試験JavaScript-Developer-I-JPN トピック1 問題83 スレッド

* Synchronous execution order JavaScript executes code in a single thread, following a well-defined order:
* All synchronous code runs first, line by line.
* Asynchronous callbacks (like those scheduled with setInterval or setTimeout) are placed into the event queue and executed only after the current call stack is empty.
Let's follow the code step by step:
* Line 01:
* let total = 10;
total is initialized with the value 10.
* Line 02:
* const interval = setInterval(() = > {
* total++;
* clearInterval(interval);
* total++;
* }, 0);
setInterval schedules the callback function to run repeatedly after a delay of at least 0 milliseconds, but it does not run immediately. The callback is added to the timer queue and will be invoked after the current synchronous script finishes and the event loop gets to process timer callbacks.
At this point, interval holds the interval ID, but the callback has not executed yet.
* Line 07:
* total++;
This is still synchronous, so it runs before any scheduled callbacks.
total was 10, now it becomes 11.
* Line 08:
* console.log(total);
At this moment, the interval callback has still not run (because the event loop has not yet processed the timer queue).
So total is 11, and console.log(total); outputs 11.
Therefore, the value printed at line 08 is 11, making option A correct.
* What happens after the log (for understanding, not affecting the answer) After the main script finishes, the event loop processes the timer callback for setInterval:
Callback:
() = > {
total++; // from 11 to 12
clearInterval(interval); // cancels further executions
total++; // from 12 to 13
}
So eventually total becomes 13, but this happens after console.log(total) has already executed. Since the question asks specifically for the output at line 08, the asynchronous updates do not change that line's output.
* Why other options are incorrect
* Option B (12): This would require the callback to run before the log, which does not happen because asynchronous callbacks are queued and executed after the current stack finishes.
* Option C (10): Ignores the total++ on line 07.
* Option D (13): This is the final value after the callback finishes, but it occurs after the console.log line executes, not at the time line 08 runs.
JavaScript knowledge references (descriptive, no links):
* JavaScript is single-threaded and uses an event loop with a call stack and task queues.
* setInterval schedules callbacks to run asynchronously after a minimum delay; the callback never runs before the current synchronous code finishes.
* Synchronous statements like total++ on line 07 execute before any queued interval callback.