On the whole global variables are bad, and unintentional global variables can be terrible.
programmers.stackexchange.com/qu … te-so-evil
So can we please avoid create variables outside of constructors - not this in the top level of the code
var iAmGlobal=[];This will immediately clash with anybody else creating a variable in the top level with this name anywhere else. It will also make a variable called iAmGlobal available in scope for everything (its global).
Surprisingly something like this can also create a global variable
MyThing.prototype.myMethod = function() {
var myNumbers = [0,1,2,3,4,5,6,7,8,9];
for (vari=0;i<10;i++) {
myNumber=myNumers[i];
}
}
the variable ‘myNumber’ is actually defined in the global scope and will clash with any other variable call myNumber anywhere else.
If you put ‘use strict’; at the top of your code (like I have been trying to introduce) then the second example is actually an error and will not work - which is a good thing 
Anyway can we please try to avoid global variables.