# Live DOM mirror `GetDOM` hands you the document as it was when you asked. To find out whether it still looks like that, you have to ask again — which is why [`GetDOMHash`](/docs/guides/reading#getdomhash--the-change-detector) exists, and why the usual shape is a polling loop that re-serializes the whole page to notice that one attribute changed. A mirror inverts that. You get the top of the tree once, and from then on the browser tells you what moved — but only inside the part you actually expanded. A page rewriting a list sixty times a second inside a collapsed subtree costs you one number per batch instead of a re-serialized document. This is what makes a DevTools-style tree view practical against a live page. > **TL;DR** > > - `MirrorDom` returns a `DomMirror` holding the tree, plus a handler called after every change. > - **One mirror covers the whole page.** An `