Let’s break down the key differences between your three setInterval examples, since the core issue here is how setInterval interacts with asynchronous operations in its callback.

First, a critical baseline: setInterval does NOT wait for its callback to finish executing. It will attempt to run the callback every X milliseconds, starting from when the interval was first scheduled. If the previous callback is still running, most environments (browsers, Node.js) will skip that scheduled trigger instead of queueing it up.

Now let’s walk through each case:

1. setInterval(async()=>{ fetchFile() }, 1000) #

• The callback is an async function, but we’re not awaiting fetchFile(). That means the callback itself will finish executing immediately after starting fetchFile—it doesn’t wait for the async operation to complete.
• setInterval will stick strictly to its 1000ms schedule: every second, it fires the callback, which starts a new fetchFile instance. If fetchFile takes longer than 1 second to run (say, 1.5s), you’ll end up with overlapping fetchFile operations running in parallel.
• Error handling gotcha: If fetchFile throws an error (or rejects its promise), there’s no await to capture that rejection. This will result in an unhandled Promise rejection, which might trigger warnings in your console or even terminate your Node.js process in some cases.

2. setInterval(async()=>{ await fetchFile() }, 1000) #

• Here, we’re awaiting fetchFile() inside the async callback. Now the callback’s execution time is tied directly to how long fetchFile takes to complete.
• Let’s say fetchFile takes 1.5s. The first callback runs at 0ms, starts fetchFile, and waits 1.5s for it to finish. Meanwhile, setInterval tries to fire again at 1000ms—but since the first callback is still running (it’s awaiting), this second trigger is skipped. The next trigger will happen at 2000ms, right after the first callback finishes.
• This means you won’t get overlapping fetchFile operations, but your actual interval between fetchFile runs could be longer than 1000ms if fetchFile takes longer than the interval.
• Error note: While await will catch the rejection if fetchFile fails, you still need a try/catch block inside the callback to handle the error properly—otherwise, it’ll still become an unhandled rejection.

3. setInterval(()=>{ fetchFile() }, 1000) #

• This is almost identical to the first case. The callback is a regular synchronous function, but when you call fetchFile() (an async function), it returns a Promise and runs in the background immediately. The callback itself finishes right away, just like in the first example.
• setInterval will still fire every 1000ms, starting new fetchFile instances regardless of whether previous ones are done. The only tiny difference is that the callback isn’t marked as async—but since we’re not handling the Promise from fetchFile, this doesn’t change the behavior at all.
• Same error risk as the first case: unhandled rejections if fetchFile fails.

Key Takeaways #

• If you don’t await the async function in the callback: setInterval runs on strict schedule, potentially causing overlapping async operations and unhandled rejections.
• If you do await the async function: setInterval will skip triggers when the callback is still running, preventing overlaps but making the actual interval between async operations variable.
• For more control over periodic async tasks (especially if you want consistent gaps between the end of one task and the start of the next), consider using a recursive setTimeout instead of setInterval—it lets you explicitly wait for the async operation to finish before scheduling the next run.

内容的提问来源于stack exchange，提问作者Ruoxuan.Wang