How to edit the function with the output of MQTT OUT

As the figure 1 below, I am going to use the MQTT OUT node that sends the downlink data to TTN. But in the debug window, I have two signal humidity and light in msg.payload.payload-fields, but I just want to send one signal in downlink to TTN, so I use a function node, which included the payload of humidity seen in figure 2.
However in TTN console, the downlink payload format is wrong, what I want is a float number like payload_fields.humidity, so I guess the problem is from the function I edits, the first line maybe wrong, how can I write it in the correct Javascript code?
Thanks so much!!!