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.

Pretty useful, thank you!

I thought setting a readOnly private property with just the getter would avoid the need to write the setter. In case anyone else is wondering about this, TypeScript would fail to compile if trying to access a private property even when there's a getter with that same name. So, yep, the setter is necessary.

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);