If you’ve been programming for any length of time, you’ve probably come across the advice “don’t use global variables”.
Why are global variables so often advised against?
The reason is that global variables make a program less understandable. When you’re looking at a piece of code that uses a global variable, you don’t know if you’re seeing the whole picture. The code isn’t self-contained. In order to understand your piece of code, you potentially have to venture to some outside place to have a look at some other code that’s influencing your code at a distance.
The key idea is scope. If a local variable is defined inside of a function, for example, then that variable’s scope is limited to that function. Nobody from outside that function can see or mess with that variable. As another example, if a private instance variable is defined for a class, then that variable’s scope is limited to that class, and nobody from outside that class can see or mess with the variable.
The broader a variable’s scope, the more code has to be brought into the picture in order to understand any of the code that involves that variable. If I have a function that depends on its own arguments and nothing else, then that function can be understood in isolation. All I need in order to understand the function (at least in terms of causes and effects, as opposed to conceptual understanding which may require outside context) is the code inside the function. If alternatively the function involves a class instance variable, for example, then I potentially need to look at the other places in the class that involve the instance variable in order to understand the behavior of the function.
The maximum scope a variable can have is global scope. In terms of understanding, a global variable presents the biggest burden and requires the most investigative work. That’s why global variables are so often cautioned against.
Having said that, it’s actually a little simplistic to say “global variables are bad”. It would be more precise to say “global variables are costly”. There are some scenarios where the cost of a global variable is worth the price. In those cases, the idea of a global variable could be said to be good because it’s less costly than the alternatives.
But in the vast majority of cases, it’s good to keep the scope of variables as small as possible. The smaller the scopes of your variables are, the more it will aid the understandability of your code.