Added important scope information

docs/03.reference/01.functions/isnull/function.md

@@ -8,4 +8,24 @@ categories:
 description: Determines whether given object is null or not
 ---
 
-Determines whether given object is null or not
+Determines whether given object is null or not.
+
+**IMPORTANT** — When testing the presence of `null` values unscoped variables will use scope precedence to determine if the variable exists in any scope. This behavior differs from Adobe ColdFusion. So when testing when local variables in a function are `null`, it’s important to prefix the variable with the `local` scope. 
+
+For example, in the following code the variable `name` would return `false` for the `isNull()` if `name` ends up in a user supplied scope, such as the `URL` or `FORM` scopes:
+
+```
+function getName(){
+ var name = nullValue(); 
+ return isNull(name) ? "is null" : "is not null";
+}
+```
+
+In order to make sure that only the local `name` variable is checked, you would change the code to:
+
+```
+function getName(){
+ var name = nullValue(); 
+ return isNull(local.name) ? "is null" : "is not null";
+}
+```