Decorator Pattern — Add behaviour at runtime without subclassing
Wrap the same interface to add behaviour without subclass explosion. · Decorators can stack — each delegates inward.
Watch
Watch, then scroll down for code and practice.
In code
interface Beverage {
cost(): number;
describe(): string;
}
class Coffee implements Beverage {
cost() {
return 40;
}
describe() {
return "Coffee";
}
}
class Milk implements Beverage {
constructor(private inner: Beverage) {}
cost() {
return this.inner.cost() + 10;
}
describe() {
return this.inner.describe() + " + milk";
}
}📘 Key ideas
The pattern
Wrap an object with another object that has the same interface. The wrapper delegates to the inner object and adds behaviour before/after.
Avoids class explosion
Without Decorator, MilkCoffee, SugarCoffee, MilkSugarCoffee, WhipCoffee... → 2^N classes. With Decorator: just add wrappers at runtime.
Key rule
Every decorator must accept ANY object of the base type, including other decorators. This is what enables stacking.
Real-world
Java's BufferedInputStream wraps FileInputStream. HTTP middleware chains. Express/Koa middleware. All are Decorator.
🧠 Practice — Apply What You Learned
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