Function computed

computed<T>(fn: () => T): ReadonlySignal<T>
Define a read-only signal computed from the values of other signals. Dependencies are tracked automatically: whatever signals fn reads during evaluation become its dependencies, re-tracked on every evaluation (so conditional reads narrow or widen them dynamically).

Evaluation is lazy and memoized: fn does not run until the computed is first read, and re-runs only when a read (or an effect/subscription) finds a dependency actually changed. A re-evaluation that produces an equal value (strict equality) stops propagation — dependents do not re-evaluate.

fn must be pure and synchronous: writing to a signal during a computed evaluation throws a TypeError. Computed signals cannot be written to; calling one with an argument throws a TypeError.
Type Parameters
- T
Parameters
- fn: () => T
Returns ReadonlySignal<T>
Example
```
const a = signal(1);
const b = signal(2);
const sum = computed(() => a() + b());
sum();  // 3 — evaluated on this first read
a(2);   // nothing evaluated yet
sum();  // 4 — evaluated now
```
- Defined in computed.ts:39