Read only property ES6

read-only-property-es6.js

/**
 * Class that holds a read only property.
 */
class PropertyBag {

  /**
   * Accessor.
   *
   * @return {string}
   *   The value. This annotation can be used for type hinting purposes.
   */
  static get readOnly() {
    return 'This property can only be read.';
  }

  /**
   * Mutator.
   *
   * @param {*} value
   *   The passed in value.
   *
   * @throws {Error}
   *   Inconditionally.
   * @return {void}
   */
  static set readOnly(value) {
    throw new Error(`The readOnly property cannot be written. ${value} was passed.`);
  }

}

console.log(PropertyBag.readOnly); // This property can only be read.
PropertyBag.readOnly = 'Ha!'; // Error: The readOnly property cannot be written. Ha! was passed.

class Todo {
        constructor(data) {
                this.__getId__ = function () { return data.id };
        }

        get id() { return this.__getId__(); }
}

var todo = new Todo({id: 123456});

console.log(todo.id);        // return 123456

todo.id = 654321;

console.log(todo.id);        // return 123456

OR

class Todo {
        constructor(data) {
                this.__getId__ = function () { return data.id };
        }

        get id() { return this.__getId__(); }
        
        set id(v) { throw new Error('Todo id property is read only'); }

}

var todo = new Todo({id: 123456});

console.log(todo.id);        // return 123456

todo.id = 654321;        // throw error

console.log(todo.id);        // return 123456

The problem with this is that we cannot even change the value from within the class.

class Todo {
  #data
  constructor(data) {
    this.#data = data
  }

  get findData() {
    return this.#data;
  };

  set findData(value) {
    throw new Error("Todo findData property readonly")
  } 
}

const newTodo = new Todo({
  id: 1,
  name: "lorem",
  description: "lorem ipsum"
});

newTodo.findData = {
  id: 2,
  name: "lorem 2",
  description: "lorem ipsum 2"
}

console.log(newTodo.findData);