abbr abbr - 1 year ago 241
Javascript Question

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

Suppose you maintain a library that exposes a function

. Your users call it to get actual data:

var output = getData();

Under the hood data is saved in a file so you implemented
using Node.js built-in
. It's obvious both
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,
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);
console.log('ok... ' + new Date);
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.

Answer Source

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;
      ret = "hello";
  while(ret === undefined) {
  return ret;    

var output = AnticipatedSyncFunction();
//expected: output=hello (after waiting for 3 sec)
//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.)

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