JavaScript async/await playground

async-await-playground.js

// utility
function stamp_time(text){
  console.log(new Date().toLocaleTimeString(), text);
}

// mock process
function make_double(v){
  return new Promise(function(res, rej){
    setTimeout(x => {
      res(v*2); // double value
    }, v*1000); // take v sec to process
  }).then( out => {
    stamp_time(`Job ${v} done`);
    return out;
  });
}

async function async_way(){
  stamp_time(`Job start`);
  /*
  var a = await make_double(1); // +1 sec
  var b = await make_double(2); // +2 sec
  return a + b;                 // total 3 sec
  */
  var a = make_double(1);
  var b = make_double(2);
  return await a + await b;     // start a, b same time
                                // a done in 1 sec wait b
                                // b done in 2 sec
                                // total 2 sec
}

function promise_all_way(){
  stamp_time(`Job start`);
  var a = make_double(1);
  var b = make_double(2);
  return Promise.all([a, b]).then((o) => {
    return o[0] + o[1];
  });
}

// test
function test1(){
  console.log(`===== async way =====`);
  async_way().then(o => {
    console.log('Output => ' + o);
  }).then(x => {
    console.log(`===== Promise.all way =====`);
    promise_all_way().then(o => {
      console.log('Output => ' + o);
      console.log('===========================');
    });
  });
}

async function test2(){
  console.log(`===== async way =====`);
  var o = await async_way();
  console.log('Output => ' + o);
  //
  console.log(`===== Promise.all way =====`);
  var o = await promise_all_way();
  console.log('Output => ' + o);
  //
  console.log('===========================');
}

// run test
// test1();
test2();