What are Promises in JavaScript?

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A Promise is an object that represents the eventual result of an asynchronous operation. It acts as a placeholder for a value that is not yet available, letting you attach callbacks that run when the value finally arrives — or when the operation fails. Promises replaced the old callback-based style and became the foundation for async / await . The three states Pending — initial state, neither fulfilled nor rejected. Fulfilled — the operation succeeded with a value. Rejected — the operation failed with a reason (usually an Error). A promise transitions from pending to either fulfilled or rejected exactly once. Subsequent attempts to change its state are ignored, which is why promises are called settled once resolved or rejected. Creating a promise const p = new Promise((resolve, reject) => { setTimeout(() => resolve("done"), 500); }); p.then((value) => console.log(value)) // "done" .catch((err) => console.error(err)) .finally(() => console.log("cleanup")); Chaining .then() always returns a new promise. Returning a value inside a .then() passes it to the next link; returning a promise waits for it. A thrown error jumps to the nearest .catch() . fetch("/api/user") .then((res) => res.json()) .then((user) => fetch(`/api/posts/${user.id}`)) .then((res) => res.json()) .catch(console.error); Combinators Promise.all — resolves when every input promise resolves; rejects on the first rejection. Promise.allSettled — waits for all to settle, regardless of outcome. Promise.race — settles with the first promise to settle, fulfilled or rejected. Promise.any — resolves with the first fulfillment; rejects only if all reject. Relationship to async/await async functions always return a promise; await pauses execution until the promise settles. Under the hood it is still promise plumbing, just written to read like synchronous code. Error handling Always end a chain with .catch() or wrap await in try/catch . An unhandled rejection will trigger the unhandledrejection event and, in Node, can crash the process in newer versions. Know all three states (pending, fulfilled, rejected). Show chaining (callback hell solution). Know all four static methods: all (parallel, fail-fast), allSettled (all complete), race (first settled), any (first fulfilled). Promise.allSettled is often missed.