JS: Class Private Fields and Methods

What is private elements

in the body of a class,

class ClassName {body}

Private elements are names that start with hash #.

example

• #var_name;
• #var_name = value;
• #method_name (params) {body}
• get #getter_name () {body}
• set #setter_name (params) {body}

static elements can also be private.

Scope of private elements

Private elements are only visible within the class body.

Access private elements

Private instance elements

• this.#var_name for instance variable
• this.#method_name(arg) for instance methods.

// basic example of private variable
class Burger {
 #secret_sauce = "eggs and milk";

 show_sauce() {
  return this.#secret_sauce;
 }
}

// create a instance
const my_burger = new Burger(1);

console.log(my_burger.show_sauce());
// "eggs and milk"

Private static elements

• Constructor_name.#var_name for static variable
• Constructor_name.#method_name(arg) for static methods.

🛑 warning: for private static elements, this also works this.#var_name but breaks in subclass.

// basic example of static private variable
class Burger {
 static #secret_sauce = "eggs and milk";

 show_sauce() {
  return Burger.#secret_sauce;
 }
}

const my_burger = new Burger(1);

console.log(my_burger.show_sauce());
// "eggs and milk"

Private elements are not properties

Private elements are not properties. You cannot use the bracket notation such as this[#name].

// verify private elements are not properties
class Burger {
 constructor(x) {
  this.meat = x;
 }

 // static private variable
 static #secret_cheese = 1;

 // static private method
 static #add_cheese(x) {
  return Burger.#secret_cheese + x;
 }

 // private variable
 #secret_sauce = 1;

 // private method
 #add_sauce(x) {
  return this.#secret_sauce + x;
 }
}

// s------------------------------

// create a instance
const my_burger = new Burger(1);

// private elements are not a property of any object

console.assert(
 Reflect.has(Burger, "#secret_sauce") === false,
 Reflect.has(Burger, "#add_sauce") === false,
 Reflect.has(Burger, "#secret_cheese") === false,
 Reflect.has(Burger, "#add_cheese") === false,
);

console.assert(
 Reflect.has(my_burger, "#secret_sauce") === false,
 Reflect.has(my_burger, "#add_sauce") === false,
 Reflect.has(my_burger, "#secret_cheese") === false,
 Reflect.has(my_burger, "#add_cheese") === false,
);