async_test.js

//https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Statements/async_function #20190526_1018

var timestamp = () => (new Date().getSeconds() + ' ' + new Date().getMilliseconds());
var resolveAfter2Seconds = function() {
  console.log("starting slow promise " + timestamp());
  return new Promise(resolve => {
    setTimeout(function() {
      resolve("slow " + timestamp());
      console.log("slow promise is done " + timestamp());
    }, 5000);
  });
};

var resolveAfter1Second = function() {
  console.log("starting fast promise " + timestamp());
  return new Promise(resolve => {
    setTimeout(function() {
      resolve("fast " + timestamp());
      console.log("fast promise is done " + timestamp());
    }, 1000);
  });
};

var sequentialStart = async function() {
  console.log('==SEQUENTIAL START==');

  // 1. Execution gets here almost instantly
  const slow = await resolveAfter2Seconds();
  console.log(slow); // 2. this runs 2 seconds after 1.

  const fast = await resolveAfter1Second();
  console.log(fast); // 3. this runs 3 seconds after 1.
}

var concurrentStart = async function() {
  console.log('==CONCURRENT START with await==');
  const slow = resolveAfter2Seconds(); // starts timer immediately
  const fast = resolveAfter1Second(); // starts timer immediately

  //Execution gets here almost instantly
  console.log('Execution 1.');
  console.log(await slow); // 2. this runs 2 seconds after 1.
  console.log('Execution 2.');
  console.log(await fast); // 3. this runs 2 seconds after 1., immediately after 2., since fast is already resolved
}

var concurrentPromise = function() {
  console.log('==CONCURRENT START with Promise.all==');
  return Promise.all([resolveAfter2Seconds(), resolveAfter1Second()]).then((messages) => {
    console.log(messages[0]); // slow
    console.log(messages[1]); // fast
  });
}

var parallel = async function() {
  console.log('==PARALLEL with await Promise.all==');
  
  // Start 2 "jobs" in parallel and wait for both of them to complete
  await Promise.all([
      (async()=>console.log(await resolveAfter2Seconds()))(),
      (async()=>console.log(await resolveAfter1Second()))()
  ]);
}

// This function does not handle errors. See warning below!
var parallelPromise = function() {
  console.log('==PARALLEL with Promise.then==');
  resolveAfter2Seconds().then((message)=>console.log(message));
  resolveAfter1Second().then((message)=>console.log(message));
}