bevy_mod_js_scripting

bevy_mod_js_scripting is an experimental scripting integration crate for writing javascript/typescript scripts and running them in bevy with full access to the ECS values like components and resources.

Example

// type safe access to resources and values
type Scoreboard = { score: number };
const Scoreboard: BevyType<Scoreboard> = { typeName: "breakout::Scoreboard" };

// script-local variables can be used for easy cross-frame state
let i = 0;

function run() {
    // increment score every 60 frames
    if(i % 60 == 0) {
        let score = world.resource(Scoreboard)!;
        score.score += 1;
        // logging works via `trace`, `debug`, `info`, `warn`, `error`
        info(score.score);
    }

    // query components
    for (const item of world.query(Transform, Aabb)) {
        let [transform, aabb] = item.components;
        info("Translation:", transform.translation.toString());
        info("AABB Center:", aabb.center.toString());

        // call methods on value references (requires app code setup, see headless.rs)
        let normalized = transform.scale.normalize();
    }
}

export default {
    // execute the `run` function in the update stage
    update: run,
}

More examples can be found in the examples folder. Also check out the punchy wiki page on scripting, which uses bevy_mod_js_scripting.

Current Status

Currently supported operations are

• resource access (world.resource(Time))
• world information (world.components, world.resources, world.entities)
• queries (world.query(Ball, Velocity).map(({ entity, components }) => components[1]))
• component insertion (world.insert(value))
• dealing with ecs value references (Value.create, Value.patch)

Design decisions

Types

In bevy_ecs, the common methods for dealing with ECS values all take a type parameter, like

world.resource::<T>(); // or
world.query<(Entity, &Component)>();

Ideally we would be able to write

let time = world.resource<Time>();

as well in typescript, but since typescript just transpiles to javascript without adding any new runtime capabilities, we cannot associate any runtime values with the Time type.

Instead, what we need to do is write type definition with an associated variable of type BevyType<T>, which contains the referenced type's type name.

type Transform = {
  translation: Vec3,
  rotation: Quat,
  scale: Vec3,
};
const Transform: BevyType<Transform> = {
    typeName: "bevy_transform::components::global_transform::GlobalTransform"
};

// `world.resource` is typed so that typescript can infer `transform` to be of type `Transform | undefined`
let transform = world.resource(Transform);

Similarly, queries list their types like

for (item of world.query(Ball, Velocity)) {
    info(item.entity);
    let [_ball, velocity] = item.components;
    // velocity is properly typed
}

Currently, there is a pregenerated list of bevy types in ./types/bevy_types.ts, and you can also just define your own ones.

In the future we may include a utility for automatically generating the typescript definitions for your game in a build.rs script, so that you don't need to manually write or re-generate them.

Javascript Values & ECS Value references

When you call world.resource (or any other method returning references to ECS values), what you get is not just a simple javascript object corresponding to the rust value, but instead a Proxy which defers all accesses/modifications to the actual value inside the bevy world.

Only leaf values, like transform.translation.x, which can be natively represented as a javascript primitive, are automatically converted to/from the rust representation on gets and sets.

This means that

let transform = world.resource(Transform);
let translation = transform.translation;
// typeof translation.x == "number"
translation.x = 3.0;

If you want to create a new value reference, for example for inserting a new resource, the current APIs to do that are Value.create and Value.patch.

let transform = Value.create(Transform);
let vec3 = Value.create(Vec3, { x: 0.0, y: 1.0, z: 2.0 });
transform.translation = vec3;
world.insertResource(Transform, transform);

Expect to see changes in this area as we figure out the best way to deal with the interaction of javascript objects and value references.

Web support

bevy_mod_js_scripting can run in the browser using its native javascript execution environment. To try it out, download and configure wasm-server-runner and run

cargo run --example breakout --target wasm32-unknown-unknown