I think there are two concepts being confused here - this is not the same as `var` vs. `const`/`let`. Immutable.js implements persistent data structures, which

I think there are two concepts being confused here - this is not the same as `var` vs. `const`/`let`. Immutable.js implements persistent data structures, which are entirely runtime concepts. --- Deep copying isn't very common because it's usually a pretty obvious flag that performance might suffer, however it's the only other way to implement this same concept and that pattern is definitely found in applications which desire persistent data. You might here this referred to as "defensive copying".