Javascript 合并具有嵌套属性的对象

让我们看一下下面的例子：

var ref = {
    "fullName": {
        "rules": {
            "type": "string",
            "minLength": 4,
            "maxLength": 64
        },
        "description": "Full name of a user."
    }
};

var user = {
    "fullName": {
        "rules": {
            "required": true,
            "maxLength": 128
        },
        "message": "You have submitted a wrong full name."
    }
};

现在我想要的是这样的：

1. 合并对象和属性。
2. 保留第二个对象的属性（如果它们已设置）（maxLength）

以下是我期望的结果：

var res = {
    "fullName": {
        "rules": {
            "required": true,
            "maxLength": 128
            "type": "string",
            "minLength": 4
        },
        "description": "Full name of a user.",
        "message": "You have submitted a wrong full name."
    }
};

我尝试过的：

function mergeNestedObjects(firstObject, secondObject) {
    var finalObject = {};
    
    for (var propertyKey in firstObject) {
        var propertyValue = firstObject[propertyKey];
        
        if (typeof(propertyValue) === "object") {
            finalObject[propertyKey] = mergeNestedObjects(firstObject[propertyKey], secondObject[propertyKey]);
        } else if (secondObject[propertyKey] === undefined) {
            finalObject[propertyKey] = firstObject[propertyKey];
        } else {
            finalObject[propertyKey] = secondObject[propertyKey];
        }
    }
    
    return finalObject;
}

上面的函数合并但不知何故不嵌套属性。

更新与解答让它工作起来，我忘了迭代第二个对象，多么愚蠢。感谢@AnthonyGrist

function mergeProperties(propertyKey, firstObject, secondObject) {
    var propertyValue = firstObject[propertyKey];

    if (typeof(propertyValue) === "object") {
        return mergeNestedObjects(firstObject[propertyKey], secondObject[propertyKey]);
    } else if (secondObject === undefined || secondObject[propertyKey] === undefined) {
        return firstObject[propertyKey];
    }
    
    return secondObject[propertyKey];
}

function mergeNestedObjects(firstObject, secondObject) {
    var finalObject = {};
    
    // Merge first object and its properties.
    for (var propertyKey in firstObject) {
        finalObject[propertyKey] = mergeProperties(propertyKey, firstObject, secondObject);
    }

    // Merge second object and its properties.
    for (var propertyKey in secondObject) {
        finalObject[propertyKey] = mergeProperties(propertyKey, secondObject, firstObject);
    }
    
    return finalObject;
} 

function mergeProperties(propertyKey, firstObject, secondObject) {
    var propertyValue = firstObject[propertyKey];

    if (typeof(propertyValue) === "object") {
        return mergeNestedObjects(firstObject[propertyKey], secondObject[propertyKey]);
    } else if (secondObject[propertyKey] === undefined) {
        return firstObject[propertyKey];
    }

    return secondObject[propertyKey];
}

function mergeNestedObjects(firstObject, secondObject) {
    var finalObject = {};

    // Merge first object and its properties.
    for (var propertyKey in firstObject) {
        finalObject[propertyKey] = mergeProperties(propertyKey, firstObject, secondObject);
    }

    // Merge second object and its properties.
    for (var propertyKey in secondObject) {
        finalObject[propertyKey] = mergeProperties(propertyKey, secondObject, firstObject);
    }

    return finalObject;
}