declaring a variable in CF

I thought when a variable is needed, CF developer can just set it on the fly, <cfset var MyVar="assign a value here">
I'm writing a simple UDFs, and did not declare a variable. The error I got was Variable is undefined. Should I declared the variable when it is used within a function?

<cffunction name="InUser">
<cfargument name="EmpID" required="true" type="numeric">
<cfargument name="DeptArray" required="true" type="array">

<cfset NewUserID = 0> ----????????? If Omitted this, I got error saying NewUserID is Undefined.
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<CFSET NewUserID = GetNewUser(UserName)>
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Then I use the value of NewUserID for something else down here.
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</cffunction>

<cffunction name="GetNewUser">
select statement to get userID based on userName

<CFRETURN #GetID.UserID#>
</cffunction>