Virtualization is the process of keeping track of which portions of the data are visible and rendering only those parts. This reduces the number of DOM elements in the document tree and improves performance dramatically.
The FlexGrid exposes the visible part of the data through the viewRange property. Whenever the user resizes the screen or scrolls the grid, the viewRange is updated and the grid updates its child DOM elements.
The sample below uses the viewRange property to implement a simple type of 'infinite scrolling'. When the user scrolls to the bottom of the grid, the code adds items to the grid's itemsSource.
If you inspect the DOM, you will notice that no matter how large the itemsSource gets, the number of DOM elements remains constant. The data is 'virtualized'.