You now know how to design a spreadsheet.
Architecture: Grid + SelectionOverlay + floating editor.
Data model: Sparse storage for efficiency. Only store non-empty cells.
Virtualization: Render visible cells only. Handle variable dimensions.
Formulas: Parse, build dependency graph, evaluate in topological order.
Interactions: Full keyboard navigation, copy/paste, undo/redo.
Performance: Virtualize, cache calculations, use Web Workers.
Interview tip: Focus on virtualization and the dependency graph for formulas.