ReadOnlyProperty

ReadOnlyProperty<T> (from scenerystack/axon) is the base class that Property extends. It implements everything about an observable value — .value/.get(), link(), lazyLink(), unlink(), dispose(), PhET-iO serialization — except the ability to set the value from outside: its set()/value= accessors are protected, and its constructor is protected too, so you never instantiate ReadOnlyProperty directly. Subclasses fall into two families: Property (and its typed siblings BooleanProperty, NumberProperty, ...) re-expose set/value= as public, making the value externally settable; DerivedProperty and DynamicProperty leave them protected, so the only way their value changes is through their own internal logic.

import { ReadOnlyProperty, DerivedProperty, NumberProperty } from 'scenerystack/axon';

const widthProperty = new NumberProperty( 4 );
const heightProperty = new NumberProperty( 3 );

const areaProperty: ReadOnlyProperty<number> = new DerivedProperty(
  [ widthProperty, heightProperty ],
  ( width, height ) => width * height
);

areaProperty.isSettable(); // false - DerivedProperty never overrides this
// areaProperty.value = 100; // compile error: 'value' is protected on ReadOnlyProperty

Why public APIs say ReadOnlyProperty<T>, not Property<T>

When a class exposes a derived or gated value — isValidProperty, areaProperty, anything built with DerivedProperty or DynamicProperty — its public field should be typed ReadOnlyProperty<T> (or the narrower TReadOnlyProperty<T> interface), never Property<T>. Typing it as Property<T> would be a lie: it advertises a settable .value= that either doesn't exist at runtime (TypeScript would happily compile areaProperty.value = 5, since Property widens the type) or, worse, silently succeeds and desyncs from the derivation. Reserve the Property<T> type annotation for fields you actually construct as a plain, externally-settable Property (or subclass).

Public API

ReadOnlyProperty provides the full read side of the Property API; only set/value= are missing (protected) compared to Property:

Member	Effect
value (getter) / get()	The current value
link( listener ) / lazyLink( listener )	Subscribe to changes, with or without an immediate callback
unlink( listener ) / unlinkAll()	Remove one or all listeners
hasListener( listener ) / hasListeners()	Inspect subscriptions
isSettable()	Returns false on ReadOnlyProperty and any subclass that doesn't override it (e.g. DerivedProperty); Property overrides it to true
isValueValid( value ) / getValidationError( value )	Check a candidate value against the Property's validator without setting it
linkAttribute( object, attributeName )	Convenience: sets object[attributeName] = value on every change
toString() / debug( name )	Console-debugging helpers
dispose()	Removes all listeners and marks the instance disposed

Use isSettable() to branch on capability at runtime

Because both Property and ReadOnlyProperty share this same base type, isSettable() is the correct runtime check when code receives a TReadOnlyProperty<T> and needs to know whether it's safe to attempt a cast-and-set (some PhET-iO and Studio tooling does exactly this). Don't rely on instanceof Property for this — prefer the method, since it's what the class itself uses internally to guard PhetioStateEngine value-setting.