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Run JavaScript

Last updated View as MarkdownAgent setup

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

Prerequisites

  1. Sign up for a Cloudflare account ↗︎.
  2. Install Node.js ↗︎.

Node.js version manager

Use a Node version manager like Volta ↗︎ or nvm ↗︎ to avoid permission issues and change Node.js versions. Wrangler, discussed later in this guide, requires a Node version of 16.17.0 or later.

Run JavaScript

  1. Create a Worker project:

    npm create cloudflare@latest -- 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:

    {
	"$schema": "node_modules/wrangler/config-schema.json",
	"name": "sandbox-dynamic-worker",
	"main": "src/index.ts",
	// Set this to today's date
	"compatibility_date": "2026-10-01",
	"observability": {
		"enabled": true,
	},
	"upload_source_maps": true,
	"worker_loaders": [
		{
			"binding": "LOADER",
		},
	],
}
    "$schema" = "node_modules/wrangler/config-schema.json"
name = "sandbox-dynamic-worker"
main = "src/index.ts"
# Set this to today's date
compatibility_date = "2026-10-01"
upload_source_maps = true

[observability]
enabled = true

[[worker_loaders]]
binding = "LOADER"

  4. Generate types for the binding:

    npx wrangler types

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

    src/index.jsjs
    export default {
	async fetch(request, env) {
		const { code } = await request.json();

		const sandbox = env.LOADER.load({
			compatibilityDate: "2026-10-01",
			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("Code").evaluate();
		return Response.json({ result });
	},
};
    src/index.tsts
    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: "2026-10-01",
			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:

    npx 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

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