Detailed explanation of the use of EventLoop in JS

This time I will bring you a detailed explanation of the use of EventLoop in JS. What are the precautions for using EventLoop in JS? Here are practical cases, let’s take a look.

Imagine, for example, the browser is running a complex image conversion algorithm. Because it is single-threaded, the browser process is blocked at this time and cannot render the interface or run other codes. Your The application interface will lose interaction with the user.

This is usually not a big problem, but when the browser is running multiple similar algorithms at the same time, this problem becomes serious.

Most js developers with certain experience understand the asynchronous execution of code, especially the use of ajax.

// ajax(..) is some arbitrary Ajax function given by a library
var response = ajax('https://example.com/api');
console.log(response);
// `response` won't have the response

This response will not get the result you want.

Instead, you need to get the results through the callback function like a face-to-face method

ajax('https://example.com/api', function(response) {
  console.log(response); // `response` is now available
});

Also, a reminder here, the following code, async: false is never a good idea.

// This is assuming that you're using jQuery
jQuery.ajax({
  url: 'https://api.example.com/endpoint',
  success: function(response) {
    // This is your callback.
  },
  async: false // And this is a terrible idea
});

Through the above example, we should understand that asynchronous functions can help us solve problems similar to browser blocking.

Of course, you can also implement the same logic through setTimeout(callback, milliseconds). If you understand asynchronous, what will be output when the following code is executed?

function first() {
  console.log('first');
}
function second() {
  console.log('second');
}
function third() {
  console.log('third');
}
first();
setTimeout(second, 1000); // Invoke `second` after 1000ms
third();

What is the principle of the asynchronous processing mechanism now? Here we will introduce our event loop Event Loop

Event Loop has a simple Job(task) - monitors Call Stack and Callback Queue. If the call stack is empty, it will take the first event from the queue and push it onto the call stack, effectively running it.

This iteration is called a Tick in the event loop. Each event is just a function callback.

console.log('Hi');
setTimeout(function cb1() { 
  console.log('cb1');
}, 5000);
console.log('Bye');

Execute this code

Note that setTimeout(...) does not automatically place your callback in the event loop queue.

It sets a timer. When the timer expires, the browser puts your callback into the event loop so that some future tick will execute. However, there may be other events in the queue that have been added - your callback will not execute immediately.

There are many articles and tutorials about getting started with asynchronous code in JavaScript and it is recommended that you use setTimeout(callback, 0).
Now you know how Event Loop is done and how setTimeout works.

You can better understand the following code

console.log('Hi');
setTimeout(function() {
  console.log('callback');
}, 0);
console.log('Bye');

Although the wait time is set to 0 ms, the result in the browser console looks like this:

Hi

Bye

callback

I believe you have mastered the method after reading the case in this article. For more exciting information, please pay attention to other related articles on the php Chinese website!

Recommended reading:

Detailed explanation of the steps for using slot and slot-scope in vue

Removing the marker overlay on Baidu map Methods

The above is the detailed content of Detailed explanation of the use of EventLoop in JS. For more information, please follow other related articles on the PHP Chinese website!