I'm willing to believe you, but I didn't make that up, I was just cribbing from the link: To reduce the time required to access JavaScript properties, V8 does not use dynamic lookup to access properties. Instead, V8 dynamically creates hidden classes behind the scenes.

I gather a "hidden class" is not an idiom of C++ as I'd assumed, rather something V8 people made up, but that's what they call it.