Option Explicit

The Option Explicit directive enforces stricter variable definitions in your script. When this statement is placed at the beginning of your script, all variables must be declared before they are used with the Dim statement. If a variable is not defined, the script will throw a runtime error.

The use of Option Explicit is highly encouraged since it allows you to check for variables which you forgot to define. You should make sure and declare all of the variables that are used in your script because it will increase the performance.

It is especially important to declare all of the variables used inside of a subroutine or function. If you fail to declare these variables, they will assume that they are in the global variable scope and could cause conflicts or weird errors in your script.