What is a Proxy in JavaScript and what are its use cases?

A Proxy wraps an object and intercepts fundamental operations (get, set, delete, etc.) via trap functions. Syntax: const proxy = new Proxy(target, handler);. Common traps: get(target, prop): Intercept property access. set(target, prop, value): Intercept property assignment. has(target, prop): Intercept the in operator. deleteProperty(target, prop): Intercept delete. Use cases:

1. Validation: Throw errors when invalid values are set on an object.
2. Logging/debugging: Log all property accesses.
3. Default values: Return a default when a property doesn't exist (avoid undefined).
4. Reactive systems: Vue.js 3's reactivity system is built on Proxy.
5. Immutable objects: Prevent writes in the set trap. LWC's internal reactivity for @track uses Proxy-like mechanisms under the hood.