全局变量javascript，java，php，所有语言

Question

I always read on the web that global variables are bad, and I understand that they are bad because each function has access to them and if each function in the application modifies the global variable, in a large application, it can become very difficult to trace the state of this variable at a particular point in the code.

But my question is this, if I have a variable that each function is going to need to use anyway, isn't it better to declare it as global? instead of having to instantiate my variable every time inside each function?

Also, when should I use global variables as opposed to local variables?

Answer 1

Every variable represents some real or imaginable entity. If you are sure that entity is unique, then it's OK to have static variable for it. For example, screen/keybord/mouse devices are represented with static variable of type java.awt.Toolkit (accessible via getter). But such cases a rare. Usually, programmer thinks of single instance of entity, and declares a static variable for it, and then a need for another similar entity arise, which ends in laborious refactoring.

Answer 2

This is all about memory utilization while code is running. You always try to keep less number of object into the memory while code running.Object with less scope(method scope) your object will live less in memory hence good memory utilization. So need to keep variable as less scope as it is necessary. But your statement for global variable is not fully correct as in java we have access modifier (public,private,default etc) and if any global variable(class variable) has private modifier then it cannot be modified by outside the class.