Promises and async/await

Anything that takes time in the browser, such as a network request or a timer, is asynchronous. Your code starts it and carries on, and the result arrives later. Promises stand for that future result.

Each one starts pending, then is either fulfilled with a value or rejected with an error. Async functions and await let you write this as straight line code: const res = await fetch(url) pauses that one function, not the whole page. Handle failure with try and catch.

Run independent requests together with Promise.all, instead of awaiting them one after another. The two classic traps are forgetting await, so you get a promise instead of data, and having no catch, so failures vanish silently.