Observing text change in the entire HTML DOM - How to do this?

Question

I want to monitor the complete HTML for the following changes:

• Whenever the "text" in a tag changes
• Whenever the "text" in a tag is added
• Whenever a new tag is added with a "text"

So, I just want to monitor for the text changes in the DOM. It could be because a new node was added with some text, it could be because a text in some existing tag changed from a to b, or it could be because the text was added in the otherwise empty tag <h1></h1> --> <h1>hello</h1>

How could I do this? I tried MutationObserver, but I find it very confusing and I am not able to use it correctly. I want to detect the newly added text and transform it via a transformation function. So for example,

<span>apple</span>

was added, it will be transformed as:

const transformedText = transform("apple");
// update the text
<span>transformedText</span>

Here is what I tried:

const config = {
    attributes: false,
    childList: true,
    subtree: true,
    characterData: true,
    characterDataOldValue: true
};
const callback = function (mutationsList, observer) {
    // Use traditional 'for loops' for IE 11
    const dict = transformDictionary();
    for (const mutation of mutationsList) {
        if (mutation.type === "childList") {
            for (const node of mutation.addedNodes) {
                if (node.setAttribute) {
                    // TRANSFORM
                }
            }
            if (mutation.target) {
                // TRANSFORM
            }
        } else if (mutation.type === "characterDataOldValue") {
            console.log(mutation);
        } else if (mutation.type === "characterData") {
        }
    }
};

const o = new MutationObserver(callback).observe(document.body, config);

But it appears there are a lot many triggers for the node which is not a text node. For example a trigger will also be received for a node such as this complete div :

<div>
  wow
   <div>
     ok
      <p>
          <span>hello</span>
      </p>
   </div>
</div>

How can I achieve this?

Answer 1

"use strict";
const NOT_AVAILABLE = "NOT AVAILABLE";

let dictionary = new Map();
dictionary.set("apple", "transformedAppleText");
dictionary.set("pickle", "transformedPickleText");
dictionary.set("pumpkin", "transformedPumpkinText");

const callback = function(mutationsList, observer) {

  const transform = (node) => {
    let transformedText = dictionary.get(node.textContent);
    node.textContent = transformedText !== undefined ? transformedText : NOT_AVAILABLE;
  };

  for (let mutation of mutationsList) {
    if (mutation.type === "childList") {
      for (let node of mutation.addedNodes) {
        // Thanks to:https://developer.mozilla.org/en-US/docs/Web/API/Node/nodeType
        if (node.nodeType === Node.TEXT_NODE) {
          transform(node);
        } else if (node.childNodes.length > 0) {
          for (let childNode of node.childNodes) {
            if (childNode.nodeType === Node.TEXT_NODE) {
              transform(childNode);
            }
          }
        }
      }
    }

  }
  // clear out any changes made during this function - prevents bouncing
  observer.takeRecords();
};

const config = {
  childList: true,
  subtree: true
};

const o = new MutationObserver(callback).observe(document.getElementById("main"), config);

// DOM updates for testing
const div = document.createElement("div");
document.body.appendChild(div);
const apple = document.createTextNode("apple");
const p = document.createElement("p");
p.id = "apple";
p.appendChild(apple);
document.getElementById("div").appendChild(p);
const orange = document.createTextNode("orange");
const p2 = document.createElement("p");
p2.appendChild(orange);
document.getElementById("div").appendChild(p2);
document.getElementById("span").textContent = "pumpkin";
setTimeout(() =>
  document.getElementById("apple").textContent = "pickle", 4000);

<main id="main">
  <h1>Demo</h1>
  <p>New line</p>
  <div id="div">
    <p>Some text</p>
    <div>cat
      <span id="span"></span>
    </div>
  </div>
</main>

The .takeRecords method reads all the mutations in the queue and returns them. It also empties the queue. I used it to discard any mutations caused by the code in the observer changing the text. https://developer.mozilla.org/en-US/docs/Web/API/MutationObserver/takeRecords