Implementing a Sort for an Array
Sorting is hard.


console.log( sortArray([ '123', '9' , '1' , '11' ]) ); returns ["1", "9", "11", "123"], that does not sound good. After removing the numeric test I get ["11", "123", "1", "9"] which is better but not intuitive, I would have expected the 1 up front. This shows that you cannot just write the code, you need to build a set of tests to validate your code. Especially during interviews.
String.prototype.getFirstCase is very cute, but it needs commenting for lesser readers.
You are mixing var and let, I am not too excited about that
If you remove the numeric test, then you no longer need isNumeric as well
compareCaseSenitiveStrings should be compareCaseSensitiveStrings
I would convert 2 line comments to 1 line comment for readability, and an overall review of what comments really add value
Other than the code was very readable