Global state is prone to the side effects from parts of the application far away. These side effects will change the results of the code you run. Functional programming has caught on in recent years, as one of the tenants is to reduce side effects, as they can cause unpredictable and undesirable behavior in a system. Instead, you should strive to break down your code into what are known as pure functions. Pure functions take an input and produce an output. For any given input, the output will always be the same.