MutationRecord

MutationRecord is the data structure used by the MutationObserver API to describe a single DOM mutation. When a MutationObserver detects changes in an observed subtree, its callback receives an array of MutationRecord objects, each representing one mutation event. The records provide details about what changed, where it changed, and, when applicable, what the previous value was.

Each MutationRecord includes a type field that indicates the kind of mutation. The type is one of "attributes", "characterData", or "childList". The target field points to the Node on which the mutation occurred. For mutations of type "childList", addedNodes and removedNodes are NodeLists containing the nodes that were added or removed, and previousSibling and nextSibling describe the siblings adjacent to the affected nodes within their parent. For mutations of type "attributes", attributeName and attributeNamespace identify the changed attribute. For mutations of type "characterData", the mutated node is the target.

The oldValue field holds the previous value of the mutated attribute or character data, but only if the observer was created with the corresponding option: attributeOldValue for attributes, or characterDataOldValue for characterData. If those options are not enabled, oldValue may be null or absent.

MutationRecord objects are delivered in batch to the callback, allowing developers to examine each change and respond accordingly. They form the core data units used to track DOM changes with MutationObserver.