[straws]

There's a benefit to teasing apart two ideas here:

- writing functions that expect immutable data (you get referential transparency and value equality → a system that's easier to reason about) - using persistent data structures (makes it cheap and efficient to create new changes to your data over some messy Object.assign helpers)

Javascript doesn't promote applications written in that style though, so you're definitely going to want to use a library like Immutable.js everywhere for those kinds of guarantees.