JavaScript and web standards
The Workers runtime provides the following standardized APIs for use by Workers running on Cloudflare’s global network.
JavaScript standards
Cloudflare Workers uses 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 the latest JavaScript features, with no need for transpilers.
All of the standard built-in objects supported by the current Google Chrome stable release are supported, with a few notable exceptions:
eval()
is not allowed for security reasons.new Function
is not allowed for security reasons.Date.now()
returns the time of the last I/O; it does not advance during code execution.
Web global APIs
The following methods are available per the Worker Global Scope:
Base64 utility methods
Timers
- Schedules a function to execute every time a given number of milliseconds elapses.
- Cancels the repeated execution set using
setInterval()
.
- Cancels the repeated execution set using
- Schedules a function to execute in a given amount of time.
- Cancels the delayed execution set using
setTimeout()
.
- Cancels the delayed execution set using
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 return0
.
- Returns the high resolution time origin. Workers uses the UNIX epoch as the time origin, meaning that
performance.now()
- Returns a
DOMHighResTimeStamp
representing the number of milliseconds elapsed sinceperformance.timeOrigin
. Note that Workers intentionally reduces the precision ofperformance.now()
such that it returns the time of the last I/O and does not advance during code execution. Effectively, because of this, and becauseperformance.timeOrigin
is always,0
,performance.now()
will always equalDate.now()
, yielding a consistent view of the passage of time within a Worker.
- Returns a
EventTarget
and Event
The EventTarget
and Event
API allow objects to publish and subscribe to events.
AbortController
and AbortSignal
The AbortController
and AbortSignal
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.
The TextEncoderStream
and TextDecoderStream
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
Compression Streams
The CompressionStream
and DecompressionStream
classes support the deflate, deflate-raw and gzip compression methods.
Refer to the MDN documentation for more information
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.
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.
navigator.userAgent
When the global_navigator
compatibility flag is set, the navigator.userAgent
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
event is emitted by the global scope when a JavaScript promise is rejected without a rejection handler attached.
The rejectionhandled
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.jsaddEventListener('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(...)
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');