Classes and Prototypes

JavaScript objects inherit behavior through prototype chains. Class syntax is a clearer way to define constructors and shared prototype methods.

Define an instance contract

01class Course {
02  #completed = 0;
03
04  constructor(title, lessonCount) {
05    this.title = title;
06    this.lessonCount = lessonCount;
07  }
08
09  completeLesson() { this.#completed += 1; }
10  get progress() { return this.#completed / this.lessonCount; }
11}
12
13const html = new Course("HTML", 14);

Methods are shared through Course.prototype. Private fields are inaccessible outside the class body.

Composition before inheritance

Inheritance with extends can model a true subtype, but deep class trees couple behavior tightly. Small functions and composed objects are often easier to test.

01const withProgress = (lessonCount) => ({ completed: 0, lessonCount });
02const course = { title: "CSS", progress: withProgress(18) };

Keep this

Know that classes use prototypes, keep instance state intentional, and prefer composition unless inheritance expresses a real relationship.