Closure CavesISSUE #41 of 61

classes · inheritance · extends · super

LambdaVSThe Mutator

Lambda saysextends creates a subclass; super calls the parent constructor and methods.

class Admin extends User inherits everything from User. In the subclass constructor you must call super(...) before using this. super.method() calls the parent's version, so you can extend behavior rather than replace it. Favor shallow hierarchies — deep inheritance gets brittle.

Power-ups you unlock

extends inherits from a parent class
Call super(...) before this in the constructor
super.method() reaches the parent method
Keep hierarchies shallow

The Mutator attacks — common mistakes

Using this before super() (throws)
Deep inheritance chains that are hard to follow
Overriding without calling super when you meant to extend

Boss battleSubclass a base class, call super in the constructor, and extend a method.

Example code

<!doctype html><html><head><meta charset="utf-8"></head>
<body style="background:#06040d;color:#e6e0ff;font-family:monospace;padding:20px"><pre id="o"></pre>
<script>class User{constructor(n){this.n=n;} role(){return 'user';}}
class Admin extends User{ role(){ return super.role()+' (admin)'; } }
document.getElementById('o').textContent=new Admin('drew').role();</script></body></html>

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