Develop Reference

How can Promise.resolve().then execute later?

Using native Javascript Promise:
Promise.resolve(1).then(it => console.log(it))
console.log(2)
This is logged:
2
1
Question: how is it possible for 2 to execute before 1? JS being event-driven, what is the event that is executing the callback given to then when the original caller has already left that execution tree? Is the engine doing some kind of behind-the-scene magic here?

JavaScript maintains something called a callstack. This is used to keep track of whereabouts in the script we are. When you call a function, it gets added to the callstack, and when the function returns/finishes, it gets removed/popped off the callstack. It is also helpful to think of your entire script also as being in its own "function", and so, when your script first begins its executing, we add "script" to the callstack:
Stack:
- Script
When your Promise resolves, it executes its associated .then() method and adds the callback to something called the micro-task queue. This queue (along with the macro-task queue) is used so that JavaScript can manage asynchronous events. As a result, once you run:
Promise.resolve(1).then(it => console.log(it))
the state of your queues/stacks looks like so (note, this is the state after Promise.resolve(1) and .then() have been added/popped off the callstack):
Stack:
- Script
Micro task queue:
- it => console.log(it)
The callback in the micro-task queue will only execute once it gets added to the Stack. This happens through the use of the event-loop. The event-loop will pull tasks off the micro-task queue only when the callstack is empty. Currently, we are still running "script", so the callstack isn't empty yet.
Next, you encounter your console.log(2), as this is a function call, it gets added to the call stack, and once it returns it gets popped off. This is the reason why you see 2 in the console appear first, as the Promise's .then() callback hasn't executed yet, as it's sitting in the micro-task queue waiting for the main script to finish. One the main script finishes, "script" gets popped off the stack:
Stack:
- (empty) <----------------<
| --- gets moved to the stack by the event-loop
Micro task queue: |
- it => console.log(it) ---^
the event loop then moves the task(s) from the micro-task queue now that the callstack is empty. Once the task is moved to the callstack, it executes, running the second console.log(it). This is why you see 1 logged in the console after the 2.

What is the difference between microtask and macrotask in javascript? [duplicate]

I've just finished reading the Promises/A+ specification and stumbled upon the terms microtask and macrotask: see http://promisesaplus.com/#notes
I've never heard of these terms before, and now I'm curious what the difference could be?
I've already tried to find some information on the web, but all I've found is this post from the w3.org Archives (which does not explain the difference to me): http://lists.w3.org/Archives/Public/public-nextweb/2013Jul/0018.html
Additionally, I've found an npm module called "macrotask": https://www.npmjs.org/package/macrotask
Again, it is not clarified what the difference exactly is.
All I know is, that it has something to do with the event loop, as described in https://html.spec.whatwg.org/multipage/webappapis.html#task-queue
and https://html.spec.whatwg.org/multipage/webappapis.html#perform-a-microtask-checkpoint
I know I should theoretically be able to extract the differences myself, given this WHATWG specification. But I'm sure that others could benefit as well from a short explanation given by an expert.

One go-around of the event loop will have exactly one task being processed from the macrotask queue (this queue is simply called the task queue in the WHATWG specification).
After this macrotask has finished, all available microtasks will be processed, namely within the same go-around cycle. While these microtasks are processed, they can queue even more microtasks, which will all be run one by one, until the microtask queue is exhausted.
What are the practical consequences of this?
If a microtask recursively queues other microtasks, it might take a long time until the next macrotask is processed. This means, you could end up with a blocked UI, or some finished I/O idling in your application.
However, at least concerning Node.js's process.nextTick function (which queues microtasks), there is an inbuilt protection against such blocking by means of process.maxTickDepth. This value is set to a default of 1000, cutting down further processing of microtasks after this limit is reached which allows the next macrotask to be processed)
So when to use what?
Basically, use microtasks when you need to do stuff asynchronously in a synchronous way (i.e. when you would say perform this (micro-)task in the most immediate future).
Otherwise, stick to macrotasks.
Examples
macrotasks: setTimeout, setInterval, setImmediate, requestAnimationFrame, I/O, UI rendering
microtasks: process.nextTick, Promises, queueMicrotask, MutationObserver

