Immutability is one of the building blocks of functional programming.

Since the JavaScript language does not support immutability by design, you have to adopt immutability by convention and discipline (and that includes immutability.js). In other languages like Smalltalk and Python and Go, you can also adopt immutability by convention and discipline. You can also write an immutability library for these languages. In other words, programming in Smalltalk can be every bit as safe as in functional languages.