---
title: "Signals, computed state, linkedSignal, and resources"
chapter: "04"
---

# Signals, computed state, linkedSignal, and resources

Signals are Angular's granular synchronous reactive state model.

## Writable and computed

```ts
items = signal<Order[]>([]);
query = signal('');
visible = computed(() =>
  this.items().filter(item => item.name.includes(this.query()))
);
```

Store source state in writable signals. Derive state with `computed`; do not
copy derived values into another writable signal.

## Effects

Effects synchronize state with non-reactive side effects such as logging,
storage, or imperative APIs. They are not the default way to propagate state.
Avoid writing signals inside effects when a computed relationship exists.

## linkedSignal

`linkedSignal` models writable state that depends on another source—for example,
a selected option that resets only when it becomes invalid after options
change.

## resource and httpResource

Resources combine reactive parameters with asynchronous loading, cancellation,
status, value, and error signals. `resource` and `httpResource` are stable
since Angular 22. `httpResource` uses HttpClient and interceptors. Guard
`value()` with `hasValue()` because an error-state read can throw.

Use HttpClient methods directly for mutations. A resource is naturally suited
to reads whose parameters are reactive.

## Equality and mutation

Signals notify on updates according to equality. Mutating an array in place can
hide change. Create the next value:

```ts
this.items.update(items => [...items, created]);
```

## RxJS interop

`toSignal` subscribes to an Observable within lifecycle ownership.
`toObservable` exposes signal changes as an Observable. Convert once at a
boundary and keep the internal model consistent.

## Feynman check

A signal is a labeled value with subscribers. A computed signal is a formula.
An effect is a messenger leaving Angular's reactive graph. A resource is a
managed async request with state.