What is the concept of js execution mechanism? Implementation method of js execution mechanism

What this article brings to you is the concept of js execution mechanism? The implementation method of js execution mechanism has certain reference value. Friends in need can refer to it. I hope it will be helpful to you.

In actual development, after the request is completed, you always want to assign the data obtained by the request to a certain object or variable. If the value is not assigned in the callback function of the request, but assigned below the request statement , we will find that the data obtained by the request is normal, but the variable after assignment is undefined. Why is this?
First of all, we need to understand a concept, which is Synchronization and asynchronousness. As we all know, js is a single-threaded language, that is to say, js can only execute one task at a time. If there are multiple tasks, then they will be executed in the order of the tasks. But if one of the tasks takes a lot of time, such as falling into an infinite loop, then other tasks cannot be executed, causing the browser to become unresponsive. So how does js solve it? That is to divide tasks into synchronous and asynchronous modes for execution. Synchronization is as above, and asynchronous means that there is more than one callback function. When the task ends, it does not execute the next task but the callback function. So some people may have questions, what is the difference between doing this and synchronization? The difference is that the latter task does not need to wait for the previous task to be completely executed before executing it. Therefore, the program execution order we get is not the order of tasks.

setTimeout(function(){
    console.log('第一个延时调用');
});
console.log('哈哈哈');
new Promise(function(resolve){
    console.log('promise任务吗');
    resolve();
}).then(function(){
    console.log('回调函数？？？')
});
console.log('嘻嘻嘻');
setTimeout(function(){
    console.log('第二个延时调用');
});

What is the execution result?
Hahaha
promise task
heeheehee
Callback function? ? ?

The first delayed call
The second delayed call
Why is this? Why is setTimeout executed at the end? It is obviously 0ms.
is because in addition to the synchronous and asynchronous modes, we have further divided tasks into macro tasks and micro tasks.
Macro task: including the overall code script, setTimeout, setInterval
Micro task: Promise, process.nextTick
When executing, after entering the macro task, start the first loop, then execute all micro tasks, and then Performing the next cycle of the macro task.
In the above code, enter the macro task, register setTimeout as the macro task and put it into the queue, and then execute Hahaha. When encountering new Promise, execute it directly. The callback function places the microtask queue, and then executes hehehe. The second A setTimeout is as above. Then execute the micro task, there is only hee hee hee here. Finally enter the next macro task.

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