In my flow, I read and run (for-each) an array of JSON objects. I insert each object in the same file with APPEND type.
I want to store each JSON with indent as false (one line by JSON) and a carriage-return like this example:
{"hello:"world1"}
{"hello:"world2"}
{"hello:"world3"}
I use this:
%dw 2.0
output application/json indent=false
---
payload ++ '\r'
But it returns an error about cannot coerce Object to String. How can I solve this?

application/json is technically an object, not a string. So you cannot concatenate directly.
This works for me, to get the desired result:
write as json first to use the writer property to remove indentation then convert to string and concatenate and output as String application/java