Parallel Serial problem in async/await

there are two functions 

var a = function () {
  return new Promise((resolve, reject)=>{
    setTimeout(()=>{
      resolve("a")
    }, 3000)
  })
}
var b = function () {
  return new Promise((resolve, reject)=>{
    setTimeout(()=>{
      resolve("b")
    }, 2000)
  })
}

if executed in this way 

;(async()=>{
  console.time("test")
  var aa = await a()
  var bb = await b()
  console.log(`$(aa)-${bb}`)
  console.timeEnd("test")
})()

the result is 

$(aa)-b
test: 5010.7548828125ms  //

but if you execute

like this 
;(async()=>{
  console.time("test")
  var promiseA = a()
  var promiseB = b()
  var aa = await promiseA
  var bb = await promiseB
  console.log(`$(aa)-${bb}`)
  console.timeEnd("test")
})()

the result is 

$(aa)-b
test: 3001.277099609375ms  //

Why is this?

Javascript
Mar.10,2021

in fact, this kind of grammatical candy does not quite understand the principle. It is recommended that you read the code after converting it to ES5 through babel, which is easy to understand.
such as the above code body, after conversion:

Serial: 

while (1) {
  switch (_context.prev = _context.next) {
    case 0:
      console.time('test');
      _context.next = 3;
      return a(); //  a

    case 3:
      aa = _context.sent; // a 
      _context.next = 6;
      return b(); 

    case 6:
      bb = _context.sent;

      console.log('$(aa)-' + bb);
      console.timeEnd('test');

    case 9:
    case 'end':
      return _context.stop();
  }
}

parallel: 

_asyncToGenerator( /*-sharp__PURE__*/regeneratorRuntime.mark(function _callee() {
  var promiseA, promiseB, aa, bb;
  return regeneratorRuntime.wrap(function _callee$(_context) {
    while (1) {
      switch (_context.prev = _context.next) {
        case 0:
          console.time('test');
          //  a,bajax
          
          // await promiseA, await promiseB
          promiseA = a(); // a 
          promiseB = b(); // b
          
          _context.next = 5; // 5 
          return promiseA;

        case 5:
          aa = _context.sent;
          _context.next = 8;
          return promiseB;

        case 8:
          bb = _context.sent;

          console.log('$(aa)-' + bb);
          console.timeEnd('test');

        case 11:
        case 'end':
          return _context.stop();
      }
    }
  }, _callee, undefined);
}))();

because you call
at the same time, the original logic is: a call to wait a and then b execute wait b two seconds
now a calls waiting a while b has been executed in two seconds, so to b 

var promiseA = a()
var promiseB = b()

try this 

;(async () => {
    console.time('test')
    var promiseA = a()
    var aa = await promiseA
    var promiseB = b()
    var bb = await promiseB
    console.log(`$(aa)-${bb}`)
    console.timeEnd('test')
})()

the first case is serial, and the second case is that the a () of promiseA and the b () of promiseB are executed at the same time, that is, the execution of promiseB does not wait until the execution of promiseA, that is, parallel, so the time is shorter than that of the first case

.

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