Why does setTimeout sometimes execute before a synchronous loop finishes in JavaScript?

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Why does setTimeout sometimes execute before a synchronous loop finishes in JavaScript?

Asked on Jan 01, 2026

Answer

In JavaScript, "setTimeout" is asynchronous and runs in the event loop, which allows it to execute after the current call stack is clear, even if a synchronous loop is still running. Here's a simple example to illustrate this behavior:

<!-- BEGIN COPY / PASTE -->
        console.log("Start");

        setTimeout(() => {
            console.log("Timeout");
        }, 0);

        for (let i = 0; i < 1000000000; i++) {
            // Simulating a long-running loop
        }

        console.log("End");
        <!-- END COPY / PASTE -->

Additional Comment:

The "setTimeout" function schedules the "Timeout" message to be logged after 0 milliseconds, but it waits for the current call stack to finish.
The "for" loop is synchronous and blocks the call stack until it completes.
"Start" and "End" are logged before "Timeout" because the loop must finish before the event loop can process the "setTimeout" callback.

✅ Answered with JavaScript best practices.

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Why does setTimeout sometimes execute before a synchronous loop finishes in JavaScript?

Asked on Jan 01, 2026

Answer

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