There are definitely costs to doing immutable updates of objects and arrays. The engine has to allocate a new object in memory, copy over all the existing keys,

There are definitely costs to doing immutable updates of objects and arrays. The engine has to allocate a new object in memory, copy over all the existing keys, and copy any new keys / values you're assigning. For very large objects, yes, this can become noticeable. But, for most apps, this isn't a concern given the kinds of data and state updates that are common. There's a specific library called Immutable.js, which implements some complex internal data structures to optimize copying objects. However, the library is relatively large, and complicated some common usage patterns. My opinion is that it's rarely worth using that lib.