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                <div id="change_txt" contenteditable="true">編集可能な要素です。何か入力してください。</div>
<p id="result"></p>
<button id="stop_btn" onclick="this.disabled = true;">監視をストップする</button>


                #change_txt {
    background-color: #fff;
    margin: 20px 20px 0;
    padding: 10px;
    border: 1px solid #ccc;
    border-radius: 8px;
#stop_btn {
    max-width: 200px;
    padding: 10px 14px;
    margin: 20px auto;
    display: block;
#result {
    margin: 10px 20px;


                const result = document.querySelector('#result');
const stopBtn = document.querySelector('#stop_btn');

const options = {
    // childList: true, //直接の子の変更を監視
    characterData: true,  //文字の変化を監視
    // characterDataOldValue: true, //属性の変化前を記録
    // attributes: true,  //属性の変化を監視
    subtree: true, //全ての子要素を監視
function callback(mutationsList, observer) {
    for(const mutation of mutationsList) {
        result.textContent =;
const target = document.querySelector('#change_txt');
const obs = new MutationObserver(callback);
obs.observe(target, options);

stopBtn.addEventListener("click", function() {