Fix edge case of facet effects without facets

Author: pirelenito

packages/@react-facet/core/src/hooks/useFacetEffect.spec.tsx

@@ -4,6 +4,26 @@ import { useFacetEffect } from './useFacetEffect'
 import { createFacet } from '../facet'
 import { NO_VALUE } from '../types'
 
+it('triggers the effect on mount, even if no facet is provided', () => {
+  const cleanup = jest.fn()
+  const callback = jest.fn().mockReturnValue(cleanup)
+
+  const ComponentWithFacetEffect = () => {
+    useFacetEffect(callback, [], [])
+
+    return null
+  }
+
+  const result = render(<ComponentWithFacetEffect />)
+  expect(callback).toHaveBeenCalledTimes(1)
+  expect(cleanup).not.toHaveBeenCalled()
+
+  callback.mockClear()
+  result.rerender(<></>)
+  expect(callback).not.toHaveBeenCalled()
+  expect(cleanup).toHaveBeenCalledTimes(1)
+})
+
 it('triggers the effect on mount with the initial value and on any update of the facet', () => {
   const demoFacet = createFacet({ initialValue: 'initial value' })
 

packages/@react-facet/core/src/hooks/useFacetEffect.tsx

@@ -14,6 +14,10 @@ export const createUseFacetEffect = (useHook: typeof useEffect | typeof useLayou
     useHook(() => {
       let cleanup: void | Cleanup
 
+      if (facets.length === 0) {
+        return effectMemoized()
+      }
+
       if (facets.length === 1) {
         const unsubscribe = facets[0].observe((value) => {
           if (cleanup !== undefined) {