Understanding the JavaScript 'typeof' Operator

When working with JavaScript, you'll often encounter the typeof operator that's used to determine the data type of a given expression. An interesting feature of JavaScript is its ability to create and utilize classes and extending other classes. But what happens when you try to extend a class with multiple classes as seen in the question?

The Correct Answer: "undefined"

If you try to evaluate the expression in the question, JavaScript will return "undefined". Let's break down why this happens.

In the provided expression typeof (new (class F extends (String, Array) { })).substring, we're trying to create a new instance of class F, which is declared to extend from both String and Array.

But here's the catch: in JavaScript, a class can only extend a single class, not multiple. JavaScript uses a comma operator here, causing it to discard the first operand (String) and use the second operand (Array) for the extends keyword. So, essentially, the class F is extending the Array class, not the String class.

Now, when you try accessing the substring method, which is a method of the String prototype and NOT the Array prototype, it will be undefined as it's not available in the Array prototype.

Practical Application

It's important to remember that typeof operator in JavaScript detects the data type of its operand. Harnessing typeof can allow you to write scripts that can process data differently depending on its type.

However, the key takeaway is understanding that JavaScript only supports single inheritance - a class can only extend from another class. If you require functionality from multiple different classes, consider using features like Mixins.

Best Practices and Additional Insights

When you're dealing with inheritance in JavaScript, make sure to clearly understand the single-inheritance model. If you need more functionality from different class types, there's usually much more maintainable ways to design your code using different design patterns like Mixins or composition.

Understanding exactly how the typeof operator works is crucial for avoiding potential bugs or confusion down the line. It's always a good idea to experiment with typeof and other operators so that you can better understand their output and behavior.