JavaScript 程序，让变量在键盘按下时向上/向下计数。

以下是使用 JavaScript 在按键盘 ↑/↓ 键时使变量递增或递减的代码 −

示例

<!DOCTYPE html>
<html lang="en">
<head>
<meta charset="UTF-8" />
<meta name="viewport" content="width=device-width, initial-scale=1.0" />
<title>>Document</title>
<style>
   body {
      font-family: "Segoe UI", Tahoma, Geneva, Verdana, sans-serif;
   }
   .result {
      font-size: 20px;
      font-weight: 500;
   }
</style>
</head>
<body>
<h1>Changing variable value on keyboad up/down keypress</h1>
<div style="color: green;" class="result"></div&g;
<h3>
Press up or down arrow key to increment/decrement variable
</h3>
<script>
   let dayVal = document.querySelector(".day");
   let resEle = document.querySelector(".result");
   let a = 0;
   resEle.innerHTML = a;
   document.body.addEventListener("keydown", (event) => {
      if (event.keyCode === 38) {
         resEle.innerHTML = ++a;
      }
      else if (event.keyCode === 40) {
         resEle.innerHTML = --a;
      }
   });
</script>
</body>
</html>