The MutationRecord read-only property oldValue contains the character data or attribute value of an observed node before it was changed.

A string representing the old value of an attribute which has been changed, if:

• the attributeOldValue parameter to MutationObserver.observe() is true
• the attributes parameter to MutationObserver.observe() is true or omitted
• the mutation type is attributes.

A string representing the old value of a CharacterData node that has been changed, if:

• the characterDataOldValue parameter to MutationObserver.observe() is true
• the characterData parameter to MutationObserver.observe() is true or omitted
• the mutation type is characterData.

Otherwise this property is null.

In the following example, there is a button that changes the color of an h1 to a random new color. A MutationObserver is used to observe the target node (h1) for changes to the attribute; when a change is detected, the observer calls a function, logOldValue().

The logOldValue() function is passed the mutationRecords array, which contains the MutationRecord objects. The oldValue property of the MutationRecord object is then displayed, in the color of the old value.

HTML

<h1 id="h1" style="color:rgb(0, 0, 0);">Hi, Mom!</h1>
<button id="changeColorButton">Change color</button>
<p id="log"></p>

JavaScript

const h1 = document.getElementById("h1");
const changeValueButton = document.getElementById("changeColorButton");
const log = document.getElementById("log");

changeColorButton.addEventListener("click", () => {
  // Random 6 character hexadecimal number to use as the hex color value
  const newColor = Math.floor(Math.random() * 16777215).toString(16);
  h1.style.color = `#${newColor}`;
});

function logOldValue(mutationRecordArray) {
  for (const record of mutationRecordArray) {
    log.textContent = `Old value: ${record.oldValue}`;
    log.style.cssText = record.oldValue;
  }
}

const observer = new MutationObserver(logOldValue);
observer.observe(h1, {
  attributes: true,
  attributeOldValue: true,
});

Result