Function effect

Creates a reactive effect that automatically tracks signal dependencies.

Effects automatically re-run when any signals they read change. This is the foundation of reactive programming - side effects that stay in sync with state.

Isomorphic: Works identically across all JavaScript runtimes. Performance: Only re-runs when dependencies actually change. Cleanup: Automatic dependency cleanup when effect re-runs.

// Basic reactive effect
const count = signal(0);

const dispose = effect(() => {
  console.log('Count is:', count());
});

count.set(1); // Logs: "Count is: 1"
count.set(2); // Logs: "Count is: 2"

dispose(); // Stop the effect
count.set(3); // No log (effect disposed)
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// Async effects work seamlessly
const data = signal(null);

effect(async () => {
  if (data()) {
    await fetch('/api/track', {
      method: 'POST',
      body: JSON.stringify(data())
    });
  }
});
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