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Proxy Pattern

Proxy Pattern

Introduction

The Proxy pattern provides a surrogate that controls access to another object. JavaScript has built-in Proxy for this.

Implementation

const user = { name: 'Alice', role: 'user' };

const protectedUser = new Proxy(user, {
  get(target, prop) {
    if (prop === 'password') {
      throw new Error('Access denied');
    }
    return target[prop];
  },

  set(target, prop, value) {
    if (prop === 'role' && target.role === 'admin') {
      throw new Error('Cannot change admin role');
    }
    target[prop] = value;
    return true;
  }
});

console.log(protectedUser.name);    // 'Alice'
// protectedUser.password;          // Error: Access denied

// Caching proxy
function createCacheProxy(fn) {
  const cache = new Map();
  return new Proxy(fn, {
    apply(target, thisArg, args) {
      const key = JSON.stringify(args);
      if (cache.has(key)) return cache.get(key);
      const result = target.apply(thisArg, args);
      cache.set(key, result);
      return result;
    }
  });
}

Summary

Controls access to target objects
Built-in JavaScript Proxy API
Use cases: validation, caching, logging, access control

Further Reading