A Promise represents the eventual completion (or failure) of an asynchronous operation and its resulting value.
Promise States
A Promise is always in one of three mutually exclusive states:
- pending: Initial state, neither fulfilled nor rejected.
- fulfilled: Operation completed successfully (
resolve(value)). - rejected: Operation failed (
reject(error)).
Constructor & Basic Chaining
const loadUser = (id) => {
return new Promise((resolve, reject) => {
fetch(`/api/users/${id}`)
.then((res) => {
if (!res.ok) throw new Error(`HTTP ${res.status}`);
return res.json();
})
.then((data) => resolve(data))
.catch((err) => reject(err));
});
};
loadUser(42)
.then((user) => console.log('Loaded:', user.name))
.catch((err) => console.error('Error:', err.message))
.finally(() => console.log('Request finished'));
Static Resolvers
// Immediately fulfilled promise
const p1 = Promise.resolve('Immediate value');
// Immediately rejected promise
const p2 = Promise.reject(new Error('Immediate failure'));
Promise Combinators
| Method | Behavior | Fails When |
|---|---|---|
Promise.all([p1, p2]) |
Waits for all to fulfill | Any promise rejects (short-circuits) |
Promise.allSettled([p1, p2]) |
Waits for all to finish (never rejects) | Never rejects |
Promise.race([p1, p2]) |
Settles with first settled promise | First settled promise rejects |
Promise.any([p1, p2]) |
Fulfills with first fulfilled promise | All promises reject (AggregateError) |
// Promise.all: Wait for multiple independent requests
const [users, posts] = await Promise.all([
fetchUsers(),
fetchPosts()
]);
// Promise.allSettled: Inspect statuses of all requests
const results = await Promise.allSettled([reqA(), reqB()]);
results.forEach((res) => {
if (res.status === 'fulfilled') console.log('Value:', res.value);
else console.error('Reason:', res.reason);
});
Async / Await Equivalent
async function getUserData(id) {
try {
const res = await fetch(`/api/users/${id}`);
const data = await res.json();
return data;
} catch (err) {
console.error('Failed to retrieve user:', err);
throw err;
}
}