Basic concepts in spec:
An event loop has one or more task queues.(task queue is macrotask queue)
Each event loop has a microtask queue.
task queue = macrotask queue != microtask queue
a task may be pushed into macrotask queue,or microtask queue
when a task is pushed into a queue(micro/macro),we mean preparing work is finished,so the task can be executed now.
And the event loop process model is as follows:
when call stack is empty,do the steps-
select the oldest task(task A) in task queues
if task A is null(means task queues is empty),jump to step 6
set "currently running task" to "task A"
run "task A"(means run the callback function)
set "currently running task" to null,remove "task A"
perform microtask queue
(a).select the oldest task(task x) in microtask queue
(b).if task x is null(means microtask queues is empty),jump to step (g)
(c).set "currently running task" to "task x"
(d).run "task x"
(e).set "currently running task" to null,remove "task x"
(f).select next oldest task in microtask queue,jump to step(b)
(g).finish microtask queue;
jump to step 1.
a simplified process model is as follows:
run the oldest task in macrotask queue,then remove it.
run all available tasks in microtask queue,then remove them.
next round:run next task in macrotask queue(jump step 2)
something to remember:
when a task (in macrotask queue) is running,new events may be registered.So new tasks may be created.Below are two new created tasks:
promiseA.then()'s callback is a task
promiseA is resolved/rejected: the task will be pushed into microtask queue in current round of event loop.
promiseA is pending: the task will be pushed into microtask queue in the future round of event loop(may be next round)
setTimeout(callback,n)'s callback is a task,and will be pushed into macrotask queue,even n is 0;
task in microtask queue will be run in the current round,while task in macrotask queue has to wait for next round of event loop.
we all know callback of "click","scroll","ajax","setTimeout"... are tasks,however we should also remember js codes as a whole in script tag is a task(a macrotask) too.

I think we can't discuss event loop in separation from the stack, so:
JS has three "stacks":
standard stack for all synchronous calls (one function calls another, etc)
microtask queue (or job queue or microtask stack) for all async operations with higher priority (process.nextTick, Promises, Object.observe, MutationObserver)
macrotask queue (or event queue, task queue, macrotask queue) for all async operations with lower priority (setTimeout, setInterval, setImmediate, requestAnimationFrame, I/O, UI rendering)
|=======|
| macro |
| [...] |
| |
|=======|
| micro |
| [...] |
| |
|=======|
| stack |
| [...] |
| |
|=======|
And event loop works this way:
execute everything from bottom to top from the stack, and ONLY when the stack is empty, check what is going on in queues above
check micro stack and execute everything there (if required) with help of stack, one micro-task after another until the microtask queue is empty or don't require any execution and ONLY then check the macro stack
check macro stack and execute everything there (if required) with help of the stack
Micro stack won't be touched if the stack isn't empty. The macro stack won't be touched if the micro stack isn't empty OR does not require any execution.
To sum up: microtask queue is almost the same as macrotask queue but those tasks (process.nextTick, Promises, Object.observe, MutationObserver) have higher priority than macrotasks.
Micro is like macro but with higher priority.
Here you have "ultimate" code for understanding everything.
console.log('stack [1]');
setTimeout(() => console.log("macro [2]"), 0);
setTimeout(() => console.log("macro [3]"), 1);
const p = Promise.resolve();
for(let i = 0; i < 3; i++) p.then(() => {
setTimeout(() => {
console.log('stack [4]')
setTimeout(() => console.log("macro [5]"), 0);
p.then(() => console.log('micro [6]'));
}, 0);
console.log("stack [7]");
});
console.log("macro [8]");
/* Result:
stack [1]
macro [8]
stack [7], stack [7], stack [7]
macro [2]
macro [3]
stack [4]
micro [6]
stack [4]
micro [6]
stack [4]
micro [6]
macro [5], macro [5], macro [5]
--------------------
but in node in versions < 11 (older versions) you will get something different
stack [1]
macro [8]
stack [7], stack [7], stack [7]
macro [2]
macro [3]
stack [4], stack [4], stack [4]
micro [6], micro [6], micro [6]
macro [5], macro [5], macro [5]
more info: https://blog.insiderattack.net/new-changes-to-timers-and-microtasks-from-node-v11-0-0-and-above-68d112743eb3
*/

