JavaScript Is Not a Pure Functional Language

JavaScript is not a pure functional language. Currently it doesn't have, let's say for example immutable data structures. It might gain some if and when the records and tuples proposal becomes officially part of JavaScript or ECMAScript but currently it doesn't. So effectively you're simulating an immutable data structure on top of data structures that aren't intrinsically or inherently immutable. How do you deal with that? Well first of all, TypeScript makes it much easier because with TypeScript we can kind of get the TypeScript compiler to tell us that it's immutable.