Is there any kind of test or samples you can provide on this? I don't have a big problem with this as is since it gets rid of a file, but I'd like to understand a bit more about the background and (scale of) impact if possible

Inside wrapAsync, it is implemented using setImmediate. In this case, it will create a new macro task at the end of the JavaScript task queue. The actual I/O operation will only commence when the macro task is executed.

That is to say, when the user initiates a readFile request, it will be inserted at the end of the JavaScript task queue. The actual I/O operation will only start after the current JavaScript task queue has been completely executed.

When using microtasks (Promise), it is equivalent to inserting a high-priority task into the task queue. It will be executed immediately at the end of the current macro task instead of being placed at the end of all macro tasks.

When there is a for loop that executes readFile within the loop and there are a large number of setTimeout calls in the program, the execution speed will become very slow.

test.a.js

function wrap(callback) {
  setImmediate(() => {
    callback();
  });
}
function readFile(file, callback) {
  wrap(() => {
    // nothing
    callback();
  });
}

let timer = null;
function wait() {
  timer = setTimeout(() => {
    const date = Date.now();
    while (Date.now() - date < 100) {
      // nothing
    }
    wait();
  });
}

(async () => {
  wait();
  console.time('A');
  for (let i = 0; i < 1000; i++) {
    await new Promise(resolve => {
      readFile('a.txt', resolve);
    });
  }
  console.timeEnd('A');

  clearTimeout(timer);
})();

test.b.js

function wrap(callback) {
  Promise.resolve().then(() => {
    callback();
  });
}
function readFile(file, callback) {
  wrap(() => {
    // nothing
    callback();
  });
}

let timer = null;
function wait() {
  timer = setTimeout(() => {
    const date = Date.now();
    while (Date.now() - date < 100) {
      // nothing
    }
    wait();
  });
}

(async () => {
  wait();
  console.time('B');
  for (let i = 0; i < 1000; i++) {
    await new Promise(resolve => {
      readFile('a.txt', resolve);
    });
  }
  console.timeEnd('B');

  clearTimeout(timer);
})();

You can try to execute the above two test scripts：

node test.a.js && node test.b.js

Note that do not execute two tests in one file, as some mechanisms in js make the one that is executed first slower

@G-Rath
Do I need to add any more information?
I changed the PR name according to the rules, but it seems that CI didn't recheck? Could you help trigger this manually and give it a try? Or could you tell me how to change the PR name? 😅

🎉 This PR is included in version 4.17.2 🎉

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• npm package (@latest dist-tag)
• GitHub release

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