Mutation Observer

Summary

An API that watches a DOM subtree and fires a callback when nodes, attributes or text content change — useful for reacting to DOM changes made outside your own code. It is particularly useful at integration boundaries where another library, browser feature, or third-party script changes DOM content your code needs to observe.

• 1Watch a container element and log whenever children are added or removed. Validate your understanding by applying it in a small practical example.
• 2Compare this to React's own re-render model — you rarely need it in React apps directly. Write down when you would choose each option and what trade-off drives that choice.
• 3Learn a realistic use case: detecting when a third-party script injects DOM content.