Reference

JavaScript / ECMAScript

Cloudflare Workers use the V8 JavaScript engine from Google Chrome. The Workers runtime is updated at least once a week, to at least the version that is currently used by Chrome’s stable release. This means you can safely use latest JavaScript features, with no need for “transpilers”.

Standard APIs

This is a summary of the JavaScript APIs available to Cloudflare Worker scripts, and links to their documentation on MDN. Cloudflare currently implements a subset of these APIs, as shown on this table. We are adding support for more APIs all the time – let us know if we’re missing one you need.

API	Support
ECMAScript Builtins	Everything supported by current Google Chrome stable release. `eval()` and `new Function()` are disallowed for security reasons. `WebAssembly` is currently disabled (stay tuned). `Date.now()` returns the time of last I/O; it does not advance during code execution.
Service Worker API	`"fetch"` event
Web Global APIs	Base64 utility methods Timers¹
Encoding API	UTF-8
URL API	http://, https:// schemes
Fetch API¹	`fetch()`, Headers, Request, and Response classes (some features inapplicable to the edge, such as CORS-related properties, are not implemented)
Streams API¹	ReadableStream class (enough to support the Fetch API) WritableStream class (constructed via TransformStream) TransformStream class (zero-argument constructor)
Web Crypto API	Cryptographically-secure random number generation¹ Digest (SHA family) Sign and verify (HMAC with SHA family) Encrypt and decrypt (AES-GCM, AES-CBC) Key derivation (PBKDF2) Raw key import/export for all of the above algorithms

Some APIs are only available inside a request context. A request context is active during a "fetch" event callback, and, if you pass a Response promise to FetchEvent.respondWith(), any asynchronous tasks which run before the Response promise has settled. Any attempt to use such APIs during script startup will throw an exception.

For example:
```
const promise = fetch("https://example.com/")       // ERROR

addEventListener("fetch", event => {
  event.respondWith(fetch("https://example.com/"))  // OK
})
```
This code snippet will throw during script startup, and the "fetch" event listener will never be registered.
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