Why are global variables considered bad practice?

Question

I keep seeing warnings not to use global variables in JavaScript, but it seems that the only reason people say that is because the clogs up the global namespace. I can imagine this being easily fixed by putting all of the variables into one big object. Now the question is: are there any other reasons not to use global variables other than convenience sake? Are there any performance or compatibility issues involved with them?

Answer 1

In a nutshell, Global variables cause (and more) the following issues.

1) Variable naming collisions - If you're working on a team and both yourself and your co-worker use the same variable name on the global scope, the variable defined last will overwrite the initial variable. This obvious can have devastating consequences.

2) Security - Specifically on the web, every user has access to the Window (or global) object. By putting variables on the global scope, you give any user the ability to see or change your variables.

3) Slower - This is arguably negligible, but it still exists. The way JavaScript variable lookups work is the JavaScript engine will do a lookup on the current scope the variable is being looked up in. If it can't find it, it will do a look up on the next parent scope. If it doesn't find it there, it will continue looking upward until it reaches the global object looking for that variable. If all of your variables are located on the global scope, the JavaScript engine will always have to go through every scope in order to finally reach the global scope to find the variable.