promise.js

'use strict';

// Promise is a JavaScript object for asynchronous operation
// State: pending -> fulfilled or rejected
// Producer vs Consumer

// 1. Producer
// when new Promise is created, the executer runs automatically
const promise = new Promise((resolve, reject) => {
    // doing some heavy work (network, read files)
    console.log('doing something...');
    setTimeout(() => {
        // resolve('sill');
        reject(new Error('no network'));
    }, 2000);
});

// 2. Consumer: then, catch, finally
promise // 
    .then((value) => {
        console.log(value);
    })
    .catch(error => {
        console.log(error);
    })
    .finally(() => {
        console.log('finally')
    });

// 3. Promise chaning
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));

    // 4. Error Handling
const getHen = () =>
    new Promise((resolve, reject) => {
        setTimeout(() => resolve(`닭`), 1000);
    });
const getEgg = hen => 
    new Promise((resolve, reject) => {
        setTimeout(() => reject(new Error(`error! ${hen} => 달걀`)), 1000); // resolve(`${hen} => 달걀`), 1000);
    });
const cook = egg => 
    new Promise((resolve, reject) => {
        setTimeout(() => resolve(`${egg} => 달걀 프라이`), 1000);
    });    

getHen()
 .then(getEgg) // hen => getEgg(hen)
 .catch(error => { // 달걀을 받아오는 함수의 에러를 처리하고 싶을 때는 그 다음 라인에 catch를 작성!
     return '빵';
 })
 .then(cook) // (egg => cook(egg)
 .then(console.log) // meal => console.log(meal)
 .catch(console.log);