Macro tasks include keyboard events, mouse events, timer events (setTimeout) , network events, Html parsing, changing Urletc. A macro task represents some discrete and independent work. micro task queue has higher priority so macro task will wait for all the micro tasks are executed first.
Microtasks, are smaller tasks that update the application state and should be executed before the browser continues with other assignments such as
re-rendering the UI. Microtasks include promise callbacks and DOM mutation changes. Microtasks enable us to execute certain actions before the UI is re-rendered, thereby avoiding unnecessary UI rendering that might show an inconsistent application state.
Separation of macro and microtask enables the
event loop to prioritize types of tasks; for example, giving priority to performance-sensitive tasks.
In a single loop iteration, one macro task at most is processed
(others are left waiting in the queue), whereas all microtasks are processed.
Both task queues are placed outside the event loop, to indicate that the act of adding tasks to their matching queues happens outside the
event loop. Otherwise, any events that occur while JavaScript code is
being executed would be ignored. The acts of detecting and adding
tasks are done separately from the event loop.
Both types of tasks are executed one at a time. When a task starts executing, it’s executed to its completion. Only the browser can stop
the execution of a task; for example, if the task takes up too much
time or memory.
All microtasks should be executed before the next rendering because their goal is to update the application state before rendering occurs.
The browser usually tries to render the page 60 times per second,
It's accepted that 60 frames per second are the rate at which
animations will appear smooth. if we want to achieve smooth-running
applications, a single task, and all microtasks generated by that task
should ideally complete within 16 ms. If a task gets executed for more
than a couple of seconds, the browser shows an “Unresponsive script”
message.
reference John Resig-secrets of JS Ninja

I created an event loop pseudocode following the 4 concepts:
setTimeout, setInterval, setImmediate, requestAnimationFrame, I/O, UI rendering are part of the macrotask queue. One macrotask item will be processed first.
process.nextTick, Promises, Object.observe, MutationObserver are part of the microtasks queue. The event loop will process all of the items in that queue including once that were processed during the current iteration.
There is another queue called animation queue which holds animation changes task items which will be processed next. All tasks which exists in this queue will be processed (not including new one which were added during the current iteration). It will be called if it is time for rendering
The rendering pipeline will try to render 60 times a second (every 16 ms)
while (true){
// 1. Get one macrotask (oldest) task item
task = macroTaskQueue.pop();
execute(task);
// 2. Go and execute microtasks while they have items in their queue (including those which were added during this iteration)
while (microtaskQueue.hasTasks()){
const microTask = microtaskQueue.pop();
execute(microTask);
}
// 3. If 16ms have elapsed since last time this condition was true
if (isPaintTime()){
// 4. Go and execute animationTasks while they have items in their queue (not including those which were added during this iteration)
const animationTasks = animationQueue.getTasks();
for (task in animationTasks){
execute(task);
}
repaint(); // render the page changes (via the render pipeline)
}
}

event loop prefers microtask queue over callback queue?

I was testing out concepts of asynchronous code in JS . Got confused between callback queue & microtask queue order. Whenever promise objects gets resolved , the fulfillment method { then } is pushed into microtask queue while the callbacks of browser timer functions such as setTimeout is pushed into callback queue. Event loop continuously checks queue and pushes functions from queue into call stack whenever call stack gets empty. Event loop should prefer microtask queue over normal callback queue but in the example : https://jsfiddle.net/BHUPENDRA1011/2n89ftmp/ it's happening otherwise.
function display(data) {
console.log('hi back from fetch');
}
function printHello() {
console.log('hello');
}
function blockfor300ms() {
for (let i = 0; i < 300; i++) {
// just delaying logic
}
}
// this sets the printHello function in callback queue { event loop queue }
setTimeout(printHello, 0);
const futureData = fetch('https://api.github.com/users/xiaotian/repos');
// after promise is resolved display function gets added into other queue : Microtask queue { job queue}
futureData.then(display);
// event loop gives prefrence to Microtask queue ( untill its complete)
blockfor300ms();
// which runs first
console.log('Me first !')
expected output
Me first !
hi back from fetch
hello
actual output :
Me first !
hello
hi back from fetch
Kindly let me know how it's happening over here.
Thanks

