javascript settimeout cancel

JavaScript is a popular programming language that can be used to write interactive elements and logic in web applications. Among them, setimeout is a built-in function in JavaScript, which can execute the specified code after a certain delay. However, sometimes it is necessary to cancel the setTimeout timer during execution. This article will introduce how to cancel the setTimeout timer using JavaScript language.

1. Basic overview of the setTimeout function

Before introducing how to cancel setTimeout, let’s first understand the basic usage and concepts of the setTimeout function.

The setTimeout function is a global function in JavaScript. Its function is to delay the execution of the specified code after a certain period of time. The specific usage is as follows:

setTimeout(function(){
  //执行的代码
},延迟的毫秒数)

Among them, function represents the function to be delayed, and the number of milliseconds to delay is a numeric type parameter, indicating the time to be delayed, in milliseconds. For example, if you want to execute a function after 500 milliseconds, you can use the following code:

setTimeout(function(){
  console.log('延迟500毫秒后执行的代码。')
},500)

2. How to use the clearTimeout function to cancel the setTimeout timer

The delay time in the setTimeout function is too long Or when in doubt, you may need to cancel the timer during the run. At this time, you can use the clearTimeout function of the window object. This function can cancel the pause of the setTimeout function that has not yet been executed.

The clearTimeout function is used as follows:

var timer = setTimeout(function(){
  console.log('延迟执行的代码。')
},3000)

//取消计时器
clearTimeout(timer);

In the above code example, a variable named timer is defined to store the last1 timer, and then the clearTimeout function is used to cancel the timer. This way, if you need to cancel the delay before executing the code, you can use the clearTimeout function.

3. The timer of setTimeout can be canceled, but the internal code cannot be stopped

It should be noted that the timer can be canceled through the clearTimeout function, but the code executed within the timer cannot be stopped. This means that if you define a function that is always executed inside the setTimeout function, even if the timer is canceled, the function will continue to execute until the function is completed.

For example, in the following code example, a setInterval function is defined that is executed once every second. Even if the timer is canceled, the function will stop after executing once.

var timer = setTimeout(function(){
  setInterval(function(){
    console.log('每秒执行一次的代码。')
  },1000)
},5000)

//取消计时器
clearTimeout(timer)

4. Practical application of setTimeout and clearTimeout

Now let us take a look at the practical application of setTimeout and clearTimeout.

In actual applications, the setTimeout function and clearTimeout function are usually used in scenarios that require delayed execution of code, code that requires polling, and providing feedback to users. For example:

1. Delayed execution code

In some scenarios that require waiting for a period of time before execution, you can use the setTimeout function to achieve this, such as automatically completing a search after the user inputs Tip:

var timer;
function handleChange(){
  clearTimeout(timer)
  timer = setTimeout(function(){
    console.log('自动完成搜索提示')
  },500)
}

2. Polling code

In some scenarios that require real-time updates, you can use the setInterval function to implement polling. For example, update data from the server every once in a while:

var timer = setInterval(function(){
  console.log('从服务器轮询数据')
},5000)

//取消轮询
clearInterval(timer)

3. Feedback code

In some scenarios where feedback needs to be provided to users, the setTimeout function can be used. . For example, countdown, perform certain operations after waiting, etc.:

var count = 10;
var timer = setInterval(function(){
  count--
  console.log('还有' + count + '秒...')
  if(count == 0){
    clearInterval(timer)
    console.log('倒计时结束')
  }
},1000)

Taken together, the setTimeout function and the clearTimeout function are very simple, but they can play a big role in practical applications, making the web page more functional and user-friendly. More friendly.

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