I'm having a difficult time to understand , how to implement the behavior of an enemy in a tile based game, 

* movement
* attack
* behavior per enemy level ( higher level enemies should be hard to kill)
* defend
* other behavior

since the game is a tile game written in javascript, 
the behavior of the enemies should be implement as functions like

* `Chaser` - hunts its opponents
* `Brave` - does not start running if with low HP
* `Sensitive` - when damaged might take additional damage

and using the standard entity class , based on the configuration of the enemy, could `extend`  the behavior?

Or write like standard enemy behavior and additionally write separate scripts per enemy on how they should behave against the opponent, surroundings, etc..