Is google apps script synchronous?

While Google Apps Script implements a subset of ECMAScript 5, there's nothing forcing it to be asynchronous.

While it is true that JavaScript's major power is its asynchronous nature, the Google developers appear to have given that up in favor of a simpler, more straightforward API.

UrlFetchApp methods are synchronous. They return an HttpResponse object, and they do not take a callback. That, apparently, is an API decision.

I'm guessing from Google's point of view, that parallel processing two tasks (for example, that simply had Utilities.sleep(3000)) would require multiple threads to run in the server cpu, which may not be manageable and may be easy to abuse.

Whereas parallel processing on the client or other companies server (e.g., Node.js) is up to that developer or user. (If they don't scale well it's not Google's problem)

However there are some things that use parallelism

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UrlFetchApp.fetchAll

UrlFetchApp.fetchAll() will asynchronously fetch many urls. Although this is not what you're truly looking for, fetching urls is a major reason to seek parallel processing.

I'm guessing Google is reasoning this is ok since fetchall is using a web client and its own resources are already protected by quota.

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FirebaseApp getAllData

Firebase I have found is very fast compared to using a spreadsheet for data storage. You can get many things from the database at once using FirebaseApp's getAllData:

function myFunction() {
  var baseUrl = "https://samplechat.firebaseio-demo.com/";
  var secret = "rl42VVo4jRX8dND7G2xoI";
  var database = FirebaseApp.getDatabaseByUrl(baseUrl, secret);

  // paths of 3 different user profiles
  var path1 = "users/jack";
  var path2 = "users/bob";
  var path3 = "users/jeane";

  Logger.log(database.getAllData([path1, path2, path3]));
}

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HtmlService - IFrame mode

HtmlService - IFrame mode allows full multi-tasking by going out to client script where promises are truly supported and making parallel calls back into the server. You can initiate this process from the server, but since all the parallel tasks' results are returned in the client, it's unclear how to get them back to the server. You could make another server call and send the results, but I'm thinking the goal would be to get them back to the script that called HtmlService in the first place, unless you go with a beginRequest and endRequest type architecture.

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tanaikech/RunAll

This is a library for running the concurrent processing using only native Google Apps Script (GAS). This library claims full support via a RunAll.Do(workers) method.

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I'll update my answer if I find any other tricks.

Please note that this hasn't really changed since the introduction of V8 runtime for google app scripts.

While we are on the latest and greatest version of ECMAScript, running a Promise.all(func1, func2) I can see that the code in the second function is not executed until the first one is completed.

Also, there is still no setTimeout() global function to use in order to branch the order of execution. Nor do any of the APIs provide callback functions or promise-like results. Seems like the going philosophy in GAS is to make everything synchronous.