To deep copy or not to deep copy - and why should ngrx's state be immutable, anyway?
- Deep copy
Our goal to keep our apps as fast as possible what means to reduce calculation when it's not needed or redundant.
Angular has 2 change detection strategies onPush and Default, the first one checks pointers of variables passed into inputs, the second one does deep check and quite heavy on heavy objects. But they have one thing - they rerender only when data has been changed.
Deep copy is bad because it causes the same data to be presented under new object pointers, this causes render cycles and because data is the same rerendered result will be the same too, unless it's time dependent app.
- Immutable state
The best way for the data flow is from top to bottom, and bottom can notify top to change the data.
In such circumstances we can always find how data came here via checking parents and what they do with the data, and we have a single point where data is changing and we can find there who causes the change.
If we mutate data where we need the change instead of notifying the top, with time we can't easily find who does it anymore because these places will be everywhere in the code and we need to check all of them to find the issue.