页面的head部分，需引入一个CSS样式文件，代码如下：

<link type="text/css" rel="stylesheet" href="css/style.css">

页面的body部分，用了多个div的嵌套结构，代码如下：

<div class="container">
  <div class="setters">
    <div class="minutes-set">
      <button data-setter="minutes-plus">+</button>
      <button data-setter="minutes-minus">-</button>
    </div>
    <div class="seconds-set">
      <button data-setter="seconds-plus">+</button>
      <button data-setter="seconds-minus">-</button>
    </div>
  </div>
   <div class="circle">
      <svg width="300" viewBox="0 0 220 220" xmlns="http://www.w3.org/2000/svg">
         <g transform="translate(110,110)">
            <circle r="100" class="e-c-base"/>
            <g transform="rotate(-90)">
               <circle r="100" class="e-c-progress"/>
               <g id="e-pointer">
                  <circle cx="100" cy="0" r="8" class="e-c-pointer"/>
               </g>
            </g>
         </g>
      </svg>
   </div>
   <div class="controlls">
      <div class="display-remain-time">00:30</div>
      <button class="play" id="pause"></button>
   </div>
</div>

页面的底部，这里使用了部分ES6的语法，里面已经提供了部分注释了，代码如下：

<script type="text/javascript">
//圆形开始
let progressBar = document.querySelector('.e-c-progress');
let indicator = document.getElementById('e-indicator');
let pointer = document.getElementById('e-pointer');
let length = Math.PI * 2 * 100;

progressBar.style.strokeDasharray = length;

function update(value, timePercent) {
	var offset = - length - length * value / (timePercent);
	progressBar.style.strokeDashoffset = offset; 
	pointer.style.transform = `rotate(${360 * value / (timePercent)}deg)`; 
};
//圆形结束

const displayOutput = document.querySelector('.display-remain-time')
const pauseBtn = document.getElementById('pause');
const setterBtns = document.querySelectorAll('button[data-setter]');

let intervalTimer;
let timeLeft;
let wholeTime = 0.5 * 60; //设置时间 
let isPaused = false;
let isStarted = false;


update(wholeTime,wholeTime); //刷新进度条
displayTimeLeft(wholeTime);

function changeWholeTime(seconds){
  if ((wholeTime + seconds) > 0){
    wholeTime += seconds;
    update(wholeTime,wholeTime);
  }
}

for (var i = 0; i < setterBtns.length; i++) {
    setterBtns[i].addEventListener("click", function(event) {
        var param = this.dataset.setter;
        switch (param) {
            case 'minutes-plus':
                changeWholeTime(1 * 60);
                break;
            case 'minutes-minus':
                changeWholeTime(-1 * 60);
                break;
            case 'seconds-plus':
                changeWholeTime(1);
                break;
            case 'seconds-minus':
                changeWholeTime(-1);
                break;
        }
      displayTimeLeft(wholeTime);
    });
}

function timer (seconds){ //按秒计时
  let remainTime = Date.now() + (seconds * 1000);
  displayTimeLeft(seconds);
  
  intervalTimer = setInterval(function(){
    timeLeft = Math.round((remainTime - Date.now()) / 1000);
    if(timeLeft < 0){
      clearInterval(intervalTimer);
      isStarted = false;
      setterBtns.forEach(function(btn){
        btn.disabled = false;
        btn.style.opacity = 1;
      });
      displayTimeLeft(wholeTime);
      pauseBtn.classList.remove('pause');
      pauseBtn.classList.add('play');
      return ;
    }
    displayTimeLeft(timeLeft);
  }, 1000);
}
function pauseTimer(event){
  if(isStarted === false){
    timer(wholeTime);
    isStarted = true;
    this.classList.remove('play');
    this.classList.add('pause');
    
    setterBtns.forEach(function(btn){
      btn.disabled = true;
      btn.style.opacity = 0.5;
    });

  }else if(isPaused){
    this.classList.remove('play');
    this.classList.add('pause');
    timer(timeLeft);
    isPaused = isPaused ? false : true
  }else{
    this.classList.remove('pause');
    this.classList.add('play');
    clearInterval(intervalTimer);
    isPaused = isPaused ? false : true ;
  }
}

function displayTimeLeft (timeLeft){ //显示时间
  let minutes = Math.floor(timeLeft / 60);
  let seconds = timeLeft % 60;
  let displayString = `${minutes < 10 ? '0' : ''}${minutes}:${seconds < 10 ? '0' : ''}${seconds}`;
  displayOutput.textContent = displayString;
  update(timeLeft, wholeTime);
}
pauseBtn.addEventListener('click',pauseTimer);
</script>