Detailed explanation of examples of js event loop

I have read some event loop blogs before, but after not reading them for a while, I realized that I had forgotten them all, so I decided to write a blog to summarize them!

First of all, let’s explain what the event loop is:

As far as we know, the browser’s js is single-threaded, that is to say , at most only one code segment is executing at the same time, but the browser can handle asynchronous requests very well, so why? Let’s take a look at a picture first

We can see from the above picture that the js main thread has an execution stack, and all js code will run in the execution stack. During the execution of the code, if you encounter some asynchronous code (such as setTimeout, ajax, promise.then and user clicks, etc.), the browser will put these codes into a thread (here we call it a behind-the-scenes thread). Waiting does not block the execution of the main thread. The main thread continues to execute the remaining code in the stack. When the code in the background thread is ready (for example, the setTimeout time is up and the ajax request is responded to), the thread will process it. The callback function is placed in the task queue and waits for execution. When the main thread finishes executing all the code in the stack, it will check Task Queue to see if there is a task to be executed. If there is a task to be executed, then the task will be put into the execution stack for execution. If the current task queue is empty, it will continue to wait in a loop for the task to arrive. Therefore, this is called an event loop.

So, here comes the question. If there are many tasks in the task queue, which task should be executed first?

In fact (as shown in the picture above), js has two task queues, one is called Macrotask Queue (Task Queue), and the other is called Microtask Queue

• The former is mainly used to carry out some relatively large-scale work, common ones include setTimeout, setInterval, user interaction operations, UI rendering, etc.

• The latter is mainly used to carry out some relatively small-scale work, which is common There are Promise, process.nextTick(nodejs)

So, what are the specific differences between the two? Or, if two tasks appear at the same time, which one should be chosen?
In fact, what the event loop does is as follows:

1. Check whether the Macrotask queue is empty. If not, proceed to the next step. If it is empty, jump to 3

2. Take the first task from the Macrotask queue (the one with the longest time in the queue) and execute it in the execution stack (only one). After execution, go to the next step

3. Check whether the Microtask queue is empty, if not, go to the next step, otherwise, jump to 1 (start a new event loop)

4. Get from the Microtask queue The task at the head of the queue (with the longest time in the queue) goes into the event queue for execution. After execution, it jumps to 3

. Among them, the microtask task added during the execution of the code will be in the current It is executed within the event loop period, and the newly added macrotask task can only wait until the next event loop to execute (an event loop only executes one macrotask)
First, let’s look at a piece of code

console.log(1)
setTimeout(function() {
  //settimeout1
  console.log(2)
}, 0);
const intervalId = setInterval(function() {
  //setinterval1
  console.log(3)
}, 0)
setTimeout(function() {
  //settimeout2
  console.log(10)
  new Promise(function(resolve) {
    //promise1
    console.log(11)
    resolve()
  })
  .then(function() {
    console.log(12)
  })
  .then(function() {
    console.log(13)
    clearInterval(intervalId)
  })
}, 0);

//promise2
Promise.resolve()
  .then(function() {
    console.log(7)
  })
  .then(function() {
    console.log(8)
  })
console.log(9)

You What do you think the result should be?
The results I output in the node environment and chrome console are as follows:

1
9
7
8
2
3
10
11
12
13

In the above example
The first event loop:

1. console.log(1) is executed, output 1

2. settimeout1 is executed, and added to the macrotask queue

3. setinterval1 is executed and added to the macrotask queue

4. settimeout2 is executed and added to the macrotask queue

5. promise2 is executed and its two then functions are added to the microtask Queue

6. console.log(9) is executed and outputs 9

7. According to the definition of the event loop, the new microtask task will be executed next , execute console.log(7) and console.log(8) in the order of entering the queue, output 7 and 8
   microtask queue is empty, return to the first step, enter the next event loop, at this time the macrotask queue For: settimeout1,setinterval1,settimeout2

The second event loop:

1. Get the queue located in the macrotask queue The first task (settimeout1) is executed and the output 2
   microtask queue is empty. Go back to the first step and enter the next event loop. At this time, the macrotask queue is: setinterval1,settimeout2

The third event loop:

1. Get the task at the head of the team (setinterval1) from the macrotask queue and execute it, output 3, and then generate a new The setinterval1 is added to the macrotask queue
   The microtask queue is empty, return to the first step and enter the next event loop. At this time, the macrotask queue is: settimeout2,setinterval1

Four event loops:

1. Take the task at the head of the team (settimeout2) from the macrotask queue and execute it, output 10, and execute the function in new Promise (in new Promise The function is a synchronous operation, not an asynchronous operation), outputs 11, and adds its two then functions to the microtask queue

2. From the microtask queue, take the task at the head of the queue and execute it until it is empty. Therefore, the two newly added microtask tasks are executed in order, outputting 12 and 13, and clearing setinterval1
   At this time, both the microtask queue and the macrotask queue are empty, and the browser will always check whether the queue is empty and wait for new The task is added to the queue.

Here, you may wonder, in the first loop, why isn't macrotask executed first? Because according to the process, shouldn't we first check whether the macrotask queue is empty, and then check the microtask queue?
Reason: Because the task running in the js main thread at the beginning is the macrotask task, and according to the process of the event loop, only one macrotask task will be executed in an event loop. Therefore, after executing the code of the main thread, it will go from Take the first task from the microtask queue and execute it.

Note:

When executing a microtask task, it will only enter the next event loop when the microtask queue is empty. Therefore, if it Continuously generating new microtask tasks will cause the main thread to be executing microtask tasks, and there is no way to execute macrotask tasks. In this way, we will not be able to perform UI rendering/IO operations/ajax requests. Therefore, we should avoid this situation. occur. In process.nexttick in nodejs, you can set the maximum number of calls to prevent blocking the main thread.

With this, we introduce a new problem, the timer problem. Is the timer real and reliable? For example, if I execute a command: setTimeout(task, 100), will it be executed exactly after 100 milliseconds? In fact, based on the above discussion, we can know that this is impossible.
I think everyone should know the reason, because after you execute setTimeout(task,100), you actually just ensure that the task will enter the macrotask queue after 100 milliseconds, but it does not mean that it can run immediately. Maybe The main thread is currently performing a time-consuming operation, or there may be many tasks in the microtask queue, so this may be the reason why everyone has been criticizing setTimeout, hahahahahahahahahahahahahahahahahahahahahahahahahahahahahahahahahahahahahahahahahahahahahahahahahahahahahahahahahahahahahahahahahahahahahahahahahahahahahahahahahahahahahahahahahahaha

The above is just my personal view of the event loop Some opinions and references from other excellent articles

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