Classes & objects

Use classes to bundle configuration, state and behaviour. Create objects with new at the top of a file:

Enemy.ts

class Patrol {
  speed = 2;
  direction = 1;

  constructor(public left: number, public right: number) {}

  update() {
    me.x += this.speed * this.direction;
    if (me.x > this.right || me.x < this.left) this.direction = -this.direction;
  }
}

class FastPatrol extends Patrol {
  constructor(left: number, right: number) {
    super(left, right);
    this.speed = 5;
  }
}

const patrol = new FastPatrol(-150, 150);

whenFlag(() => {
  forever(() => patrol.update());
});

How objects map to Scratch

Code	Scratch
const patrol = new Patrol(-150, 150)	variables patrol.speed, patrol.direction, patrol.left, patrol.right
patrol.update()	custom block patrol.update (each object gets its own copy)
this.speed inside a method	the object's variable
export const boss = new Enemy()	the object's variables are global, shared by all sprites
a clone of the sprite	gets its own copy of every object, so each clone has independent state

The last row is what makes classes useful for games: give the sprite an object, create clones, and every clone runs the same methods on its own data.

const health = new Health(3);
onClone(() => {
  forever(() => {
    if (touching("Bullet")) health.damage(1);   // each clone tracks its own health
    if (health.dead()) deleteClone();
  });
});

What's supported

• Fields with literal or constant initial values, including lists (items: number[] = []).
• Constructors with parameters and defaults, parameter properties (constructor(public speed = 2)) and super(...).
• Methods with parameters and return values, calling other methods with this.method().
• extends with overrides: a method called on a Boss uses Boss's version, even when called from a base-class method.
• Classes in src/lib/ and in engine modules (new Anim.Animation("walk", 2, 8)).

Rules

• Objects are created when the project is built, so constructors can only do this.field = value with constants or constructor arguments, and super(...). Put anything else in a method (e.g. init()) and call it from whenFlag.
• Create objects with new at the top level of a file (not inside functions or loops). For many objects at runtime, use clones.
• Objects can't be passed around as values. Call their methods and read their fields directly.
• No static members, getters or setters. Use top-level functions and constants instead.