Sure, there's a way to use reduce and do this all in one line. This v2 is 477 bytes as is.

 test data:
 const sample_input = [[7,8,100,101,222,223],
                [ 2,3,88,89,90,103,177 ],
                [ 2,3,6,7,10,11 ],
                [ 1 ],
                [ 1,2 ]]

const sample_output = [ "4/5","5/6","3/6","1/1","1/2" ]

and test with:
console.log(app(sample_input).toString() === sample_output.toString() ? "success" : "fail")