n and m values are reversed in examples

JS: Node v14 should be used along with its appropiate assertion tools (Mocha + Chai).
Refer to this and this

JS: Most test cases in the random test expect "No result"

JS: Node v12 should be used along with its appropiate assertion tools (Mocha + Chai).
Refer to this and this

JS: Node v12 should be used along with its appropiate assertion tools (Mocha + Chai).
Refer to this and this

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Please review and approve.

There're already approved katas asking to find the most frequent value in the input.

Please use Test.assertEquals instead of Test.assertSimilar. The quoting and escaping is driving me crazy.

Returning inconsistent datatypes is not a best practice. Consider requesting -1 as a return value, or an Error be thrown.

Please specify that "the interval from a to b" is inclusive.

that is maximal after division of its digits.

You should specify that the division is only between tens and units (tens / units, and only in that way).
Explaining some of the example results would make it clearer imho.

If multiple letters appear with the maximum frequency, return the letter that appears earliest in the alphabet.

However, the tests apparently ask for the letter that occurs earliest in the input string.

If it has to be the alphabet, please specify whether A comes before or after a (should they appear with the same frequency). And fix the tests.

Otherwise, please update the description.

Too many 'No result' in random tests.
Can you make it appear less? ;-)