Docs / Reference / Reactivity

    signal()#

    import { signal } from 'janux';

const count = signal(0);

    The reactive primitive everything else is built on: a mutable cell that tracks who reads it. signal<T>(initial: T): Sig<T>.

    Member Type What it does
    value T (get/set) Reading inside a watch or computed subscribes to it. Writing notifies subscribers.
    peek() () => T Reads without subscribing — the targeted version of untrack.
    readers() () => number Live subscriber count. 
    import { signal, watch } from 'janux';

const count = signal(0);

watch(() => console.log('count is', count.value));   // logs immediately: 0
count.value = 1;                                     // logs: 1
count.peek();                                        // 1 — no subscription created

    Writes are deduped by Object.is#

    Assigning the same value notifies nobody, so an idempotent write is free:

    const name = signal('ada');

name.value = 'ada';   // no notification at all
name.value = 'grace'; // notifies

    Object.is semantics mean NaN is stable (NaN → NaN is not a change) while 0 and -0 are different. Objects compare by identity: mutating a nested field of a signal's object value does not notify — assign a new object, or model that state with schema inside a component, where every field is tracked for you.

    You rarely need this directly#

    Inside a component or store, state is already a fine-grained reactive object and derived is already a computed — both built on signals, both projected to agents. Reach for raw signals when you're writing framework-level code: a custom integration, a foreign-runtime bridge, or a helper that must be reactive outside any component.

    Component state (preferred — schema-typed, agent-visible, auditable):

    state: schema({ count: int() }),

    A raw signal (local, invisible to agents, yours to manage):

    const scrollY = signal(0);

    readers() and ownership#

    readers() exists so an owner can reclaim a signal nobody observes anymore. Subscriptions are removed automatically when the reading effect is disposed — see ownership.

    Related: computed · watch · batch · untrack