Asynchronous code and waiting

Some operations take time, like fetching data from a server, reading a file or waiting for a reply. JavaScript doesn't stop and wait for them to finish: it keeps running the rest of the code and comes back to the result when it's ready.

A Promise is a value that will arrive later, and the keyword await means "wait here until the value arrives". Any function that uses await must be declared with async.

That's why an interface needs a loading state the user can see between sending the request and getting the result. Otherwise they'll think something broke.