While it is true, what "kib" stated:
"your function blockfor300ms doesn't block the thread long enough for
the fetch to receive a response"
sadly this is irrelevant, because even if you did block execution until after you received a response to your fetch call, you would still see the same result...
(see sample code snippet below, you can block execution with an alert box or a long loop of non-async XMLHttpRequest calls, I received the same result)
Unfortunately, fetch does not work as described by all the blogs and resources I've found... which state that
Fetch will add it's promise chain to the micro-tasks queue and run before any callbacks on the next tick of the event loop.
From the sample code below, it appears fetch does not simply add it's resolve handler function to the micro-tasks queue as described by others, because as Lewis stated since it requires network activity it is being handled by the Network Task Source. But, I don't believe this is due to printHello "blocking" the network task, because fetch is being fired prior to printHello in my sample code below, and the network response would occur before the timer completed as well.
As you can see in the example below I have the printHello delayed to be added to the Task Queue long after the fetch response has been received (2000ms), yet if we block the code execution for longer than 2000ms (so that there is still running execution context) then "Hello" will be printed first. Meaning the fetch resolve() handler is NOT being simply added to the Micro-Task Queue where it would have fired along with the other promise handlers.
So, why is it still being logged after the callback if the response is received and theoretically added to the Task Queue prior to the timer task completing (at 2000ms)? Well, my guess is that the timer task source must be receiving priority over that of the network task source. And therefore both are sitting in their task queue, but the timer task queue is firing before the network task queue...
Links to specs:
Timer Task Source - https://html.spec.whatwg.org/multipage/timers-and-user-prompts.html#timer-task-source
Networking Task Source - https://html.spec.whatwg.org/multipage/webappapis.html#networking-task-source
function display(data){console.log("Fetch resolved!")}
function printHello(){console.log("Callback Time")}
function blockExecution() {
console.log("Blocking execution...");
alert('Wait at least 2000ms before closing');
}
const futureData = fetch('https://jsonplaceholder.typicode.com/todos/1');
futureData.then(response => response.json()).then(display);
setTimeout(printHello, 2000);
const p = new Promise(
// this is called the "executor"
(resolve, reject) => {
console.log("I'm making a promise...");
resolve("Promise resolved!");
console.log("Promise is made...");
}
);
p.then(
// this is called the success handler
result => console.log(result)
);
blockExecution();
console.log("Execution ended!");

futureData is actually a fetch promise so there is absolutely a network task queued into task queue when fetch is called. As the result, printHello will definitely be executed before the network task since they're both tasks. And method display will only be put into microtask queue when the promise of network task is resolved. Microtasks, by definition, are only executed at the end of each task. So display will be called at the end of the network task when printHello has already been called long time before.
If you want display to be called before printHello, futureData must only queue microtasks. Let's modify your example a bit.
function display(data) {
console.log('hi back from fetch');
}
function printHello() {
console.log('hello');
}
let now = Date.now();
function executeFutureDataWithMicrotasksOnly() {
// Execute microtasks continually in 300ms.
return Promise.resolve().then(() => Date.now() - now < 300 && executeFutureDataWithMicrotasksOnly());
}
function blockfor300ms() {
for (let i = 0; i < 300; i++) {
// just delaying logic
}
}
// this sets the printHello function in callback queue { event loop queue }
setTimeout(printHello, 0);
const futureData = executeFutureDataWithMicrotasksOnly();
// after promise is resolved display function gets added into other queue : Microtask queue { job queue}
futureData.then(display);
// event loop gives prefrence to Microtask queue ( untill its complete)
blockfor300ms();
// which runs first
console.log('Me first !')
As you can see from the above example, if you replace fetch with a method having only microtasks, the execution order is changed as expected though both fetch and executeFutureDataWithMicrotasksOnly are executed in a similar time interval. When futureData no longer queues tasks, all microtasks including display will be executed at the end of the currently executing task, which is the previous task of task printHello.

