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'use strict';

class Amount {

// Abstraction with mutable state

#value = 0; // Private state, Hiding, Encapsulation

constructor(initial = 0) {

this.#value = initial;

}

update(delta) {

// Public interface: method

this.#value += delta; // Implementation, Encapsulation

return this.#value;

}

get value() {

// Public interface: getter

return this.#value; // Implementation, Encapsulation

}

}

class Spending extends EventTarget {

// Inheritance

#amount = new Amount(); // Structural Composition

update(delta = 1) {

const value = this.#amount.update(delta); // Delegation

const detail = { value, delta };

this.dispatchEvent(new CustomEvent('change', { detail }));

return value;

}

get value() {

return this.#amount.value; // Delegation

}

}

const config = { available: 10000 }; // Immutable configuration

const budget = new Amount(config.available); // Value Object

const spending = new Spending(); // Entity

const balance = new Amount(config.available);

const report = (purchase, logger) => {

// Dependency Injection

for (const [key, instance] of Object.entries(purchase)) {

logger(`${key}: ${instance.value}`); // Polymorphism

}

};

spending.addEventListener('change', (event) => {

const { value, delta } = event.detail;

balance.update(-delta); // Tell Don't Ask

console.log({ event: event.type, value, delta });

});

spending.update(1023);

spending.update(1507);

spending.update(1010);

const purchase = { budget, spending, balance }; // Aggregation

report(purchase, console.log); // Dependency Injection