Angular signals are reactive values that let Angular track where state is read and update affected parts of an application when that state changes. Start with signal() for writable state, use computed() for derived values, and reserve effect() for synchronizing with imperative systems such as storage or third-party APIs.
What Angular signals do
A signal wraps a value. Reading it means calling it like a function; Angular can track that read as a dependency and respond when the value changes. This dependency tracking helps Angular target rendering updates, but signals do not eliminate change detection or guarantee that every application will be faster. See the Angular Signals overview.
For example, a component can keep a count as writable state and derive a display value from it:
import { computed, signal } from '@angular/core';
const count = signal(0);
const doubled = computed(() => count() * 2);
count.set(1);
count.update(value => value + 1);
console.log(count()); // 2
console.log(doubled()); // 4count() reads the current value. .set(value) replaces it, while .update(fn) calculates a replacement from the current value. The function-call syntax applies to reads; it is not property access such as count.value.
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Choose the signal API that matches the job
| Need | Use | What it means |
|---|---|---|
| Writable source state | signal(initialValue) |
Read by calling the signal; update with .set() or .update(). |
| A value derived from other signals | computed(() => ...) |
Read-only, lazy, memoized, and dependent on the signals read during its latest evaluation. |
| Derived state that must also be manually adjustable | linkedSignal() |
Maintains a value related to other state while allowing that value to be set manually. |
| Synchronization with a non-signal, imperative system | effect(() => ...) |
Runs in response to the signals it reads; register cleanup for work that must stop before a rerun or destruction. |
| General asynchronous work | resource() |
Loads using reactive parameters and exposes readable data and status. |
| Reactive HTTP requests through Angular’s HttpClient | httpResource() |
HTTP-oriented resource that uses the HttpClient stack and supports its features, including interceptors. |
Derive values with computed()
Use computed() when a value can be calculated from other signals and should not be independently written. Angular evaluates a computed value lazily, caches its result, and recalculates it when a dependency changes. Its dependencies are dynamic: if the calculation reads signals inside a conditional branch, only the signals read in the latest evaluation are dependencies. A change in the selected branch can therefore add or remove dependencies.
For example, a filtered list or a total based on a cart’s item signals generally belongs in a computed value, rather than in a separate writable signal kept in sync by an effect. That keeps the source state and its derivation distinct.
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Use linkedSignal() for dependent state that users can change
Sometimes a value depends on other state but also needs to be changed directly. In that case, linkedSignal() is a better fit than either a read-only computed value or an effect that copies data between signals. Angular’s guidance distinguishes it from computed() on this basis: computed values are derived and read-only, while linked signals support derived state that can also be manually set. See the effects guide for Angular’s recommendation.
Use effects for imperative side effects—not state propagation
An effect is appropriate when a signal change must be synchronized with something outside the signal graph, such as logging, browser storage, custom DOM work, or a third-party rendering library. Effects track signal reads dynamically and execute asynchronously as part of Angular’s change-detection process. They support cleanup before they rerun or when their owning context is destroyed.
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Do not use an effect as the usual way to copy one signal into another or to maintain a derived value. Angular warns that propagating state through effects can cause ExpressionChangedAfterItHasBeenChecked errors, circular updates, and unnecessary change-detection cycles. Prefer a single source of truth and derive the dependent value with computed(), or use linkedSignal() when the dependent value must also be manually adjustable. Angular’s tutorial summarizes the rule in its signal effects tutorial.
Effects created in a component, directive, or service are associated with that context by default. Creating one outside an injection context requires providing an injector. If DOM work must happen after Angular has finished rendering and committed changes, Angular provides afterRenderEffect() for that timing.
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Use resource() when asynchronous work should respond to reactive parameters. It accepts parameters and an async loader, then exposes data and status as signals so a template can represent loading, success, and failure states. The resource guide describes this API and its status model.
For requests that should use Angular’s HTTP infrastructure, httpResource() is the HTTP-oriented alternative. It uses the HttpClient stack, exposes response and status as signals, and supports HttpClient features such as interceptors. See the httpResource guide.
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One server-rendering caution matters for resources: do not serialize user-specific resource data into HTML that could be cached or shared with another user. Keep the data and the caching boundary appropriate to the user-specific nature of the response.
Use signals in component inputs
Angular’s input() API returns a read-only signal for an input received by a component. Angular recommends it for new projects; decorator-based @Input remains supported. Use model() when the component can write a value and propagate it back to a parent through two-way binding. These APIs cover different ownership needs: input state is read-only to the receiving component, while a model supports component writes. Details are in Angular’s signals inputs guide.
Signal reads in templates and object updates
When an OnPush component template reads a signal, Angular tracks that dependency and marks the component for update when the signal changes. This is targeted update behavior, not a promise that signals remove change detection altogether. The tracking model and rendering behavior are described in the Signals overview.
A read-only signal prevents callers from using its signal API to assign a new value; it does not prevent deep mutation of an object held by that signal. Prefer immutable replacement or update patterns for arrays and objects so that changes are explicit and observable. For instance, create a new array when adding an item rather than mutating the existing array in place.
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Angular’s v20 roadmap records that the fundamental reactivity primitives—including signal, effect, linkedSignal, signal-based queries, and inputs—graduated to stable in v20, with the milestone marked completed in Q2 2025. That is a dated v20 stability milestone, not evidence that v20 is the latest Angular release. See the Angular v20 roadmap.
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