeted nodejs v0.1.x in early 2011 when JavaScript on the server looked a lot different. Since then async/await, Promises, and Generators were standardized and the ecosystem as a whole has moved in that direction.

I'll continue to support newer versions of nodejs as long as possible but v8 and nodejs are extraordinarily complex and dynamic platforms. It is inevitable that one day this library will abruptly stop working and no one will be able to do anything about it.

I'd like to say thank you to all the users of fibers, your support over the years has meant a lot to me.

Answer 9

As the commenters (Pointy, Wayne) say,

It's simply not possible to make a browser block and wait. They just won't do it.

Other than moving to async/await, there is no way to let the async function run to completion and resume execution where you left off. Unless you relinquish the main thread, the async code has nowhere to run.

What might work would be to use web workers. It's not only a nodejs thing, it works in the browser too. Web workers run in a separate thread, so there won't be any blocking. The difficulty lays in communicating with the web worker, because the preferred way to get data in and out of it is to use asynchronous messaging.

Here's how to talk to your web worker synchronously. You'd be using techniques that were often used in the webassembly world, see for example

Communication with the main thread happens by reading and writing data in SharedArrayBuffers which are shared between the main browser thread and the worker, allowing the emulator's main loop to run continuously without ever yielding to the event loop.

1. https://blog.persistent.info/2021/08/worker-loop.html
2. https://jamesfriend.com.au/basilisk-ii-classic-mac-emulator-in-the-browser#input

This has the following (surpassable) difficulties

1. The SharedArrayBuffer functionality has been disabled in response to Spectre/Meltdown and later browser releases reenabled it only for sites that set certain CORS headers, https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/SharedArrayBuffer#security_requirements

2. In nodejs, one may use Atomics.wait even on the main thread. In the browsers however, it is necessary to use async api or to busywait

   • https://github.com/WebAssembly/threads/issues/106
   • https://github.com/WebAssembly/threads/issues/177

3. Browser does not have the receiveMessageOnPort function (that pont's answer relies on), meaning that it's not possible to do a sync nonblocking receive on a message from a web worker. It's necessary to serialize (JSON.stringify) the data into a SharedArrayBuffer and use this shared memory for communicating.

4. Sending the data into worker using postMessage did not work in my testing, the messages were only delivered after my code relinquished the main loop, so I was not able to busywait for the message to be sent and answer delivered.

Sample code

This has three part. First, a Python script used to serve the HTML with the required security headers set so that SharedArrayBuffers are available. Second, code for the web worker. Third, the HTML page.

serve.py

#!/usr/bin/env python3

import http.server
import socketserver
import sys


# https://stackoverflow.com/questions/21956683/enable-access-control-on-simple-http-server
class CORSRequestHandler(http.server.SimpleHTTPRequestHandler):
    def end_headers(self):
        self.send_header('Access-Control-Allow-Origin', '*')
        self.send_header('Cross-Origin-Opener-Policy', 'same-origin')
        self.send_header('Cross-Origin-Embedder-Policy', 'require-corp')
        super().end_headers()


def main():
    socketserver.TCPServer.allow_reuse_address = True

    PORT = int(sys.argv[1]) if len(sys.argv) > 1 else 8000
    with socketserver.TCPServer(("", PORT), CORSRequestHandler) as httpd:
        print("serving at port", PORT)
        httpd.serve_forever()


if __name__ == '__main__':
    main()

// fibonacci.worker.js
import {encodeToSharedBuffer, decodeFromSharedBuffer} from "./utils.js"

// sample async function that we want to call synchronously
/**
 * @param n {number}
 * @returns {Promise<number>}
 */
async function fibonacci(n) {
    if (n === 1) {
        return 1;
    } else if (n === 2) {
        return 1
    } else {
        return (await fibonacci(n-2)) + (await fibonacci(n-1));
    }
}

// the web worker onmessage handler
// this runs in a separate thread, not in the main thread
/**
 * @param event {MessageEvent}
 * @returns {Promise<void>}
 */
self.onmessage = async function (event) {
    switch (event.data.command) {
        case "start": {
            const {atomicFlag, sharedData} = event.data;

            // the onmessage handler in the page runs main()
            self.postMessage("ready for work");
            while (true) {
                const flagIntView = new Int32Array(atomicFlag)
                // wait for flag not be === 0
                Atomics.wait(flagIntView, 0, 0);

                // load arguments
                const jsonArgs = decodeFromSharedBuffer(sharedData);
                console.log(jsonArgs);
                /**@type number*/
                const args = JSON.parse(jsonArgs.trim());
                console.log("startingg computation, args=", args);
                // run async method
                const result = await fibonacci(args)
                console.log("finished computation, result=", result);
                // store the result
                encodeToSharedBuffer(JSON.stringify(result), sharedData);

                // set flag
                Atomics.store(flagIntView, 0, 0);
                // need not actually notify as we busywait in the main thread
                Atomics.notify(flagIntView, 0);
            }

            break;
        }

        default: {
            console.log(event.data);
            throw new Error("Unknown command: " + event.data.command);
        }
    }
};

<html lang="en" xmlns="http://www.w3.org/1999/html">
<head>
  <meta charset="UTF-8">
  <title></title>
  <script type="module">
    // https://developer.mozilla.org/en-US/docs/Web/API/Window/crossOriginIsolated
    // https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/SharedArrayBuffer#security_requirements
    import {decodeFromSharedBuffer, encodeToSharedBuffer} from "./utils.js";

    if (!window.crossOriginIsolated) {
      throw new Error('crossOriginIsolated is required');
    }

    const myWorker = new Worker("./fibonacci.worker.js", {name:"fibonacci worker", type: "module"});
    /// 0: the page accesses sharedData, 1: the worker accesses shared data
    const atomicFlag = new SharedArrayBuffer(4);
    const sharedData = new SharedArrayBuffer(2048);
    myWorker.postMessage({command: 'start', atomicFlag: atomicFlag, sharedData: sharedData});

    function runAsyncFunctionAsSync(arg) {
      const flagIntView = new Int32Array(atomicFlag)

