11. Promise
2020. 11. 15. 15:22ㆍJavaScript & TypeScript
- Promise is a JavaScript object for asynchronous operation
- state: pending -> fulfilled or rejected
- Producer vs Consumer
1. Producer
- when new Promise is created, the executor runs automatically
const promise = new Promise((resolve, reject) => {
// doing some heavy work (network, read files)
console.log('doing something....');
setTimeout(() => {
// resolve('ellie');
reject(new Error('no network'));
}, 2000);
});
2. Consumers: 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)); // 5
4. Error Handling
const getHen = () =>
new Promise((resolve, reject) => {
setTimeout(() => resolve('HEN'), 1000);
});
const getEgg = hen =>
new Promise((resolve, reject) => {
setTimeout(() => reject(new Error(`error! ${hen} => EGG`)), 1000);
});
const cook = egg =>
new Promise((resolve, reject) => {
setTimeout(() => resolve((`${egg} => COOK`), 1000));
});
getHen()
.then(getEgg)
.then(cook)
.then(console.log)
.catch(console.log);
// error! HEN => EGG
5. Callback Promise
class UserStorage {
loginUser(id, password) {
return new Promise ((resolve, reject) => {
setTimeout(() => {
if (
(id === 'ellie' && password === 'dream') ||
(id === 'coder' && password === 'academy')
) {
resolve(id);
} else {
reject(new Error('not found'));
}
}, 2000);
});
}
getRoles(user) {
return new Promise ((resolve, reject) => {
setTimeout(() => {
if (user === 'ellie') {
resolve({ name: 'ellie', role: 'admin' });
} else {
reject(new Error('no access'));
}
}, 1000);
});
}
}
const userStorage = new UserStorage();
const id = prompt('enter your id');
const password = prompt('enter your password');
userStorage
.loginUser(id, password)
.then(userStorage.getRoles)
.then(user => alert(`Hello ${user.name}, you have a ${user.role} roles`))
.catch(console.log);
※출처
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