Proxy and Reflect

                new Proxy(target, handler) / Reflect.get/set/has/deleteProperty(...)
              

Purpose

A Proxy wraps a target object and intercepts fundamental operations (property get/set, has/in, deleteProperty, and more) through handler functions called traps. Reflect provides the default implementation of each such operation as a plain callable function.

Syntax

const handler = {
  get(target, prop, receiver) {
    console.log(`Reading ${String(prop)}`);
    return Reflect.get(target, prop, receiver);
  },
  set(target, prop, value, receiver) {
    if (prop === 'age' && value < 0) throw new TypeError('age must be non-negative');
    return Reflect.set(target, prop, value, receiver);
  },
};

const user = new Proxy({ name: 'Ada', age: 30 }, handler);

Behavior notes

• A trap that isn’t defined in the handler falls back to the target’s default behavior automatically — a Proxy only needs to define the traps it actually wants to customize.
• Certain invariants are enforced regardless of trap logic — a get trap cannot report a different value than a non-configurable, non-writable property’s actual value; violating an invariant throws a TypeError automatically.
• Reflect’s methods mirror the low-level operations Proxy traps intercept, one-to-one — using Reflect.set inside a set trap, rather than target[prop] = value, correctly passes through the receiver argument, which matters for inherited proxies.
• Every trapped operation adds overhead compared to plain object access, and makes the object’s behavior opaque to simple inspection (console.log, a debugger) — a real cost to weigh against the interception’s benefit.

Common mistakes

• Reaching for a Proxy for simple validation where a factory function with explicit getter/setter methods would be easier to debug and reason about statically.
• Forgetting to pass receiver through in a set/get trap when delegating to Reflect, breaking correct behavior for a proxy used as another object’s prototype.

Runtime notes

Core ECMAScript (ES2015), identical behavior across every current browser and Node version, though performance characteristics can differ between engines.