I think the problem comes from the fact that your function blockfor300ms doesn't block the thread long enough for the fetch to receive a response.
There won't be anything in the job queue (yet) when the event loop will see that it can call printHello.

Using Visualization
This will help you in giving a better understanding of how javascript works.
Here in this case fetch(Promise) is taking more time than setTimeout so when the event loop cycle is running fetch(Promise) was still in progress and setTimeout is executed first as it took less time and came out of the task queue and it gets processed and when fetch(Promise) ends it comes out of the microtask queue.
If you will increase setTimeout time then the first fetch(Promise) will occur first and then setTimeout will occur. Hope this solves your question.

It seems to be easier than we all think:
It is possible for a microtask to be moved to a regular task queue, if, during its initial execution, it spins the event loop. HTML Living Standard #queue a microtask
When the loop is in the process of selecting a task form the task queue, it can choose to execute tasks that were previously queued into the microtask queue and now are part of task queue:
In that case, the task chosen in the next step was originally a microtask, but it got moved as part of spinning the event loop. HTML Living Standard #event loop processing model
Code that spins the loop is anything that includes parallel operations:
In parallel:
Wait until the condition goal is met.
HTML Living Standard #spin-the-event-loop

Why Node.js setImmediate executes after I/O callbacks?

As new member, I'm unable to comment on topics, that's why I had to create a new topic. But in this way, I can clarify the problem, so hopefully you guys can help me.
I have read quite a lot about Node.js Event Loop. And I have shaped my understanding of it based on following materials:
Node.js Event Loop
What the heck is the event loop anyway?
Why setImmediate() execute before fs.readFile() in Nodejs Event Loop's works?
(Please feel free to suggest other materials which are informative and accurate)
Especially the third link, has given me a better understanding. But keeping that in mind, I'm unable to understand Event Loop behavior for the following code:
var fs = require('fs');
var pos = 0;
fs.stat(__filename, function() {
console.log(++pos + " FIRST STAT");
});
fs.stat(__filename, function() {
console.log(++pos + " LAST STAT");
});
setImmediate(function() {
console.log(++pos + " IMMEDIATE")
})
console.log(++pos + "LOGGER");
Surprisingly, for me output is as follow:
LOGGER
FIRST STAT
LAST STAT
IMMEDIATE
screenshot of my terminal, showing output as well as node version
screenshot of output from online code compiler rextester.com
Keeping the Event Loop Diagram in mind, I guess flow should be as follow:
Interpretor firstly starts two stat operations.
Interpreter en-queues setImmedate callback (event) in the setImmedate queue
Call stack logs the logger
All event queues before I/O Poll phase are empty, so Event Loop(EL) moves on
In I/O Polling phase, EL collects the events and enqueues both the fs.stat callbacks in the "run completed I/O handlers" phase
EL checks the Check phase, and run the setImmediate callback
This round of EL ends, and second round starts
In "run completed I/O handlers", EL runs both callbacks (order of them can is onn-determinstic)
Question 1: Which part of my analysis/prediction is wrong?
Question 2: At which point, does Event Loop start working? Does it start from the beginning of the app (i.e. stage 1)? or does it start once the whole code is read by interpreter, all sync tasks are done within Call Stack, and Call Stack needs more task, i.e. between stage 3-4?
Thanks in advance,

setImmediate = execute without wait any I/O
In https://nodejs.org/docs/v8.9.3/api/timers.html#timers_setimmediate_callback_args says:
Schedules the "immediate" execution of the callback after I/O events' callbacks. Returns an Immediate for use with clearImmed
Steps:
callback for First stat is queued in I/O queue
callback for Last stat is queued in I/O queue
callback for immediate is queued in Immediates queue
LOGGER
If I/O operations (in 1 and 2) are finished the callbacks in 1 and/or 2 are marked as ready to execute
Execute the ready callbacks one by one (first timmer, then I/O, finally immediates). In your case:
First stat
Last stat
LOGGER
In the case that I/O does'nt ends at 5. then LOGGER were execute before FIRST STAT and LAST STAT.
See also: https://jsblog.insiderattack.net/timers-immediates-and-process-nexttick-nodejs-event-loop-part-2-2c53fd511bb3#f3dd

