<!DOCTYPE html>
<html lang="en">
<head>
  <meta charset="UTF-8">
  <meta http-equiv="X-UA-Compatible" content="IE=edge">
  <meta name="viewport" content="width=device-width, initial-scale=1.0">
  <title>Document</title>
</head>
<body>
  <button class="btn">버튼 1</button>
  <button class="btn">버튼 2</button>
  <button class="btn">버튼 3</button>
</body>
</html>
.btn {
  margin: 10px;
  padding-left: 10px;
  padding-right: 10px;
  font-size: 20px;
}

let btn = document.querySelectorAll(".btn");

function btnOnClick(event) {
  alert(event.target.textContent + "를 클릭하셨습니다.");
}

for(let i = 0; i < btn.length; i++){
  btn[i].addEventListener("click", btnOnClick);
}

External CSS

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External JavaScript

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