Run JavaScript

import { PackageManagers, Render, Steps, TypeScriptExample, WranglerConfig, } from "~/components";

You will POST JavaScript to a Worker and read {"result":["Ada"]}.

Prerequisites

Run JavaScript

1. Create a Worker project:

   <PackageManagers type="create" pkg="cloudflare@latest" args={"sandbox-dynamic-worker --category=hello-world --type=hello-world --lang=ts --no-deploy"} />

2. Change into the project directory:

   cd sandbox-dynamic-worker
   

3. Replace wrangler.jsonc to add a Worker Loader binding:

   {
   	"$schema": "node_modules/wrangler/config-schema.json",
   	"name": "sandbox-dynamic-worker",
   	"main": "src/index.ts",
   	"compatibility_date": "$today",
   	"observability": {
   		"enabled": true,
   	},
   	"upload_source_maps": true,
   	"worker_loaders": [
   		{
   			"binding": "LOADER",
   		},
   	],
   }
   

4. Generate types for the binding:

5. Replace src/index.ts. Your Worker reads code from the JSON body and runs it in the sandbox:

   import type { WorkerEntrypoint } from "cloudflare:workers";
   
   type CodeEntrypoint = WorkerEntrypoint & {
   	evaluate(): Promise<unknown>;
   };
   
   export default {
   	async fetch(request: Request, env: Env): Promise<Response> {
   		const { code } = (await request.json()) as { code: string };
   
   		const sandbox = env.LOADER.load({
   			compatibilityDate: "$today",
   			mainModule: "code.js",
   			modules: {
   				"code.js": `
   					import { WorkerEntrypoint } from "cloudflare:workers";
   
   					export class Code extends WorkerEntrypoint {
   						evaluate() {
   							${code}
   						}
   					}
   				`,
   			},
   			// Block `fetch()` and `connect()`
   			globalOutbound: null,
   			// Stop code that uses more than 50 milliseconds of CPU time
   			limits: { cpuMs: 50 },
   		});
   
   		const result = await sandbox
   			.getEntrypoint<CodeEntrypoint>("Code")
   			.evaluate();
   		return Response.json({ result });
   	},
   };
   

   code is the body of evaluate(), so it needs a return statement. Code that waits without using CPU can still hold the request open, so add a timeout before you run code from other people. For an example, refer to Build an AI code interpreter.

6. Run wrangler dev:

7. POST JavaScript to the URL Wrangler prints. The default is http://localhost:8787:

   curl http://localhost:8787 --request POST --json '{
     "code": "const users = [{ name: \"Ada\", role: \"admin\" }, { name: \"Grace\", role: \"user\" }]; return users.filter((u) => u.role === \"admin\").map((u) => u.name);"
   }'
   

The response body is {"result":["Ada"]}. The Worker ran the JavaScript you sent.

Next steps

• Let the code call methods that your Worker provides. Refer to Bindings.
• Run a Linux command. Refer to Run a Linux command.
• Use the Loader API for load() and get().