Demonstration of handling asynchronous functions in loops in typescript using async/await

typescriptAsyncLoops.ts

// Demonstration of different ways of running asynchronous functions in loops
import util = require("util");

const arr = ["a", "b", "c", "d"];
const setTimeoutPromise = util.promisify(setTimeout);

// Asynchronously wait a very short amount of time and then print our value
async function waitAndPrint(s: string): Promise<void> {
  await setTimeoutPromise(Math.random() * 100);
  console.log(s);
}

// This example uses foreach but doesn't have any means of waiting for the
// functions within the foreach to complete, so "done" appears before the
// results are printed
async function demoForEach(): Promise<void> {
  arr.forEach(async a => waitAndPrint("forEach " + a));

  // The below - adding "await" - doesn't help as forEach is still
  // calling each function asynchronously
  // arr.forEach(async a => await waitAndPrint("forEach " + a));

  console.log("done");

  // We know this doesn't actually "wait" properly, so cheat here to
  // avoid polluting the output of the followup functions
  await setTimeoutPromise(1000);
}

// Use a map and Promise.all to execute each function simultaneously
// but wait for them all to complete before pritning "done" and returning
async function demoPromiseAll(): Promise<void> {
  const promises = arr.map(async a => waitAndPrint("PromiseAll " + a));
  await Promise.all(promises);

  console.log("done");
}

// Use a basic for loop and embed "await" within the loop to execute
// each function one by one, waiting each time.
async function demoSyncFor(): Promise<void> {
  for (const a of arr) {
    await waitAndPrint("syncFor " + a);
  }

  console.log("done");
}

(async () => {
  // Comments show example output of each example

  // demoForEach
  // done
  // forEach a
  // forEach c
  // forEach d
  // forEach b
  console.log("\ndemoForEach");
  await demoForEach();

  // demoSyncFor
  // syncFor a
  // syncFor b
  // syncFor c
  // syncFor d
  // done
  console.log("\ndemoSyncFor");
  await demoSyncFor();

  // demoPromiseAll
  // PromiseAll d
  // PromiseAll c
  // PromiseAll a
  // PromiseAll b
  // done
  console.log("\ndemoPromiseAll");
  await demoPromiseAll();
})();