addEventListener：未在特定位置添加事件（按下键）

Question

我在文档中添加了2个事件监听器，尽管其中一个在放置在另一个之后不起作用，但是在放置在另一个之前却可以正常工作，我不知道为什么吗？指定的代码：

document.addEventListener("keydown", function() {
  alert("key was pressed!");
});

[将其放置在“ click”事件侦听器上方时，它可以正常工作，问题是当它放置在其后时，仅“ click”有效，而“ keydown”不起作用。

我的代码：

var buttonLength = document.querySelectorAll(".drum").length;

document.addEventListener("keydown", function() {
  alert("key was pressed!");
});  //Works here

for (i = 0; i <= buttonLength; i++) {
  document.querySelectorAll(".drum")[i].addEventListener("click", function() {
    var buttonInnerHTML = this.innerHTML;
    switch (buttonInnerHTML) {
      case "w":
        var tom1 = new Audio('sounds/tom-1.mp3');
        tom1.play();
        break;

      case "a":
        var tom2 = new Audio('sounds/tom-2.mp3');
        tom2.play();
        break;

      case "s":
        var tom3 = new Audio('sounds/tom-3.mp3');
        tom3.play();
        break;

      case "d":
        var tom4 = new Audio('sounds/tom-4.mp3');
        tom4.play();
        break;

      case "j":
        var snare = new Audio('sounds/snare.mp3');
        snare.play();
        break;

      case "k":
        var crash = new Audio('sounds/crash.mp3');
        crash.play();
        break;

      case "l":
        var kick = new Audio('sounds/kick-bass.mp3');
        kick.play();
        break;
      default:
        console.log(buttonInnerHTML);

    }
  });

}
document.addEventListener("keydown", function() {
  alert("key was pressed!");
}); //Doesn't work here

Answer 1

您的问题在for循环中。迭代次数过多，请更改为：

for (i = 0; i < buttonLength; i++) //(remove equal sign)  

您的代码抛出错误，因此引擎无法继续进行下一个按键事件