Since I started using ServiceNow a few years ago, I was always missing concurrency features like client-side Promises, or C#-like Task - even a basic setTimeout would be sufficient.

A can proudly say, that this search is finally over - introducing "Lib Async":

gs.info("This will execute first");
x_424426_async.setTimeout(function () {
  gs.info("This will execute third (after a second wait time)");
}, 1000);
gs.info("This will execute second");

Background Script Output:

Log Output:

More Complex Showcase with Promises:

var firstPromise = new x_424426_async.snPromise(function (resolve, reject) {
  gs.info("first promise");
  x_424426_async.setTimeout(function () {
    resolve("hello");
  }, 10000);
});

var secondPromise = new x_424426_async.snPromise(function (resolve, reject) {
  gs.info("second promise");
  resolve("world");
});

x_424426_async.snPromise.all([firstPromise, secondPromise]).then(function (results) {
  gs.info("promise result: " + results.join(","));
})

Background Script Output:

Note that we do not get a result right away because the first promise is resolved after ten second

System Log Output:

Notice the ten second difference between the outputs

Source: https://github.com/kr4uzi/ServiceNow-LibAsync