Derived

A derived value automatically updates when its dependencies change.

Creating Derived

import { signal, derived, path } from 'sairin';

const firstName = signal(path("user", "firstName"), "John");
const lastName = signal(path("user", "lastName"), "Doe");

const fullName = derived(path("user", "fullName"), () => {
  return `${firstName.get()} ${lastName.get()}`;
});

console.log(fullName.get());  // "John Doe"

How It Works

1. On first .get(), the derived runs its function
2. It subscribes to any signals read during computation
3. When any dependency changes, it marks itself dirty
4. On next .get(), it recomputes if dirty

Caching

Derived values cache their results:

const a = signal(path("a"), 1);
const b = signal(path("b"), 2);

const sum = derived(path("sum"), () => {
  console.log("Computing...");
  return a.get() + b.get();
});

sum.get();  // Logs: "Computing..." -> 3
sum.get();  // Cached -> 3 (no recompute)

a.set(10);
sum.get();  // Logs: "Computing..." -> 12

Eager Mode

Compute immediately instead of lazily:

const count = signal(path("counter"), 0);

const doubled = derived(path("counter", "doubled"), () => {
  return count.get() * 2;
}, { eager: true });  // Computes immediately

console.log(doubled.peek());  // 0

Derived API

Staleness Detection

Derived uses a version clock. Each signal has a monotonic version that increments on .set(). Derived caches each dependency’s version and only recomputes when a version is higher.