JavaScript Tutorial
Promises represent future values from asynchronous operations and are the foundation of modern async JavaScript patterns.
Without strong promise handling, async code becomes fragile and hard to maintain.
// Add syntax lines here.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.
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 wait(ms) {
return new Promise((resolve) => {
setTimeout(() => resolve("done"), ms);
});
}
wait(300).then(console.log);Wrapping callback APIs into promises modernizes async composition.
Without
// Without this feature
// ...With
// With this feature
// ...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.
Practice: Write a short example that uses JavaScript Promises: States, Chains, and Reliability and run it in the compiler.
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));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.
Try this example in our JavaScript Compiler and modify it to explore variations.