How to get the Exit Transformer to return a null in the response body

When using the Exit Transformer plugin, you may want to return a field with a null value in the response body. However, if you try to assign a field a value of null directly in your Lua code, you will encounter an error because null is not defined in Lua.

For example:

return function(status, body, headers) 
    if status > 399 then
        body = { hello = "test", goodbye = null}
    end

    return status, body, headers
end

In the above code, null is not defined, so you will get an error in your logs.

To avoid this error and return a field with a null value in the response body, we need to make a few updates to the environment variables and then change the Lua slightly to require cjson.

First, set the following environment variables:

KONG_UNTRUSTED_LUA: "sandbox"
KONG_UNTRUSTED_LUA_SANDBOX_REQUIRES: "cjson"

Review the untrusted Lua configuration reference before updating this in your environment.

Next, let’s add the following to our Exit Transformer. Saved this to a file called bodyNull.lua:

return function(status, body, headers) 
    if status > 399 then
        body = { hello = (require "cjson").null }
    end

    return status, body, headers
end

Then we need to create the Exit Transformer plugin:

curl -X POST 'http://localhost:8001/plugins' \
--header 'Kong-Admin-Token: <token>' \
--form 'config.functions=@"/<pathtofile>/bodyNull.lua"' \
--form 'name="exit-transformer"'

For this specific test I created a request-termination plugin set to a status of 400 to trigger the Lua from the Exit Transformer.

curl -X POST http://localhost:8001/plugins \
    --header 'Kong-Admin-Token: <token>' \
    --data "name=request-termination"  \
    --data "config.status_code=400" \

Now if we run a request we can see that the field hello is returned with a null value:

{
	"hello": null
}