Rebuild a damage system with arrow functions and default parameters.

Step 1: `baseDamage(level, power = 10)` is an arrow function that returns `level * power`. Step 2: `applyBonus(damage, bonus = 0)` is an arrow function that returns the damage plus the bonus. Step 3: `report(name, damage)` prints `"name deals X damage"`. Step 4: Zap is level 5. Report a plain attack (default power, no bonus), then a power 20 attack with a bonus of 15.

Expected output:

Write a generic function that keeps only the elements a callback approves, then use it with inline anonymous functions.

Step 1: `keep(list, test)` returns a new array containing only the elements for which `test(element)` returns `true`. The original array must not change. Step 2: use it to keep the levels of 10 or more, and print `"High levels: N"` with how many there are. Step 3: use it to keep the names longer than 3 characters, and print each of them on its own line. Step 4: print `"Original still has N"` with the length of `levels`, to prove it wasn't modified.

Expected output:

A battle announces events. Other code registers callbacks to react to them. Build the mechanism.

Step 1: the `battle` object has a `listeners` array (empty at first) and two methods. `onHit(callback)` adds the callback to `listeners`. `hit(attacker, damage)` calls every listener with the attacker's name and the damage. Step 2: register a first listener that prints `"attacker hits for X!"`. Step 3: register a second listener that adds the damage to `totalDamage`. Step 4: make Pyrochu hit for 30, then Zap for 25, then print `"Total: N"`.

Expected output: