JavaScript and web standards

​​ JavaScript standards

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

All of the standard built-in objects Open external link supported by the current Google Chrome stable release are supported, with a few notable exceptions:

• For security reasons, the following are not allowed:
  • eval()
  • new Function
  • WebAssembly.compile Open external link
  • WebAssembly.compileStreaming Open external link
  • WebAssembly.instantiate with a buffer parameter Open external link
  • WebAssembly.instantiateStreaming Open external link
• Date.now() returns the time of the last I/O; it does not advance during code execution.

​​ Web standards and global APIs

The following methods are available per the Worker Global Scope Open external link :

​​ Base64 utility methods

​​ Timers

​​ performance.timeOrigin and performance.now()

• performance.timeOrigin
  • Returns the high resolution time origin. Workers uses the UNIX epoch as the time origin, meaning that performance.timeOrigin will always return 0.
• performance.now()
  • Returns a DOMHighResTimeStamp representing the number of milliseconds elapsed since performance.timeOrigin. Note that Workers intentionally reduces the precision of performance.now() such that it returns the time of the last I/O and does not advance during code execution. Effectively, because of this, and because performance.timeOrigin is always, 0, performance.now() will always equal Date.now(), yielding a consistent view of the passage of time within a Worker.

​​ EventTarget and Event

The EventTarget Open external link and Event Open external link API allow objects to publish and subscribe to events.

​​ AbortController and AbortSignal

The AbortController Open external link and AbortSignal Open external link APIs provide a common model for canceling asynchronous operations.

​​ Fetch global

​​ Encoding API

Both TextEncoder and TextDecoder support UTF-8 encoding/decoding.

Refer to the MDN documentation for more information Open external link .

The TextEncoderStream Open external link and TextDecoderStream Open external link classes are also available.

​​ URL API

The URL API supports URLs conforming to HTTP and HTTPS schemes.

Refer to the MDN documentation for more information Open external link

​​ Compression Streams

The CompressionStream and DecompressionStream classes support the deflate, deflate-raw and gzip compression methods.

Refer to the MDN documentation for more information Open external link

​​ URLPattern API

The URLPattern API provides a mechanism for matching URLs based on a convenient pattern syntax.

Refer to the MDN documentation for more information Open external link .

​​ Intl

The Intl API allows you to format dates, times, numbers, and more to the format that is used by a provided locale (language and region).

Refer to the MDN documentation for more information Open external link .

​​ navigator.userAgent

When the global_navigator compatibility flag is set, the navigator.userAgent Open external link property is available with the value 'Cloudflare-Workers'. This can be used, for example, to reliably determine that code is running within the Workers environment.

​​ Unhandled promise rejections

The unhandledrejection Open external link event is emitted by the global scope when a JavaScript promise is rejected without a rejection handler attached.

The rejectionhandled Open external link event is emitted by the global scope when a JavaScript promise rejection is handled late (after a rejection handler is attached to the promise after an unhandledrejection event has already been emitted).

worker.js
addEventListener('unhandledrejection', (event) => {
  console.log(event.promise);  // The promise that was rejected.
  console.log(event.reason);  // The value or Error with which the promise was rejected.
});

addEventListener('rejectionhandled', (event) => {
  console.log(event.promise);  // The promise that was rejected.
  console.log(event.reason);  // The value or Error with which the promise was rejected.
});

​​ navigator.sendBeacon(url[, data])

When the global_navigator compatibility flag is set, the navigator.sendBeacon(...) Open external link API is available to send an HTTP POST request containing a small amount of data to a web server. This API is intended as a means of transmitting analytics or diagnostics information asynchronously on a best-effort basis.

For example, you can replace:

const promise = fetch('https://example.com', { method: 'POST', body: 'hello world' });
ctx.waitUntil(promise);

with navigator.sendBeacon(...):

navigator.sendBeacon('https://example.com', 'hello world');