      // set the arguments
      encodeToSharedBuffer(JSON.stringify(arg), sharedData);

      // notify the worker
      Atomics.store(flagIntView, 0, 1);
      Atomics.notify(flagIntView, 0);

      // if things go wrong, we would freeze the browser window, so always set a timeout
      const timeout = 5000;
      // spin-wait for the computation to complete
      const then = window.performance.now();

      // this is because `Atomics.wait` cannot be called on the main browser thread
      // NOTE: If you could flip things around and run the computation in main thread and sync-await it in the worker,
      //  then things could be made more efficient; when main thead awaits, it has to spin-loop
      while (Atomics.load(flagIntView, 0) === 1 && window.performance.now() - then < timeout) {
        // noop
      }
      if (Atomics.load(flagIntView, 0) === 1) {
        // waiting failed, submit an error into your log collector
        throw new Error("failed to wait for result")
      }

      const jsonResult = decodeFromSharedBuffer(sharedData)
      console.log(jsonResult);
      return JSON.parse(jsonResult);
    }

    // we must wait for the worker to start itself before we can safely run blocking javascript on the
    // ui thread, so the code in the page has to be in a callback like this
    myWorker.onmessage = main

    function main() {
      // we can't use `new MessageChannel()` like on nodejs, because on web we don't have `receiveMessageOnPort`
      //  https://nodejs.org/api/worker_threads.html#workerreceivemessageonportport
      // myWorker.postMessage({command: 'compute', args: [42]});

      // fibonacci(42) times out, computes for too long
      console.log("fibonacci(30) is", runAsyncFunctionAsSync(30));
    }
  </script>
</head>
<body>
</body>
</html>

// utils.js
// https://stackoverflow.com/questions/72948162/javascript-textdecoder-on-sharedarraybuffer
/**
 * @param source {string}
 * @param sharedBuffer {SharedArrayBuffer}
 */
export function encodeToSharedBuffer(source, sharedBuffer) {
    const encoder = new TextEncoder()

    const tempBuffer = new ArrayBuffer(sharedBuffer.byteLength)
    const tempView = new Uint8Array(tempBuffer)
    const sharedView = new Uint8Array(sharedBuffer)

    tempView.byteOffset

    encoder.encodeInto(source, tempView)
    sharedView.set(tempView)
}

/**
 * @param sharedBuffer {SharedArrayBuffer}
 * @returns {string}
 */
export function decodeFromSharedBuffer(sharedBuffer) {
    const decoder = new TextDecoder()

    let sharedView = new Uint8Array(sharedBuffer)
    const end = sharedView.indexOf(0)
    sharedView = sharedView.subarray(0, end)
    const tempBuffer = new ArrayBuffer(sharedView.byteLength)
    const tempView = new Uint8Array(tempBuffer)
    tempView.set(sharedView)

    return decoder.decode(tempBuffer)
}

Answer 10

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)=> {
            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.

("Why would I have written an async function if it didn't use async constructs?" 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.)

Answer 11

Using Node 16's worker threads actually makes this possible, The following example the main thread is running the asynchronous code while the worker thread is waiting for it synchronously.

Not that is is very useful, but it at least does vaguely what the original question asked by waiting for asynchronous code synchronously.

const {
    Worker, isMainThread, parentPort, receiveMessageOnPort
} = require('worker_threads');
if (isMainThread) {
    const worker = new Worker(__filename);
    worker.on('message', async () => {
        worker.postMessage(await doAsyncStuff());
    });
} else {
    console.log(doStuffSync());
}

function doStuffSync(){
    parentPort.postMessage({fn: 'doStuff'});
    let message;
    while (!message) {
        message = receiveMessageOnPort(parentPort)
    }
    return message;
}

function doAsyncStuff(){
    return new Promise((resolve) => setTimeout(() => resolve("A test"), 1000));
}

Answer 12

This ability of promises includes two key features of synchronous operations as follows (or then() accepts two callbacks). When you get the result, call resolve() and pass the final result. In case of error, call reject().

The idea is that the result is passed through the chain of .then() handlers.

const synchronize = (() => {
    let chain = Promise.resolve()
    return async (promise) => {
        return chain = chain.then(promise)
    }
})()

Answer 13

I wondered the same thing and noticed that the currently best answer contains the right idea in my mind for most use cases, but forgets to mention a couple of things. When using a global variable to lock execution, we're talking about Semaphores, and there are some packages which implement those (my recommendation: async-sema). I think this makes it a little simpler and cleaner.

import { Sema } from 'async-sema'

const sema = new Sema(1) // allow only one concurrent process

async function doSomething() {

  var data;
   
  await sema.acquire();
  // only one process gets inside here
  data = await myAsynchronousCall(param1);
  sema.release();

  return data;
}

The advantage is obviously that the rest of your program can still do other things asynchronously, only the single block is kind of forced to be synchronously. Disadvantage is that you have to be careful what and where to lock, try/catch/finally possible errors, etc.

Answer 14

In Node.js is possible to do it using the deasync

• npm install --save deasync

Code example:

function asyncToSync(promise) {
  let done = false;
  let result;

  promise.then((r) => {
    result = r;
    done = true;
  }).catch((e) => {
    console.error(e);
    done = true;
  });

  require('deasync').loopWhile(() => !done);

  return result;
}

And using the function:

const value = asyncToSync(Promise.resolve(3));
console.log(value) // 3

Answer 15

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);

Answer 16

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;
}