JavaScript Setter

​

Syntax

const entity = {
  _internalState: null,
  
  set state(value) {
    this._internalState = value;
  }
};

class Entity {
  #internalState = null; // Private field backing the setter

  set state(value) {
    this.#internalState = value;
  }
}

​

Invocation

const instance = new Entity();

// Triggers the setter, passing 'active' as the argument
instance.state = 'active'; 

​

Technical Constraints and Behavior

• Arity: A setter method must have exactly one parameter. Attempting to define a setter with zero or multiple parameters will throw a SyntaxError.
• Property Descriptors: Defining a setter creates an accessor property. In the object’s property descriptor, this property will have a set attribute (pointing to the function) instead of value and writable attributes.
• Return Value: The return value of a setter function is entirely ignored by the JavaScript engine. The result of the assignment expression is always the value that was assigned, regardless of what the setter returns.
• Naming Collisions: An object cannot have a data property and an accessor property with the same name. To prevent infinite recursion (stack overflow), a setter must not assign a value to its own property identifier. It must mutate a separate backing property (conventionally prefixed with _ or declared as a private # field).

​

Dynamic Definition

const dynamicEntity = { _count: 0 };

Object.defineProperty(dynamicEntity, 'count', {
  set(value) {
    this._count = Number(value);
  },
  enumerable: true,
  configurable: true
});

dynamicEntity.count = "5"; // Invokes the dynamically defined setter