> Please write a function in JavaScript that takes in a string as input and returns true if it contains a valid roman numeral and false otherwise.

Your question actually isn't worded concisely enough. You don't specify whether the string can merely contain the roman numeral (plus other, non-roman-numeral text), or must entirely consist of just the roman numeral. The way "if it contains" is used colloquially, could imply either.

I'd use either "if it IS a roman numeral" if it must consist only of a roman numeral, and "if there exists a roman numeral as part of the string" or some such, otherwise.