要检查按下了哪个键，请使用onkeydown属性。
您可以尝试运行以下代码来获取按下哪个键：
<!DOCTYPE html>
<html>
<body>
<p>Press a key in the below box to get which key is pressed.</p>
<input type="text" size="10" onkeydown="demoFunction(event)">
<p id="test"></p>
<script>
function demoFunction(event) {
var a = event.key;
document.getElementById("test").innerHTML = "You've pressed the key: " + a;
}
</script>
</body>
</html>