JS factory functions via a proxy

proxy-factory.js

// https://github.com/Schniz/factoree
/**
 * Creates a strict factory
 *
 * @param defaults the default values that would appear on all instances
 */
function factory(defaults) {
    return attributes => {
        const data = Object.assign(Object.assign({}, defaults), attributes);
        const proxy = new Proxy(data, {
            has(target, p) {
                return target.hasOwnProperty(p);
            },
            ownKeys(target) {
                return Reflect.ownKeys(target);
            },
            get(target, p) {
                var _a;
                if (((_a = target === null || target === void 0 ? void 0 : target.hasOwnProperty) === null || _a === void 0 ? void 0 : _a.call(target, p)) || target[p]) {
                    return target[p];
                }
                else if (isExceptionKey(p)) {
                    return undefined;
                }
                else {
                    throw new Error(`Can't access undefined key '${inspect(p)}' for object ${inspect(target)}`);
                }
            },
        });
        return proxy;
    };
}
const JEST_INTERNAL_KEYS = [
    Symbol.iterator,
    Symbol.toStringTag,
    'asymmetricMatch',
    'nodeType',
    '$$typeof',
    '@@__IMMUTABLE_ITERABLE__@@',
    '@@__IMMUTABLE_RECORD__@@',
];
const PROMISE_REQUIRED_KEYS = [
    // See https://github.com/Schniz/factoree/issues/2
    'then',
];
const EXCEPTION_KEYS_THAT_MUST_EXIST = [
    ...JEST_INTERNAL_KEYS,
    ...PROMISE_REQUIRED_KEYS,
];
function isExceptionKey(p) {
    return EXCEPTION_KEYS_THAT_MUST_EXIST.indexOf(p) > -1;
}

const createUser = factory({ twitterHandle: undefined, tester: () => 'xyz' });

const gal = createUser({ name: 'Gal', twitterHandle: 'galstar' }); // => User
const joe = createUser({}); // => User

console.log(gal.tester())