Please make it return a boolean, why return YES or NO strings?

Why not just foreach (...) { } instead of ToCharArray().ToList().ForEach(...)

This loops through only once unlike most other solutions listed.

Another one:
Expected: [[231100238855664255, 44847850151521494], [353362493576484, 44847850151521494], [7580086324199136, 44847850151521494]], instead got: [[77033412951888085, 14949283383840498], [117787497858828, 14949283383840498], [2526695441399712, 14949283383840498]]

Incorrect test:

Expected: [[80696, 136240], [87360, 136240], [102180, 136240]], instead got: [[20174, 34060], [21840, 34060], [25545, 34060]]

The given result has a lower denominator than the expected result and the fractions are equivalent, therefore the given result should be correct.

This isn't really solving it, it's just using the method they ask you to implement..

you left off the "var" for your r declaration

you don't specify the max number of allowed decimal places, is there one?

"If the test case says: "B sends three duplicates to A" that means that A sent some data to B and B received it three times."
I think you mean:
"If the test case says: "A sends three duplicates to B".."

Description is unnecessarily lengthy, just cut it down to your description of what memoizing/caching is and how it would improve the efficiency. Your comments in the answer section could be moved to the description section too and then the description would be good.

Also - my initial solution (without DOM) failed because it enclosed the href value with double quotes instead of single quotes.....

Sucks that it has to run in Node.js without any DOM implementation - the solution is much more elegant when you have access to the DOM API.

Why are you assuming it's a number?