Your solution is mutating the input array and as a consequence, the tests are misbehaving. It is indeed a kata issue because the tests should not be vulnerable to this, but at the same time your solution is not exactly correct:
This function takes the array line as a parameter and returns a new array with the tile values from line slid towards the front of the array (index 0) and merged.
Vulnerability for input mutation fixed.
Test for input mutation added.
Because the goal of our function is to implement "one swipe"(if this explanation makes the goal clearer):
A given tile can only merge once. (this limits any tile to be summed/combined only 1 time)
Your solution is mutating the input array and as a consequence, the tests are misbehaving. It is indeed a kata issue because the tests should not be vulnerable to this, but at the same time your solution is not exactly correct:
This comment is hidden because it contains spoiler information about the solution