Is there any built-in way to determine whether a setTimeout callback has already been executed?

There is no built-in way to check whether a setTimeout callback has already executed.

The value returned by setTimeout is just a timer identifier, which can only be used with clearTimeout. It doesn’t expose any state about the timer itself. That would simply add complexity and potential race conditions. You will have to track it yourself:

One way is to structure your code so that the callback itself decides whether to run:

let cancelled = false;
const id = setTimeout(() => {
  if (cancelled) return;
  console.log("Executed");
}, 1000);
// cancel
cancelled = true;
clearTimeout(id);

Or if you want to track the execution explicitly:

let executed = false;
const id = setTimeout(() => {
  executed = true;
  console.log("Executed");
}, 1000);
// later
if (!executed) {
  clearTimeout(id);
}