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<!DOCTYPE html>
<html>
<body>
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<h1>Window 对象</h1>
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<h2>setTimeout() 和 clearTimeout() 方法</h2>
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<button onclick="startCount()">开始计时！</button>
<input type="text" id="demo">
<button onclick="stopCount()">停止计时！</button>
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<p>点击“开始计时！”可启动定时器。输入字段将始终计数，从 0 开始。</p>
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<p>点击“停止计时！”可停止计数。点击“开始计时！”可再次启动定时器。</p>
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<script>
let counter = 0;
let timeout;
let timer_on = 0;
​
function timedCount() {
  document.getElementById("demo").value = counter;
  counter++;
  timeout = setTimeout(timedCount, 1000);
}
​
function startCount() {
  if (!timer_on) {
    timer_on = 1;
    timedCount();
  }
}
​
function stopCount() {
  clearTimeout(timeout);
  timer_on = 0;
}
</script>
​
</body>
</html>
​