i also dont understand. 16 + 6 = 22, not 19. I mean by 5:45 the number of chimes is 16. then at 6:00 it chimes 6 more times. dont we sum up 16 and 6?

Just to spell this out in detail, "chimes out the hour" means that it chimes once at 1:00, twice at 2:00, 3 times at 3:00, and so on, up to 12 times at 12:00.

03:38, 19 return 06:00
should notice the cuckoo clock will chimes out the hour
03:45 total chimes = 1
04:00 total chimes = 5

Use spoiler flag next time please.

Because you didn't test your code there with the input value that makes your code fail here, read the error message:

TypeError: Reduce of empty array with no initial value

Your code fails this test:

assert.strictEqual(solution(''), '');