Analysis of the principles of timers in JavaScript

In our work, many people think of the two functions setTimeout() and setInterval() when talking about the timer in javascript, but they don’t know the timer in JavaScript. What is the principle, then this article will analyze the working principle and difference of timer function from the perspective of event loop (Event Loop)!

setTimeout()

MDN defines setTimeout as:

Call a function after the specified delay time or Execute a code snippet.

Syntax

The syntax of setTimeout is very simple. The first parameter is the callback function, and the second parameter is the delay time. The function returns an ID unique identifier of numeric type. This ID can be used as a parameter of clearTimeout to cancel the timer:

var timeoutID = window.setTimeout(code, delay);

IE0+ also supports the passing of callback parameters:

var timeoutID = window.setTimeout(func, delay, [param1, param2, ...]);

setInterval()

MDN defines setInterval as:

Periodically call a function (function) or execute a piece of code.

Since the usage of setInterval and setTimeout are the same, they are not listed here anymore.

Explanation of the second parameter (delay)

Due to the event loop mechanism of javascript, the second parameter does not mean that the callback will be executed immediately after delay milliseconds. function, instead try to add the callback function to the event queue. In fact, there is a difference in the processing of setTimeout and setInterval at this point:

• setTimeout: After the delay milliseconds, no matter what, the callback function is directly added to the event queue.

• setInterval: After delaying milliseconds, first check whether there is a callback function in the event queue that has not yet been executed (the callback function of setInterval). If it exists, do not go to the event queue again. A callback function has been added.

So, when there are time-consuming tasks in our code, the timer will not behave as we think.

Let’s understand the following code through an example. I originally hoped to be able to call the callback function at 100ms and 200ms (that is, just waiting for 100ms):

var timerStart1 = now();
setTimeout(function () {
 console.log('第一个setTimeout回调执行等待时间：', now() - timerStart1);
 var timerStart2 = now();
 setTimeout(function () {
  console.log('第二个setTimeout回调执行等待时间：', now() - timerStart2);
 }, 100);
}, 100);
// 输出：
// 第一个setTimeout回调执行等待时间： 106
// 第二个setTimeout回调执行等待时间： 107

The result looks like what we expected, but once we add time-consuming tasks to the code, the result is not what we expected:

var timerStart1 = now();
setTimeout(function () {
 console.log('第一个setTimeout回调执行等待时间：', now() - timerStart1);

 var timerStart2 = now();
 setTimeout(function () {
  console.log('第二个setTimeout回调执行等待时间：', now() - timerStart2);
 }, 100);

 heavyTask(); // 耗时任务
}, 100);

var loopStart = now();
heavyTask(); // 耗时任务
console.log('heavyTask耗费时间：', now() - loopStart);

function heavyTask() {
 var s = now();
 while(now() - s < 1000) {
 }
}

function now () {
 return new Date();
}
// 输出：
// heavyTask耗费时间： 1015
// 第一个setTimeout回调执行等待时间： 1018
// 第二个setTimeout回调执行等待时间： 1000

Two The wait event of setTimeout is no longer 100ms due to the existence of time-consuming tasks! Let's describe what happened:

First, the first time-consuming task (heavyTask()) starts to execute, and it takes about 1000ms to complete.

1. Starts execution from the time-consuming task. After 100ms, the first setTimeout callback function is expected to be executed, so it is added to the event queue, but at this time the previous time-consuming task has not yet After execution, it can only wait in the queue until the time-consuming task is completed, so what we see in the result is: The first setTimeout callback execution waiting time: 1018.

2. As soon as the first setTimeout callback is executed, the second setTimeout is opened. This timer is also expected to execute its callback function after a delay of 100ms. However, there is another time-consuming task in the first setTimeout. Its plot is the same as the first timer, and it also waits for 1000ms before starting execution.

3. can be summarized by the following picture:

Let’s look at an example of setInterval:

var intervalStart = now();
setInterval(function () {
 console.log('interval距定义定时器的时间：', now() - loopStart);
}, 100);
var loopStart = now();
heavyTask();
console.log('heavyTask耗费时间：', now() - loopStart);
function heavyTask() {
 var s = now();
 while(now() - s < 1000) {
 }
}
function now () {
 return new Date();
}
// 输出：
// heavyTask耗费时间： 1013
// interval距定义定时器的时间： 1016
// interval距定义定时器的时间： 1123
// interval距定义定时器的时间： 1224

Above In this code, we expect to print a log every 100ms. Compared with setTimeout, setInterval will determine whether there are unexecuted callbacks in the queue when preparing to add the callback function to the event queue. If so, it will not add the callback function to the queue. Otherwise, multiple callbacks will be executed at the same time.

can be summarized by the following picture:

Summary:

After a brief analysis of the execution principle of the JavaScript timer, I believe that my friends have a certain understanding of the working principle of the JavaScript timer. I hope it can help you.

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Javascript timer example code

In-depth exploration of javascript timer

Complete example of javascript timer

JavaScript timer and optimized cancellation timer method

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