190 lines
6.8 KiB
Markdown
190 lines
6.8 KiB
Markdown
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# Microtasks and event loop
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Promise handlers `.then`/`.catch`/`.finally` are always asynchronous.
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Even when a Promise is immediately resolved, the code on the lines *below* your `.then`/`.catch`/`.finally` will still execute first.
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Here's the code that demonstrates it:
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```js run
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let promise = Promise.resolve();
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promise.then(() => alert("promise done"));
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alert("code finished"); // this alert shows first
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```
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If you run it, you see `code finished` first, and then `promise done`.
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That's strange, because the promise is definitely done from the beginning.
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Why did the `.then` trigger afterwards? What's going on?
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# Microtasks
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Asynchronous tasks need proper management. For that, the standard specifies an internal queue `PromiseJobs`, more often referred to as "microtask queue" (v8 term).
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As said in the [specification](https://tc39.github.io/ecma262/#sec-jobs-and-job-queues):
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- The queue is first-in-first-out: tasks enqueued first are run first.
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- Execution of a task is initiated only when nothing else is running.
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Or, to say that simply, when a promise is ready, its `.then/catch/finally` handlers are put into the queue. They are not executed yet. JavaScript engine takes a task from the queue and executes it, when it becomes free from the current code.
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That's why "code finished" in the example above shows first.
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Promise handlers always go through that internal queue.
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If there's a chain with multiple `.then/catch/finally`, then every one of them is executed asynchronously. That is, it first gets queued, and executed when the current code is complete and previously queued handlers are finished.
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**What if the order matters for us? How can we make `code finished` work after `promise done`?**
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Easy, just put it into the queue with `.then`:
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```js run
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Promise.resolve()
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.then(() => alert("promise done!"))
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.then(() => alert("code finished"));
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```
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Now the order is as intended.
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## Event loop
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In-browser JavaScript, as well as Node.js, is based on an *event loop*.
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"Event loop" is a process when the engine sleeps and waits for events, then reacts on those and sleeps again.
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Examples of events:
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- `mousemove`, a user moved their mouse.
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- `setTimeout` handler is to be called.
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- an external `<script src="...">` is loaded, ready to be executed.
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- a network operation, e.g. `fetch` is complete.
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- ...etc.
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Things happen -- the engine handles them -- and waits for more to happen (while sleeping and consuming close to zero CPU).
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As you can see, there's also a queue here. A so-called "macrotask queue" (v8 term).
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When an event happens, while the engine is busy, its handling is enqueued.
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For instance, while the engine is busy processing a network `fetch`, a user may move their mouse causing `mousemove`, and `setTimeout` may be due and so on, just as painted on the picture above.
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Events from the macrotask queue are processed on "first come – first served" basis. When the engine browser finishes with `fetch`, it handles `mousemove` event, then `setTimeout` handler, and so on.
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So far, quite simple, right? The engine is busy, so other tasks queue up.
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Now the important stuff.
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**Microtask queue has a higher priority than the macrotask queue.**
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In other words, the engine first executes all microtasks, and then takes a macrotask. Promise handling always has the priority.
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For instance, take a look:
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```js run
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setTimeout(() => alert("timeout"));
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Promise.resolve()
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.then(() => alert("promise"));
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alert("code");
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```
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What's the order?
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1. `code` shows first, because it's a regular synchronous call.
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2. `promise` shows second, because `.then` passes through the microtask queue, and runs after the current code.
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3. `timeout` shows last, because it's a macrotask.
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It may happen that while handling a macrotask, new promises are created.
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Or, vice-versa, a microtask schedules a macrotask (e.g. `setTimeout`).
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For instance, here `.then` schedules a `setTimeout`:
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```js run
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Promise.resolve()
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.then(() => {
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setTimeout(() => alert("timeout"), 0);
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})
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.then(() => {
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alert("promise");
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});
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```
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Naturally, `promise` shows up first, because `setTimeout` macrotask awaits in the less-priority macrotask queue.
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As a logical consequence, macrotasks are handled only when promises give the engine a "free time". So if we have a promise chain that doesn't wait for anything, then things like `setTimeout` or event handlers can never get in the middle.
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## Unhandled rejection
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Remember "unhandled rejection" event from the chapter <info:promise-error-handling>?
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Now, with the understanding of microtasks, we can formalize it.
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**"Unhandled rejection" is when a promise error is not handled at the end of the microtask queue.**
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For instance, consider this code:
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```js run
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let promise = Promise.reject(new Error("Promise Failed!"));
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window.addEventListener('unhandledrejection', event => {
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alert(event.reason); // Promise Failed!
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});
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```
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We create a rejected `promise` and do not handle the error. So we have the "unhandled rejection" event (printed in browser console too).
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We wouldn't have it if we added `.catch`, like this:
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```js run
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let promise = Promise.reject(new Error("Promise Failed!"));
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*!*
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promise.catch(err => alert('caught'));
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*/!*
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// no error, all quiet
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window.addEventListener('unhandledrejection', event => alert(event.reason));
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```
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Now let's say, we'll be catching the error, but after `setTimeout`:
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```js run
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let promise = Promise.reject(new Error("Promise Failed!"));
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*!*
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setTimeout(() => promise.catch(err => alert('caught')));
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*/!*
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// Error: Promise Failed!
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window.addEventListener('unhandledrejection', event => alert(event.reason));
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```
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Now the unhandled rejection appears again. Why? Because `unhandledrejection` triggers when the microtask queue is complete. The engine examines promises and, if any of them is in "rejected" state, then the event is generated.
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In the example, the `.catch` added by `setTimeout` triggers too, of course it does, but later, after `unhandledrejection` has already occurred.
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## Summary
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- Promise handling is always asynchronous, as all promise actions pass through the internal "promise jobs" queue, also called "microtask queue" (v8 term).
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**So, `.then/catch/finally` are called after the current code is finished.**
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If we need to guarantee that a piece of code is executed after `.then/catch/finally`, it's best to add it into a chained `.then` call.
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- There's also a "macrotask queue" that keeps various events, network operation results, `setTimeout`-scheduled calls, and so on. These are also called "macrotasks" (v8 term).
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The engine uses the macrotask queue to handle them in the appearance order.
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**Macrotasks run after the code is finished *and* after the microtask queue is empty.**
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In other words, they have lower priority.
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So the order is: regular code, then promise handling, then everything else, like events etc.
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