The number of tests should be increased (this incorrect solution may pass all tests occasionally). I also recommend to add small random tests (random 10 x 10 matrices).

Potentially add some random tests of a smaller grid size? My earlier solutions passed all fixed tests but failed some random, and it is difficult to debug when the grids are too large to output.

Hard coding 200_000? Better to keep working on your solution until it does the job within the given time.

Sorry, I missed something in description. Nice kata!

210 - 158 = 52 > 35, so the second row is not harmonious.

Please, test this:

assert.strictEqual(HarmoniousArray([ [ 219, 165, 229, 164, 207, 182, 186, 134, 188 ],
  [ 215, 208, 161, 222, 191, 135, 175, 155, 131 ],
  [ 146, 205, 229, 203, 223, 153, 139, 221, 212 ],
  [ 140, 158, 184, 210, 134, 160, 182, 214, 153 ],
  [ 172, 145, 226, 170, 156, 197, 146, 204, 208 ] ], 35), 5)
});
  
It should be 6 heads.

[[205, 229, 203],
[158, 184, 210]]

Thanks.

I have been trying to come up with a purely functional solution to this kata, but at last I have to turn to lodash for the deep merge.

Anybody knows how to figure this nicely without using lodash?