Return null if all properties are undefined

I don't understand what's wrong with JS. I thinked that properties property always return all properties, but in my case it's undefined!

I simply want a method returning null when all object's properties are undefined. Here is my realisation

function nullifyUndefined(obj) {
    debugger;
    for (var property in obj.properties) {
        if (obj.properties.hasOwnProperty(property) && typeof obj.properties[property] != 'undefined') {
            return obj;
        }
    }
    return null; // all properties are null, trait it as null object
}

But it doesn't work, simply becuase obj.properties is undefined. Here is a screenshot:

Why? And how to fix it?