JavaScript’s New Private Class Fields, and How to Use Them

ES6 introduced classes to JavaScript, but they’re too simplistic for complex applications. Class fields (also referred to as class properties) aim to deliver simpler constructors with private and static members. The proposal is currently at TC39 stage 3: candidate and could appear in ES2019 (ES10).
A quick recap of ES6 classes is useful before we examine class fields in more detail.
ES6 Class Basics
JavaScript’s prototypal inheritance model can appear confusing to developers with an understanding of the classical inheritance used in languages such as C++, C#, Java and PHP. JavaScript classes are primarily syntactical sugar, but they offer more familiar object-oriented programming concepts.
A class is a template which defines how objects of that type behave. The following Animal class defines generic animals (classes are normally denoted with an initial capital to distinguish them from objects and other types):
class Animal {

constructor(name = ‘anonymous’, legs = 4, noise = ‘nothing’) {

this.type = ‘animal’;
this.name = name;
this.legs = legs;
this.noise = noise;

}

speak() {
console.log(`${this.name} says “${this.noise}"`);
}

walk() {
console.log(`${this.name} walks on ${this.legs} legs`);
}

}

Class declarations execute in strict mode; there’s no need to add ‘use strict’.
A constructor method is run when an object of this type is created, and it typically defines initial properties. speak() and walk() are methods which add further functionality.
An object can now be created from this class with the new keyword:
const rex = new Animal(‘Rex’, 4, ‘woof’);
rex.speak(); // Rex says "woof"
rex.noise = ‘growl’;
rex.speak(); // Rex says "growl"

Getters and Setters
Setters are special methods used to define values only. Similarly, Getters are special methods used to return a value only. For example:
class Animal {

constructor(name = ‘anonymous’, legs = 4, noise = ‘nothing’) {

this.type = ‘animal’;
this.name = name;
this.legs = legs;
this.noise = noise;

}

speak() {
console.log(`${this.name} says "${this.noise}"`);
}

walk() {
console.log(`${this.name} walks on ${this.legs} legs`);
}

// setter
set eats(food) {
this.food = food;
}

// getter
get dinner() {
return `${this.name} eats ${this.food || ‘nothing’} for dinner.`;
}

}

const rex = new Animal(‘Rex’, 4, ‘woof’);
rex.eats = ‘anything’;
console.log( rex.dinner ); // Rex eats anything for dinner.

Child or Sub-Classes
It’s often practical to use one class as the base for another. If we’re mostly creating dog objects, Animal is too generic, and we must specify the same 4-leg and “woof” noise defaults every time.
A Dog class can inherit all the properties and methods from the Animal class using extends. Dog-specific properties and methods can be added or removed as necessary:
class Dog extends Animal {

constructor(name) {

// call the Animal constructor
super(name, 4, ‘woof’);
this.type = ‘dog’;

}

// override Animal.speak
speak(to) {

super.speak();
if (to) console.log(`to ${to}`);

}

}

super refers to the parent class and is usually called in the constructor. In this example, the Dog speak() method overrides the one defined in Animal.
Object instances of Dog can now be created:
const rex = new Dog(‘Rex’);
rex.speak(‘everyone’); // Rex says "woof" to everyone

rex.eats = ‘anything’;
console.log( rex.dinner ); // Rex eats anything for dinner.

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