Worker is currently supported as an experimental feature with limitations:

• Can not use script classes (GodotJSScript) in workers
• transferable objects are not supported
• onerror and onready events are not implemented yet

[!WARNING] The class name Worker may be changed in future versions for compatibility in different JS engines.
And, Worker scripts may need to have a specific file name suffix or reside in a designated directory.

A Simple Example

// tests/master.ts
// ...
import { Object, Node } from "godot";
import { JSWorker } from "godot.worker";

let worker = new JSWorker("tests/worker");
worker.onmessage = function (m: any) {
    console.log("master: get message", m);
}
worker.ontransfer = function (obj: Object) {
    console.assert(obj instanceof Node);
    this.add_child(obj);
    worker.terminate();
}
worker.postMessage("hello");

// tests/worker.ts
import { PackedScene, ResourceLoader } from "godot";
import { JSWorkerParent } from "godot.worker";

if (typeof JSWorkerParent !== "undefined") {
    JSWorkerParent.onmessage = function (m: any) {
        console.log("worker: get message", m);
        JSWorkerParent.postMessage("worker result");

        // [EXPERIMENTAL] instantiate a PackedScene in worker thread and transfer it back to master thread
        let asset = <PackedScene>ResourceLoader.load("res://background.tscn");
        let obj = asset.instantiate();
        console.log("[worker] transfering object:", obj);
        JSWorkerParent.transfer(obj);

        // worker can terminate itself by calling `close()`
        // close();
    }
}