Functions, Scope & Object Model

Classes, Private Fields & Methods

JavaScript classes are syntax over the prototype-based object model, with modern support for private fields and methods enforced by the runtime.

Class methods are prototype methods
Instance fields live on each object
#private fields are runtime-private
Constructors establish invariants
Classes still use JavaScript this rules

Class with runtime-private state

class RateLimiter {
  #remaining

  constructor(limit) {
    this.#remaining = limit
  }

  tryConsume() {
    if (this.#remaining <= 0) return false
    this.#remaining -= 1
    return true
  }
}

const limiter = new RateLimiter(2)

`#remaining` is enforced by the language runtime.

Class pieces
Piece	Where it lives	Notes
constructor	Called by new	Initialize instance
method	Prototype	Shared by instances
public field	Instance	Per-object state
#private field	Instance, private brand	Runtime enforced
static method	Constructor function	Class-level helper