`a.class` is explicitly legal in ES5, which is the spec that explicitly allows trailing commas in the `[,,,].join(", ")` question, so it's at least a bit confus

`a.class` is explicitly legal in ES5, which is the spec that explicitly allows trailing commas in the `[,,,].join(", ")` question, so it's at least a bit confused about which version of JavaScript it's quizzing.