JavaScript Watch helper

watch.js

/**
 * @example
 * const item = watch({}, (value, prop, source) => {
 *     if (prop === 'foo' && !value) {
 *         return 'default_value';
 *     }
 *     
 *     if (prop === 'baz') {
 *         return 'nope';
 *     }
 *     
 *     return value;
 * });
 * 
 * delete item.foo;
 * console.log(item.foo); // return 'default_value'
 * 
 * item.baz = 'ok';
 * console.log(item.baz); // return 'nope'
 * 
 * @param value
 * @param handler
 */
export function watch(value, handler = (x, y, z) => x) {
    return new Proxy(value, {
        defineProperty(target, prop, descriptor) {
            target[prop] = handler(descriptor.value, prop, target);
            return target;
        },
        deleteProperty(target, prop) {
            const value = handler(undefined, prop, target);
          
            if (value !== undefined) {
                target[prop] = value;
            }
            
            return target;
        }
    });
}