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The bug here is actually just in the condition of your while loop:

while(this.players.length !== 0 || this.enemies.length !== 0)

Your condition says to loop while there is at least one player OR there is at least one enemy. So as long as ONE of the arrays is not empty, it will continue to loop.

But when you first create this.players and this.enemies, they start at different sizes. Then when you remove one entry from each array, eventually one of the arrays is empty before the other.

Then your code has var randomEnemyIndex = Math.floor(Math.random() * this.enemies.length); which will evaluate to 0 when the array is empty. And when you do this.enemies[0], it returns undefined. When undefined is passed into weapon.attack like weapon.attack(randomPlayer, undefined), then it tries to call applyDamage(dmg) on the undefined, which throws your exception.

If you modify your code to have the following console logs, you will see the issue: