effect()

Run a side effect that re-executes when its signal dependencies change. See How Reactivity Works for the underlying mental model.

Signature

function effect(fn: () => void | (() => void)): () => void

Parameters

Param	Type	Description
fn	() => void | (() => void)	Effect function. Can return a cleanup function.

Returns

() => void — dispose function that stops the effect.

Examples

import { signal, effect } from "@whisq/core";

const count = signal(0);

const dispose = effect(() => {
  console.log(`Count: ${count.value}`);
});
// Logs: "Count: 0"

count.value = 1; // Logs: "Count: 1"
dispose();       // Stop watching
count.value = 2; // No log

With cleanup

effect(() => {
  const timer = setInterval(() => tick(), 1000);
  return () => clearInterval(timer);
});

Related primitives

• signal() — the mutable primitive; effect() re-runs when a tracked signal changes.
• computed() — derived values; prefer over effect() when you need a readable result.
• onCleanup() — component-scoped cleanup for effects started inside a component.
• batch() — batch multiple writes into one effect re-run.