How can you determine if a css class exists with Javascript?

Question

Is there a way to determine whether or not a css class exists using JavaScript?

Answer 1

Oneliner:

[].slice.call(document.styleSheets)
.reduce( (prev, styleSheet) => [].slice.call(styleSheet.cssRules))
.reduce( (prev, cssRule) => prev + cssRule.cssText)
.includes(".someClass")