--- description: POST JavaScript to your Worker and read the sandboxed result. title: Run JavaScript image: https://developers.cloudflare.com/sandbox/get-started/dynamic-workers/og.png?v=4ac2191cd371fe40 --- [Skip to content](#main-content) > Documentation Index > Fetch the complete documentation index at: https://developers.cloudflare.com/sandbox/llms.txt > Use this file to discover all available pages before exploring further. # Run JavaScript Last updated Sep 30, 2026|Copy as Markdown| [View as Markdown](https://developers.cloudflare.com/sandbox/get-started/dynamic-workers/index.md)| [Agent setup](https://developers.cloudflare.com/agent-setup/) You will POST JavaScript to a Worker and read `{"result":["Ada"]}`. ## Prerequisites 1. Sign up for a [Cloudflare account ↗︎](https://dash.cloudflare.com/sign-up/workers-and-pages). 2. Install [`Node.js` ↗︎](https://docs.npmjs.com/downloading-and-installing-node-js-and-npm).
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:npmyarnpnpm ``` npm create cloudflare@latest -- sandbox-dynamic-worker --category=hello-world --type=hello-world --lang=ts --no-deploy ``` ``` yarn create cloudflare sandbox-dynamic-worker --category=hello-world --type=hello-world --lang=ts --no-deploy ``` ``` pnpm create cloudflare@latest sandbox-dynamic-worker --category=hello-world --type=hello-world --lang=ts --no-deploy ``` 2. Change into the project directory: ```sh cd sandbox-dynamic-worker ``` 3. Replace `wrangler.jsonc` to add a Worker Loader binding: ```jsonc { "$schema": "node_modules/wrangler/config-schema.json", "name": "sandbox-dynamic-worker", "main": "src/index.ts", // Set this to today's date "compatibility_date": "2026-10-05", "observability": { "enabled": true, }, "upload_source_maps": true, "worker_loaders": [ { "binding": "LOADER", }, ], } ``` ```toml "$schema" = "node_modules/wrangler/config-schema.json" name = "sandbox-dynamic-worker" main = "src/index.ts" # Set this to today's date compatibility_date = "2026-10-05" upload_source_maps = true [observability] enabled = true [[worker_loaders]] binding = "LOADER" ``` 4. Generate types for the binding:npmyarnpnpm ``` npx wrangler types ``` ``` yarn wrangler types ``` ``` pnpm wrangler types ``` 5. Replace `src/index.ts`. Your Worker reads `code` from the JSON body and runs it in the sandbox: *src/index.jsjs* ```js export default { async fetch(request, env) { const { code } = await request.json(); const sandbox = env.LOADER.load({ compatibilityDate: "2026-10-05", 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* ```ts import type { WorkerEntrypoint } from "cloudflare:workers"; type CodeEntrypoint = WorkerEntrypoint & { evaluate(): Promise; }; export default { async fetch(request: Request, env: Env): Promise { const { code } = (await request.json()) as { code: string }; const sandbox = env.LOADER.load({ compatibilityDate: "2026-10-05", 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 }); }, }; ``` `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](https://developers.cloudflare.com/sandbox/get-started/build-an-ai-code-interpreter/). 6. Run `wrangler dev`:npmyarnpnpm ``` npx wrangler dev ``` ``` yarn wrangler dev ``` ``` pnpm wrangler dev ``` 7. POST JavaScript to the URL Wrangler prints. The default is `http://localhost:8787`: ```sh 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](https://developers.cloudflare.com/dynamic-workers/usage/bindings/). - Run a Linux command. Refer to [Run a Linux command](https://developers.cloudflare.com/sandbox/get-started/). - Use the [Loader API](https://developers.cloudflare.com/dynamic-workers/api-reference/) for `load()` and `get()`. Was this helpful? YesNo ## On this page [![](https://developers.cloudflare.com/_astro/logo.te5VL_aD.svg)Docs](https://developers.cloudflare.com/) ```json {"@context":"https://schema.org","@type":"TechArticle","@id":"https://developers.cloudflare.com/sandbox/get-started/dynamic-workers/#page","headline":"Run JavaScript","description":"POST JavaScript to your Worker and read the sandboxed result.","url":"https://developers.cloudflare.com/sandbox/get-started/dynamic-workers/","inLanguage":"en","image":"https://developers.cloudflare.com/sandbox/get-started/dynamic-workers/og.png?v=4ac2191cd371fe40","dateModified":"2026-09-30","publisher":{"@type":"Organization","name":"Cloudflare","description":"One platform for your apps, agents, and workforce. Build, secure, and scale without managing infrastructure","url":"https://www.cloudflare.com/","sameAs":["https://github.com/cloudflare","https://www.linkedin.com/company/cloudflare","https://x.com/cloudflare"],"logo":{"@type":"ImageObject","url":"https://developers.cloudflare.com/logo.svg"},"address":{"@type":"PostalAddress","streetAddress":"101 Townsend St","addressLocality":"San Francisco","addressRegion":"CA","postalCode":"94107","addressCountry":"US"},"contactPoint":[{"@type":"ContactPoint","contactType":"Customer Support","url":"https://support.cloudflare.com/","availableLanguage":["English"]},{"@type":"ContactPoint","contactType":"Sales","url":"https://www.cloudflare.com/contact/","availableLanguage":["English"]}]},"isPartOf":{"@type":"WebSite","@id":"https://developers.cloudflare.com/#website","name":"Cloudflare Docs","url":"https://developers.cloudflare.com/"}} ```