파트별 구현 옵져버 const obj = {}; Observer(obj, 'counter', (newValue, oldValue) => { console.log('LOG:', newValue, oldValue) }); obj.counter = 0; obj.counter = 1; // LOG: 0 undefined // LOG: 1 0 const Observer = (obj, property, callback) => { let value = obj[property]; Object.defineProperty( obj, property, { configurable: true, enumerable: true, set: (newValue) => { callback(newValue, value); value = n..