JavaScript Tutorial
Promises represent future values from asynchronous operations and are the foundation of modern async JavaScript patterns.
Why this matters
Without strong promise handling, async code becomes fragile and hard to maintain.
Every promise starts pending and settles once as fulfilled or rejected.
then returns a new promise, enabling linear chains instead of nested callbacks.
Errors thrown in then blocks travel to nearest catch, enabling centralized handling.
Use Promise.all when every task must succeed.
Use Promise.allSettled when you need results from all tasks regardless of individual failures.
Use Promise.race for first completion and Promise.any for first success.
function getUser(id) {
return fetch("https://jsonplaceholder.typicode.com/users/" + id)
.then((r) => r.json());
}
getUser(1)
.then((user) => user.email)
.then((email) => console.log(email))
.catch((error) => console.error(error.message));then chain keeps asynchronous flow linear and readable.
const tasks = [
Promise.resolve("A success"),
Promise.reject(new Error("B failed")),
Promise.resolve("C success"),
];
Promise.allSettled(tasks).then((result) => console.log(result));Use allSettled when partial completion is acceptable.
function wait(ms) {
return new Promise((resolve) => {
setTimeout(() => resolve("done"), ms);
});
}
wait(300).then(console.log);Wrapping callback APIs into promises modernizes async composition.
Return value/promise inside then when next step depends on it.
Always handle rejection with catch or try/catch via async/await.
Chain linearly or switch to async/await for readability.
No. Promise state is immutable once settled.
Use it when all operations are required and a single failure should fail the complete operation.
It runs cleanup logic regardless of success or failure.
No. async/await is syntax built on top of promises.