How to correctly check if an object property in JavaScript is undefined

Hello everyone! I'm new to JavaScript and encountered a problem when checking object properties. I have a user object: ```javascript const user = { name: 'Ivan', age: 25 }; ``` I want to check if the object has a property `email` and if it is `undefined`. But I'm confused about the different ways to check. For example, if I write: ```javascript if (user.email === undefined) { console.log('email is not defined'); } ``` This works, but I've heard that this is not safe because a property can be explicitly set to `undefined`. Then I tried the `in` operator: ```javascript if ('email' in user) { console.log('property exists'); } ``` But here I don't understand the difference between "property does not exist" and "property exists but is undefined". In my