Having not done any testing, I'm not even sure how I would start testing this. Any resource recommendations on testing this sort of simple function? Everything I've seen so far more relates to testing pieces inside complex apps.
Check out Mocha's in-browser test harness. It has you set up a page which pulls in the test framework Mocha and Chai (a library of assertions like equal, isTrue, lengthOf, and so on). You'll need to put your logic in a function so you can invoke it from your testcases as well as the "real" call sites, and ideally it should produce easily verifiable outputs, decoupling it from the display code; or alternatively you could look into mocking libraries like Sinon.
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