fix: ensure threshold fallbacks to options value if undefined (#416)

src/__tests__/hooks.test.tsx

@@ -1,5 +1,5 @@
 import React, { useCallback } from 'react';
-import { render } from '@testing-library/react';
+import { render, screen } from '@testing-library/react';
 import { useInView } from '../useInView';
 import { intersectionMockInstance, mockAllIsIntersecting } from '../test-utils';
 import { IntersectionOptions } from '../index';
@@ -257,3 +257,25 @@ test('should handle multiple hooks on the same element', () => {
   expect(getByTestId('item-2').getAttribute('data-inview')).toBe('true');
   expect(getByTestId('item-3').getAttribute('data-inview')).toBe('true');
 });
+
+test('should handle thresholds missing on observer instance', () => {
+  render(<HookComponent options={{ threshold: [0.1, 1] }} />);
+  const wrapper = screen.getByTestId('wrapper');
+  const instance = intersectionMockInstance(wrapper);
+  // @ts-ignore
+  instance.thresholds = undefined;
+  mockAllIsIntersecting(true);
+
+  screen.getByText('true');
+});
+
+test('should handle thresholds missing on observer instance with no threshold set', () => {
+  render(<HookComponent />);
+  const wrapper = screen.getByTestId('wrapper');
+  const instance = intersectionMockInstance(wrapper);
+  // @ts-ignore
+  instance.thresholds = undefined;
+  mockAllIsIntersecting(true);
+
+  screen.getByText('true');
+});

src/observers.ts

@@ -50,16 +50,15 @@ function createObserver(options: IntersectionObserverInit) {
   if (!instance) {
     // Create a map of elements this observer is going to observe. Each element has a list of callbacks that should be triggered, once it comes into view.
     const elements = new Map<Element, Array<ObserverInstanceCallback>>();
+    let thresholds: number[] | readonly number[];
 
     const observer = new IntersectionObserver((entries) => {
       entries.forEach((entry) => {
         // While it would be nice if you could just look at isIntersecting to determine if the component is inside the viewport, browsers can't agree on how to use it.
         // -Firefox ignores `threshold` when considering `isIntersecting`, so it will never be false again if `threshold` is > 0
         const inView =
           entry.isIntersecting &&
-          (observer.thresholds ?? [0]).some(
-            (threshold) => entry.intersectionRatio >= threshold,
-          );
+          thresholds.some((threshold) => entry.intersectionRatio >= threshold);
 
         // @ts-ignore support IntersectionObserver v2
         if (options.trackVisibility && typeof entry.isVisible === 'undefined') {
@@ -74,6 +73,13 @@ function createObserver(options: IntersectionObserverInit) {
       });
     }, options);
 
+    // Ensure we have a valid thresholds array. If not, use the threshold from the options
+    thresholds =
+      observer.thresholds ||
+      (Array.isArray(options.threshold)
+        ? options.threshold
+        : [options.threshold || 0]);
+
     instance = {
       id,
       observer,