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ObservableLimited availability

The when() method on a event target returns an Observable object, which provides a declarative API for subscribing to and operating on events. It's an alternative to addEventListener() callbacks.

Not yet supported in all major browsers — check the table and provide a fallback.

How to use ↓Compare browsers

Browser support

DesktopChrome
1–134Not supported
135+Supportedsince Apr 1, 2025
Current: 155
Edge
12–134Not supported
135+Supportedsince Apr 4, 2025
Current: 154
Firefox
✕Not supportedup to 157
Current: 157
Safari
✕Not supportedup to 27
Current: 27
MobileChrome Android
18–134Not supported
135+Supportedsince Apr 1, 2025
Current: 155
Firefox Android
✕Not supportedup to 157
Current: 157
Safari iOS
✕Not supportedup to 27
Current: 27

How to use

Not supported in Firefox, Safari, Firefox Android, Safari iOS. Use feature detection below and provide a fallback (or a polyfill) for those browsers.

Example

Full syntax and examples are on MDN: when.

Feature detection

JS · check before using
if ("Observable" in globalThis) {
  // Observable is available
} else {
  // load a polyfill or use a fallback
}

Documentation (MDN)

Specification

Sub-features

Support for each part of the feature (30). The overall table above shows when the whole feature works.

Sub-featureChromeEdgeFirefoxSafariChrome AndroidFirefox AndroidSafari iOS
EventTarget.when135135✕✕135✕✕
Observable135135✕✕135✕✕
Observable.Observable135135✕✕135✕✕
Observable.catch135135✕✕135✕✕
Observable.drop135135✕✕135✕✕
Observable.every135135✕✕135✕✕
Observable.filter135135✕✕135✕✕
Observable.finally135135✕✕135✕✕
Observable.find135135✕✕135✕✕
Observable.first135135✕✕135✕✕
Observable.flatMap135135✕✕135✕✕
Observable.forEach135135✕✕135✕✕
Observable.from_static135135✕✕135✕✕
Observable.inspect135135✕✕135✕✕
Observable.last135135✕✕135✕✕
Observable.map135135✕✕135✕✕
Observable.reduce135135✕✕135✕✕
Observable.some135135✕✕135✕✕
Observable.subscribe135135✕✕135✕✕
Observable.switchMap135135✕✕135✕✕
Show 10 more
Sub-featureChromeEdgeFirefoxSafariChrome AndroidFirefox AndroidSafari iOS
Observable.take135135✕✕135✕✕
Observable.takeUntil135135✕✕135✕✕
Observable.toArray135135✕✕135✕✕
Subscriber135135✕✕135✕✕
Subscriber.active135135✕✕135✕✕
Subscriber.addTeardown135135✕✕135✕✕
Subscriber.complete135135✕✕135✕✕
Subscriber.error135135✕✕135✕✕
Subscriber.next135135✕✕135✕✕
Subscriber.signal135135✕✕135✕✕