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What is a class in JavaScript, and is it a new kind of object model? How does the constructor method work in a class? How do you create a subclass using extends? What does the # syntax mean for class members? What are static methods and properties, and when do you use them? How do getters and setters work inside a class? How does method overriding with super work in JavaScript classes? What is a static initialization block and why would you use one? Why can classes not be called without 'new', unlike regular functions?
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What are class fields, and how do they differ from constructor-assigned properties?

Intermediate

Answer

Class fields let you declare and initialize instance properties directly in the class body, outside the constructor, which runs before the constructor body executes. They reduce boilerplate especially with inheritance and default values.
class Counter {
  count = 0; // class field
  increment() { this.count++; }
}
Real-world example Declaring default component state fields cleanly at the top of a class, without cluttering the constructor.

Common follow-ups: Are class fields defined before or after super() runs in a subclass?

Objects Property Descriptors & Immutability
All 10 Classes & Class Syntax questions