es6 해독 - async 함수

      function timeout(ms) {
          return new Promise((resolve) => {
              setTimeout(resolve, ms);
          });
       }

      async function asyncPrint(value, ms) {
          await timeout(ms);
          console.log(value);
      }

      asyncPrint('hello world', 50);

     async  Promise  ， await 。 ， 
 ：
     async function timeout(ms) {
          await new Promise((resolve) => {
              setTimeout(resolve, ms);
          });
      }

      async function asyncPrint(value, ms) {
          await timeout(ms);
          console.log(value);
      }

    asyncPrint('hello world', 50);

async 함수의 실현 원리

async  ，  Generator  ， 。

async function fn(args) {
  // ...
}

//  

function fn(args) {
  return spawn(function* () {
    // ...
  });
}

function spawn(genF) {
  return new Promise(function(resolve, reject) {
    const gen = genF();
    function step(nextF) {
      let next;
      try {
        next = nextF();
      } catch(e) {
        return reject(e);
      }
      if(next.done) {
        return resolve(next.value);
      }
      Promise.resolve(next.value).then(function(v) {
        step(function() { return gen.next(v); });
      }, function(e) {
        step(function() { return gen.throw(e); });
      });
    }
    step(function() { return gen.next(undefined); });
  });
}

비동기적으로 두루 다니다.

const asyncIterable = createAsyncIterable(['a', 'b']);
const asyncIterator = asyncIterable[Symbol.asyncIterator]();

asyncIterator
.next()
.then(iterResult1 => {
  console.log(iterResult1); // { value: 'a', done: false }
  return asyncIterator.next();
})
.then(iterResult2 => {
  console.log(iterResult2); // { value: 'b', done: false }
  return asyncIterator.next();
})
.then(iterResult3 => {
  console.log(iterResult3); // { value: undefined, done: true }
});

async promise await 실행 순서

async function async1(){
  console.log('async1 start')
  await async2()
  console.log('async1 end')
}
async function async2(){
  console.log('async2')
}
console.log('script start')
setTimeout(function(){
  console.log('setTimeout') 
},0)  
async1();
new Promise(function(resolve){
  console.log('promise1')
  resolve();
}).then(function(){
  console.log('promise2')
})
console.log('script end')

script start
async1 start
async2
promise1
script end
promise2
async1 end
setTimeout