Javascript merge object with nested properties

Question

Lets take a look at the example below:

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."
    }
};

Now what I want is this:

1. Merge objects & properties.
2. Keep the properties of the second object IF they are set already (maxLength)

Below is the result that I expect:

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."
    }
};

What I have tried:

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;
}

The function above merges but somehow doesnt nest the properties.

UPDATE & ANSWER got it working, I forgot too itterate through the second object, how dumb. Thanks to @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;
} 

Answer 1

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;
} 

Answer 2

Pretty old question, but can be useful. A little bit of recursion.

function mergeObjects(og, so) {
    for (var key in so) {
        if (typeof (og[key]) === 'object') {
            mergeObjects(og[key], so[key]);
        } else {
            if (og[key] || typeof (og[key]) === 'boolean') {
                og[key] = so[key];
            }
        }
    }
    return og;
}

mergeObjects(ref, user);