How to properly check if an object property exists in JavaScript (undefined)

Hi everyone! I started learning JavaScript and encountered a confusing situation. I have an object with user data, and I want to check if a certain property exists in it. But I'm confused between `undefined`, `null`, and the property not being declared at all. Here's an example. Suppose I have an object: ```javascript const user = { name: 'Ivan', age: 25 }; ``` I want to check if the user has an `email` property. If I write: ```javascript if (user.email === undefined) { console.log('Email not specified'); } ``` This seems to work, but I've heard that doing it this way is incorrect. Why? After all, `user.email` does return `undefined`. But what if I deliberately set a property with the value `undefined`? ```javascript const user2 = { name: 'Maria',