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const handler = { get(target, key) { return key in target ? target[key] : `No key: ${key}`; }, set(target, key, val) { if (typeof val !== "number") throw TypeError("Numbers only"); return Reflect.set(target, key, val); } }; const proxy = new Proxy({}, handler); proxy.age = 30; // OK proxy.name = "x"; // TypeError
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