How to use Date to implement stopwatch in javascript

With the development of modern society, the role of timers is becoming more and more important. Whether it is timing in daily life, or timing in sports competitions and computer competitions, it needs to be implemented as a stopwatch. Today we will introduce how to use the Date object in JavaScript to implement a stopwatch.

First, let’s take a look at JavaScript’s Date object. The Date object is a very commonly used object in JavaScript, which is used to represent time and date. We can use the new Date() statement to create a new Date object. By default, using the new Date() statement returns the current time and date.

The following is an example showing how to use a Date object to get the current time:

let now = new Date();
console.log(now);

The output result is the current time and date, for example:

Thu Oct 07 2021 12:08:20 GMT+0800 (中国标准时间)

After getting the current time, we You can use it to implement a stopwatch. The stopwatch needs to use a timer, and the timing function is implemented by setting the setInterval() function. The following is an example showing how to use the setInterval() function to implement a simple timer:

let count = 0;
let intervalId = setInterval(function() {
   count++;
   console.log(count);
}, 1000);

In this example, we use the count variable to record the number of seconds that have passed. After each second, the Count is incremented by 1 and output to the console. We used the setInterval() function and assigned it to the intervalId variable. The setInterval() function will execute the passed function at regular intervals and return a unique identifier for subsequent operations.

Next, we can add start, stop and reset functions to the stopwatch. Below is a complete implementation that includes the 3 buttons Start, Stop and Reset and can be timed correctly.

   <title>JavaScript秒表</title>


   <h1 id="JavaScript秒表">JavaScript秒表</h1>
   <div>00:00:00</div>
   <button>开始</button>
   <button>停止</button>
   <button>重置</button>

<script>
   let intervalId = null;
   let count = 0;
   let clock = document.getElementById(&#39;clock&#39;);
   let startBtn = document.getElementById(&#39;startBtn&#39;);
   let stopBtn = document.getElementById(&#39;stopBtn&#39;);
   let resetBtn = document.getElementById(&#39;resetBtn&#39;);

   function startClock(){
      intervalId = setInterval(function() {
         count++;
         let hours = Math.floor(count / 3600);
         let minutes = Math.floor((count - hours * 3600) / 60);
         let seconds = count - hours * 3600 - minutes * 60;
         clock.innerHTML = hours.toString().padStart(2, &#39;0&#39;) + &#39;:&#39; + 
                           minutes.toString().padStart(2, &#39;0&#39;) + &#39;:&#39; + 
                           seconds.toString().padStart(2, &#39;0&#39;);
      }, 1000);
   }

   function stopClock(){
      clearInterval(intervalId);
   }

   function resetClock(){
      clearInterval(intervalId);
      count = 0;
      clock.innerHTML = &#39;00:00:00&#39;;
   }

   startBtn.addEventListener(&#39;click&#39;, startClock);
   stopBtn.addEventListener(&#39;click&#39;, stopClock);
   resetBtn.addEventListener(&#39;click&#39;, resetClock);
</script>

In this example, we first created three buttons and added start, stop and reset functions to them. We use the getElementById() function to obtain the HTML elements on the page and assign them to the corresponding JavaScript variables.

In the startClock() function, we use the setInterval() function to time, and output the timing results to the page in the format of hh:mm:ss. We use the Math.floor() function to round, and the padStart() function to format the number into a two-digit string for easy output to the page. In the stopClock() and resetClock() functions, we use the clearInterval() function to stop the timer and reset the count and clock values ​​respectively.

If you want to use JavaScript to implement a stopwatch, then this example should be able to provide you with some help. You can modify and extend the code as needed to achieve richer and more complex timing functions. Okay, that’s it for now. I hope this article will be helpful to you.

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