defineProperty is being used to set a property On DATA, named corresponding to the cell's id (e.g., A3), to a descriptor. In this case the descriptor only provi

defineProperty is being used to set a property On DATA, named corresponding to the cell's id (e.g., A3), to a descriptor. In this case the descriptor only provides a getter function, inside of which is an eval done with DATA as the context. That means all evaluated variable references will assume DATA as the implicit 'this' (whereas normally that would be 'window')