Mastering JavaScript Functional Programming - Second Edition

Global state

Of all the preceding points, the most common reason for side effects is the usage of nonlocal variables that share a global state with other parts of the program. Since pure functions, by definition, always return the same output value given the same input arguments, if a function refers to anything outside its internal state, it automatically becomes impure. Furthermore, and this is a hindrance to debugging, to understand what a function has done, you must understand how the state got its current values, and that means understanding all of the past history from your program: not easy!

Let's write a function to detect whether a person is a legal adult by checking whether they were born at least 18 years ago. (Okay, that's not ...