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How do you implement the Observer pattern in JavaScript? What is the Factory pattern, and how does it differ from calling a class constructor directly? How is the Decorator pattern typically implemented in JavaScript? What is the Strategy pattern and how does it help avoid large if/else chains? What is the Command pattern and where is it useful in JavaScript apps? What is the Adapter pattern and why is it useful when integrating third-party code? When is it a mistake to force a 'classic' design pattern into idiomatic JavaScript?
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How do you implement the Proxy pattern using JavaScript's built-in Proxy object?

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Answer

JavaScript's native Proxy wraps a target object with traps (get, set, has, etc.) that intercept and can customize fundamental operations on it, letting you add validation, logging, or virtual properties transparently.
const validated = new Proxy({}, {
  set(target, prop, value) {
    if (prop === 'age' && value < 0) throw new Error('Invalid age');
    target[prop] = value;
    return true;
  }
});
validated.age = -5; // throws
Real-world example Building a reactive state object (like Vue 3's reactivity system) that tracks property reads and writes.

Common follow-ups: What performance cost does wrapping objects in Proxy typically add?

Proxy & Reflect
All 10 Design Patterns in JavaScript questions