I copied the latest code (adding the .filter((k) => k === convertToSlug(k)) correction but leaving in the Object.hasOwn method), and I do not get any error messages. I want to spend more time with your snippet, but just wanted to pop in to say that first 
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I love what you guys are working on here!
The list gets a little ridiculously long when I run it, because I don’t have a large vault but I do have a TON of unique inline keys since I used that as workaround for Obsidian not supporting description lists…I might have to script something to convert all of those to html now.
I realized after reviewing Map objects that my last encounters with JavaScript were back when ES6 had just been released, and none of my study materials/instructors covered them yet! I will definitely be playing around with my frontmatter keys snippet to use a Map object instead, like you suggested!