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Proxy intercepts operations on an object — get, set, has, delete, function calls. Used for reactive frameworks (Vue 3), validation, observability, lazy loading.

Trap methods you can implement on the handler:

• get(target, prop, receiver) — property read
• set(target, prop, value, receiver) — assignment
• has(target, prop) — in operator
• deleteProperty(target, prop) — delete obj.x
• apply(target, thisArg, args) — function call
• construct(target, args, newTarget) — new
• ownKeys, getOwnPropertyDescriptor, defineProperty, getPrototypeOf, setPrototypeOf, isExtensible, preventExtensions

Real-world: validation:

Reflect is a companion API — Reflect.get(obj, prop) mirrors obj[prop], but in function form. Useful inside Proxy traps to delegate cleanly:

React 17/Vue 3 reactivity, mobx, and Object.observe (deprecated) all build on Proxy.