ArrayAppend returning boolean instead of array - data-structures

Question

OK I have been struggling with this for sometime and I figured I would go back to the basics and try and figure out my error, as I get the same error here as I do down the line.
Here is my Code:
<cfparam name="session.cart" default="arrayNew(1)">
<cfset session.cart = arrayAppend(session.cart,structNew() )>
<cfset thisCartItem = arraylen(session.cart)>
<cfset session.cart[thisCartItem].itemID = "X">
<cfset session.cart[thisCartItem].quantity = "X">
<cfset session.cart[thisCartItem].itemName = "X">
<cfdump var="#session.cart#">
Here is my error:
Object of type class java.lang.Boolean cannot be used as an array

Answer 1

The error means exactly what it says. The variable session.cart is not an array. It is a boolean value. If you read the ArrayAppend documentation it explains why. The function modifies the array in place and returns a boolean value:
Returns True, on successful completion.
By capturing the result here, you are actually setting the cart value to true/false:
<!--- Note the addition of the pound signs in the CFPARAM --->
<cfparam name="session.cart" default="#arrayNew(1)#">
<cfset session.cart = arrayAppend(session.cart, structNew())>
Having said that, you could simplify the code a lot by using the shorthand syntax supported in all recent versions of CF ie {} - new structure and [] - new array. To create a new array:
<cfset session.cart = []>
To append one or more structures to the end of that array:
<!--- append first structure to array --->
<cfset ArrayAppend( session.cart
, { itemID = "X", quantity = "X", itemName = "X" }
)>
<!--- append second structure to array --->
<cfset ArrayAppend( session.cart
, { itemID = "Y", quantity = "Y", itemName = "Y" }
)>