One search. Connected ideas.

Output prediction requires following the order in which synchronous code and queued callbacks can execute. This example combines the current script, a promise microtask and a timer callback. A zero-delay timer becomes eligible later; it does not interrupt the running script.

The var loop creates one shared function-scoped binding for i. Every callback closes over that same binding, and the loop has already set it to 3 before the timer callbacks run. Using let in the loop header creates a separate binding for each iteration.