async/await

또 다른 비동기를 다루는 방식
async function 함수이름(){}
또는
const 함수이름 = async () => {}

// index.js
function sleep(ms){
  return new Promise(resolve => setTimeout(resolve,ms));
}

// async/await
async function process(){
  console.log('안녕하세요!');
  await sleep(1000);
  console.log('반갑습니다.');
  return true;
}

process().then(value => {
  console.log(value);
}); 
// 안녕하세요
// 반갑습니다
// true

// error catch
async function makeError(){
  await sleep(1000);
  const error = new Error();
  throw error;
}

async function process(){
  try {
    await makeError();
  } catch(e) {
    console.log(e);
  }
}

process(); 
// 1초 뒤에 Error 발생


// 바로 실행
(async function process(){
  try {
    await makeError();
  } catch(e) {
    console.log(e);
  } finally {
    console.log('end');
  }
})();

async function에서 return되는 값을 Promise.resolve 함수로 감싸서 리턴

function p(ms){
  return new Promise((resolve,reject)=>{
    setTimeout(()=>{
      resolve(ms);
      // reject(new Error('reason');
    }, ms);
  });
}

async function asyncP(){
  const ms = await p(1000);
  return "Mark: " + ms;
}

(async function main(){
  try{
    const name = await asyncP();
    console.log(name);
  } catch(error){
    console.log(error);
  })();
  
  // resolve → Mark : 1000ms
  // reject → Error 

연속된 promise와 async await

function p(ms){
  return new Promise((resolve,reject)=>{
    setTimeout(()=>{
      resolve(ms);
      // reject(new Error('reason');
    }, ms);
  });
}

// Promise
p(1000).then(()=>p(1000))
	   .then(()=>p(1000))
       .then(()=>{
  			console.log('3000ms 후 실행');
});

// async await
(async function main(){
  await p(1000);
  await p(1000);
  await p(1000);
  console.log('3000ms 후 실행');
})();

Promise.all과 Promise.race

function p(ms){
  return new Promise((resolve,reject)=>{
    setTimeout(()=>{
      resolve(ms);
      // reject(new Error('reason');
    }, ms);
  });
}

// Promise.all
(async function main(){
  const results = await Promise.all([p(1000),p(2000),p(3000)]);
  console.log(results);
})(); // [1000,2000,3000]

// Promise.race
(async function main(){
  const result = await Promise.race([p(1000),p(2000),p(3000)]);
  console.log(result);
})(); // 1000