promise가 체인이된다보면 길어지기에

동기식으로 작성하는것처럼 보여주는 장점이 있다.

clear style of using promise

1. async

   async function fetchUser() {
   	// do network request in 10 secs....
   
   	return 'ellie';
   }
   
   const user = fetchUser();
   user.then(console.log);
   console.log(user);
   

2. await

   function delay(ms) {
   	return new Promise(resolve => setTimeout(resolve, ms));
   }
   
   async function getApple() {
   	await delay(2000);
   	throw 'error';
   	return '사과';
   }
   
   async function getBanana() {
   	await delay(1000);
   	return '바나나';
   }
   
   async function pickFruits() {
   	const applePromise = getApple();
   	const bananaPromise = getBanana();
   
   	const apple = await applePromise;
   	const banana = await bananaPromise;
   	return `${apple} + ${banana}`;
   }
   
   pickFruits().then(console.log);
   

3. useful Promise APIs

   function pickAllFruits() {
   	return Promise.all([getApple(), getBanana()])
   	.then(fruits => fruits.join('+'));
   }
   
   pickAllFruits().then(console.log);
   
   function pickOnlyOne() {
   	return Promise.race([getApple(), getBanana()]);
   }
   
   pickOnlyOne().then(console.log);