Option Explicit forces the programmer to declare all variables, rather tha

Why do we use Option Explicit?

Question Posted / jim c

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Answer Posted / jim c

Option Explicit forces the programmer to declare all
variables, rather than letting them be implicitly declared
upon their first use.
This enforces good programming practices and avoids
ambiguity of variable scope.
Here are some simple code examples:
'*************************************
i = 1
Call mySub
WScript.Echo i

Sub mySub
i = 10
End Sub
'*************************************
In this first example, because the programmer is relying on
implicit variable declaration, the subroutine is modifying
global variable i, and the printed result is 10. Now, modify
the code so that the variables are declared:
'*************************************
dim i
i = 1
Call mySub
WScript.Echo i

Sub mySub
dim i
i = 10
End Sub
'*************************************
Now, we actually have two different variables named i, one
with global scope, and one with local scope. The printed
result is now 1.
Since a lot of programmers use i in For..Next loops, you can
see how this could result in unpredictable results without
explicit variable declaration

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