The Tool Desk
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Start with the shape of the data
A Signal gives you synchronous access to a current value. Reading it calls a getter, which lets Angular track where that value is used. A writable Signal can be updated, while a computed Signal derives a read-only value from other Signals. Angular describes a Signal as “a wrapper around a value that notifies interested consumers when that value changes” in its Signals overview.
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An Observable represents values delivered over time. That makes RxJS a natural fit when the sequence, timing, cancellation, or transformation of values matters. In Angular’s interop example, a changing query is converted to an Observable and passed through switchMap to issue requests. The useful distinction is therefore not “new versus old,” but current state and derivation versus stream processing and event timing.
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Choose by use case
| Situation | Practical choice | Why |
|---|---|---|
| Current component state, such as a selected tab, expanded panel, or filter | Signal | It represents a current value that can be read synchronously. |
| A display label or whether a control is enabled, based on other state | computed Signal |
Computed Signals are read-only derived values. |
Asynchronous sequences, event streams, or work needing operators such as switchMap |
RxJS Observable | Operators express stream transformations and sequencing; Angular’s query-driven request example uses switchMap. |
| A service already returns an Observable, but a component needs its latest value | Convert once with toSignal |
The component can read the latest emission synchronously as a Signal. |
| Signal state needs to enter an Observable pipeline | Convert with toObservable |
It exposes the Signal through an Observable, but propagation is asynchronous and may coalesce writes. |
| Resource-style loading, error, and value state backed by an Observable | Consider rxResource |
Its stream factory runs when parameters change and uses resource-style state APIs. |
The table applies Angular’s documented primitives to common application decisions; the examples are not claims that Angular prescribes one architecture for every application. See the Angular RxJS interop guide for the adapters and resource API.
#1 Best Overall
Use Signals for state and derivation
Keep one source of truth
For state that a component needs to inspect or update directly, a Signal is often the clearest representation. A selected item, open/closed flag, or current filter has a meaningful value at any moment. Angular can track Signal reads made by consumers and update those consumers when the value changes.
Derive values with computed
When one value depends on other state—for example, a formatted label based on a selection—use a computed Signal. It expresses the dependency as a read-only derivation instead of maintaining a second value that must be kept in sync.
Angular advises using computed Signals for state derived from other state rather than using effects to propagate changes. Effects are better reserved for synchronizing with imperative systems such as storage, logging, or custom DOM behavior. Using effects to copy state can lead to expression-changed errors, circular updates, or unnecessary change-detection work. Details are in Angular’s Signals guide.
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Use operators for asynchronous sequences
An Observable pipeline is useful when values arrive over time and the handling of that sequence is part of the requirement. For example, a query that changes repeatedly can drive requests through switchMap; the operator expresses how new query values relate to work already in progress. If the problem is fundamentally about events, ordering, or asynchronous composition, keeping it in RxJS avoids disguising stream behavior as a single current value.
Rank #3
Keep Observable-based services if they already fit
A service does not need to be rewritten just because a component uses Signals. If the service exposes an Observable and the component needs its latest value for rendering or synchronous reads, convert at the boundary with toSignal. If the consumer needs the sequence itself, keep consuming the Observable as a stream.
Convert deliberately between the APIs
Observable to Signal: toSignal
toSignal subscribes immediately, so creating one can trigger side effects in the source Observable. Create it once for a given source and reuse the returned Signal rather than calling it repeatedly. By default, Angular unsubscribes when the component or service that called it is destroyed.
Rank #4
A Signal always has a current value, but an Observable may not emit synchronously. Set initialValue when a useful starting value exists; otherwise the Signal is undefined until the first emission. Use requireSync: true only when synchronous emission is guaranteed, as with a BehaviorSubject. If the Observable errors, the error is thrown when the Signal is read; if it completes, the Signal retains its last value. Angular documents these details in the toSignal API reference and the interop guide.
Signal to Observable: toObservable
toObservable monitors a Signal with an effect and exposes its values as an Observable. The initial value may be emitted synchronously when available, but later updates propagate asynchronously. If code sets a Signal to 1, then 2, then 3 before it stabilizes, subscribers may receive only 3. Do not use this bridge when every intermediate write must be observed as a distinct event. See Angular’s toObservable API reference.
Observable-backed resource: rxResource
When the desired interface is resource-style value, loading, and error state but the data source is an Observable, consider rxResource. Its stream factory is called when parameters change. The returned Observable must emit a value or an error before completing. Consult the Angular interop guide for the API’s current details.
A quick decision checklist
- Is this a value the UI needs to read right now? Prefer a Signal.
- Is it calculated from other state? Prefer a computed Signal.
- Does the sequence, timing, cancellation, or transformation of asynchronous values matter? Keep it in RxJS.
- Does an Observable-based service already supply the data? Preserve it, and convert only where a consumer needs a different interface.
- Are you bridging APIs? Decide the initial-value behavior, subscription lifetime, and whether coalesced updates are acceptable.
Angular version note
Angular’s current toObservable API reference says the API has been stable since Angular v20.0. Check the documentation and APIs for the Angular version used by your project rather than assuming every version has identical interop support or maturity. The current references are the RxJS interop guide and the toObservable API reference.
For structured introductions, Angular also provides a Signals tutorial and Signals essentials guide.
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