Explain Promises and async/await in JavaScript.

Promises represent eventual completion (or failure) of async operation. States: pending, fulfilled, rejected. then() for success, catch() for errors, finally() always runs. Promise.all() waits for all, Promise.race() for first. async/await (ES2017) is syntactic sugar: async function returns Promise, await pauses until Promise resolves. Error handling: try/catch with await vs .catch(). async/await makes async code look synchronous. Combine: await Promise.all([...]) for concurrent operations. Never use await in loops for parallel operations; use Promise.all instead.