What is the data coming in? Add a debug node (set to display the complete msg object) to the output of the node feeding the function node.
Then copy past the results to a reply
There’s a great page in the docs (Working with messages : Node-RED) that will explain how to use the debug panel to find the right path to any data item.
Pay particular attention to the part about the buttons that appear under your mouse pointer when you over hover a debug message property in the sidebar.