Transact SQL :: Creating Comma Separated Value Logic Not Working Inside Function?

I am need to create comma separated list in sql and I am using below query.

declare @ConcatenatedString NVARCHAR(MAX)
select @ConcatenatedString = COALESCE(@ConcatenatedString + ', ', '') + CAST(rslt.Number AS NVARCHAR)
from
(
select 1 Number
union
select 2 Number
union
select 3 Number
)rslt
select @ConcatenatedString

When I use the above code inside a function, i am not getting desired output.

create function GetConcatenatedValue
AS
(
declare @ConcatenatedString NVARCHAR(MAX)
select @ConcatenatedString = COALESCE(@ConcatenatedString + ', ', '') + CAST(rslt.Number AS NVARCHAR)
from
(
select 1 Number
union
select 2 Number
union
select 3 Number
)rslt
return @ConcatenatedString
)