Very fast endless loop without blocking I/O

Is there a faster alternative to window.requestAnimationFrame() for endless loops that don't block I/O?
What I'm doing in the loop isn't related to animation so I don't care when the next frame is ready, and I have read that window.requestAnimationFrame() is capped by the monitor's refresh rate or at least waits until a frame can be drawn.
I have tried the following as well:
function myLoop() {
// stuff in loop
setTimeout(myLoop, 4);
}
(The 4 is because that is the minimum interval in setTimeout and smaller values will still default to 4.) However, I need better resolution than this.
Is there anything with even better performance out there?
I basically need a non-blocking version of while(true).

Two things that will run sooner than that setTimeout:
process.nextTick callbacks (NodeJS-specific):
The process.nextTick() method adds the callback to the "next tick queue". Once the current turn of the event loop turn runs to completion, all callbacks currently in the next tick queue will be called.
This is not a simple alias to setTimeout(fn, 0). It is much more efficient. It runs before any additional I/O events (including timers) fire in subsequent ticks of the event loop.
Promise settlement notifications
So those might be a tools for your toolbelt, doing a mix of one or both of those with setTimeout to achieve the balance you want.
Details:
As you probably know, a given JavaScript thread runs on the basis of a task queue (the spec calls it a job queue); and as you probably know, there's one main default UI thread in browsers and NodeJS runs a single thread.
But in fact, there are at least two task queues in modern implementations: The main one we all think of (where setTimeout and event handlers put their tasks), and the "microtask" queue where certain async operations are placed during the processing of a main task (or "macrotask"). Those microtasks are processed as soon as the macrotask completes, before the next macrotask in the main queue — even if that next macrotask was queued before the microtasks were.
nextTick callbacks and promise settlement notifications are both microtasks. So scheduling either schedules an async callback, but one which will happen before the next main task.
We can see that in the browser with setInterval and a promise resolution chain:
let counter = 0;
// setInterval schedules macrotasks
let timer = setInterval(() => {
$("#ticker").text(++counter);
}, 100);
// Interrupt it
$("#hog").on("click", function() {
let x = 300000;
// Queue a single microtask at the start
Promise.resolve().then(() => console.log(Date.now(), "Begin"));
// `next` schedules a 300k microtasks (promise settlement
// notifications), which jump ahead of the next task in the main
// task queue; then we add one at the end to say we're done
next().then(() => console.log(Date.now(), "End"));
function next() {
if (--x > 0) {
if (x === 150000) {
// In the middle; queue one in the middle
Promise.resolve().then(function() {
console.log(Date.now(), "Middle");
});
}
return Promise.resolve().then(next);
} else {
return 0;
}
}
});
$("#stop").on("click", function() {
clearInterval(timer);
});
<div id="ticker"> </div>
<div><input id="stop" type="button" value="Stop"></div>
<div><input id="hog" type="button" value="Hog"></div>
<script src="https://ajax.googleapis.com/ajax/libs/jquery/2.1.1/jquery.min.js"></script>
When you run that and click the Hog button, note how the counter display freezes, then keeps going again. That's because of the 300,000 microtasks that get scheduled ahead of it. Also note the timestamps on the three log messages we write (they don't appear in the snippet console until a macrotask displays them, but the timestamps show us when they were logged).
So basically, you could schedule a bunch of microtasks, and periodically let those run out and run the next macrotask.
Note: I've used setInterval for the browser example in the snippet, but setInterval, specifically, may not be a good choice for a similar experiment using NodeJS, as NodeJS's setInterval is a bit different from the one in browsers and has some surprising timing characteristics.

there are some libs that can work like cron task, e.g., https://www.npmjs.com/package/node-cron
i think that using cron should be easier, and more flexible.