What is a Promise?
A Promise represents a value that may be available now, later, or never. It has three states: pending, fulfilled, or rejected.
const promise = new Promise((resolve, reject) => {
let success = true;
if (success) {
resolve("Operation succeeded");
} else {
reject("Operation failed");
}
});then, catch, finally
Use .then() for success, .catch() for errors, and .finally() for cleanup after either.
fetch("https://api.example.com/data")
.then(response => response.json())
.then(data => console.log(data))
.catch(error => console.error("Error:", error))
.finally(() => console.log("Request complete"));Promise.all
Run multiple promises in parallel and wait for all to complete. Rejects immediately if any promise rejects.
const p1 = Promise.resolve(10);
const p2 = Promise.resolve(20);
const p3 = Promise.resolve(30);
Promise.all([p1, p2, p3]).then(values => {
console.log(values); // [10, 20, 30]
});