Producer

when new Promise is created, the executor runs automatically

const promise = new Promise((resolve, reject) ⇒ {

console.log('doing something...');
	setTimeout(() => {
		//resolve('ellie');
		reject(new Error('no network'))
	}, 2000);
});

Consumers: then, catch, finally

promise
.then((value) => {
	console.log(value);
})
.catch(error => {
	console.log(error);
})
.finally(() => {
	console.log('finally')
});

Promise chaining

const fetchNumber = new Promise((resolve, reject) => {
	setTimeout(()=> resolve(1), 1000);
});

fetchNumber
.then(num => num * 2)
.then(num => num * 3)
.then(num => {
	return new Promise((resolve, reject) => {
		setTimeout(()=> resolve(num -1), 1000);
	});
})
.then(num => console.log(num));

Error Handling

const getHen = () =>
	new Promise((resolve, reject) => {
		setTimeout(() => resolve('닭'),1000);
	});
const getEgg = hen =>
	new Promise((resolve, reject) => {
		//setTimeout(() => resolve(`${hen} => 계란`), 1000);
		setTimeout(() => reject(new Error(`error! ${hen} => 계란`)), 1000);
	});
const cook = egg =>
	new Promise((resolve, reject) => {
		setTimeout(() => resolve(`${egg} => 후라이`), 1000);
});

getHen()
.then(getEgg) //이렇게로도 가능 아래와 같음
.catch(error => {
	return '빵';
})
.then(egg => cook(egg))
.then(meal => console.log(meal));
.catch(console.log);

getHen().then(getEgg).then(cook).then(console.log);