Add IntersectionObserver utilities

app/assets/javascripts/lib/utils/intersection_observer.js

+import { memoize } from 'lodash';
+
+import { uuids } from './uuids';
+
+export const create = memoize((options = {}) => {
+  const id = uuids()[0];
+
+  return {
+    id,
+    observer: new IntersectionObserver((entries) => {
+      entries.forEach((entry) => {
+        entry.target.dispatchEvent(
+          new CustomEvent(`IntersectionUpdate`, { detail: { entry, observer: id } }),
+        );
+
+        if (entry.isIntersecting) {
+          entry.target.dispatchEvent(
+            new CustomEvent(`IntersectionAppear`, { detail: { observer: id } }),
+          );
+        } else {
+          entry.target.dispatchEvent(
+            new CustomEvent(`IntersectionDisappear`, { detail: { observer: id } }),
+          );
+        }
+      });
+    }, options),
+  };
+});

spec/frontend/lib/utils/intersection_observer_spec.js

+import { create } from '~/lib/utils/intersection_observer';
+
+describe('IntersectionObserver Utility', () => {
+  beforeAll(() => {
+    global.IntersectionObserver = class MockIntersectionObserver {
+      constructor(callback) {
+        this.callback = callback;
+
+        this.entries = [];
+      }
+
+      addEntry(entry) {
+        this.entries.push(entry);
+      }
+
+      trigger() {
+        this.callback(this.entries);
+      }
+    };
+  });
+  describe('create', () => {
+    describe('memoization', () => {
+      const options = { rootMargin: '1px 1px 1px 1px' };
+      let expectedOutput;
+
+      beforeEach(() => {
+        create.cache.clear();
+        expectedOutput = create(options);
+      });
+
+      it('returns the same Observer for the same options input', () => {
+        expect(expectedOutput.id).toBe(create(options).id);
+      });
+
+      it('creates a new Observer for unique input options', () => {
+        expect(expectedOutput.id).not.toBe(create({ rootMargin: '1px 2px 3px 4px' }));
+      });
+
+      it('creates a new Observer for the same input options in different object references', () => {
+        expect(expectedOutput.id).not.toBe(create({ rootMargin: '1px 1px 1px 1px' }));
+      });
+    });
+  });
+
+  describe('Observer behavior', () => {
+    let observer = null;
+    let id = null;
+
+    beforeEach(() => {
+      create.cache.clear();
+      ({ observer, id } = create());
+    });
+
+    it.each`
+      isIntersecting | event
+      ${false}       | ${'IntersectionDisappear'}
+      ${true}        | ${'IntersectionAppear'}
+    `(
+      'should emit the correct event on the entry target based on the computed Intersection',
+      async ({ isIntersecting, event }) => {
+        const target = document.createElement('div');
+        observer.addEntry({ target, isIntersecting });
+
+        target.addEventListener(event, (e) => {
+          expect(e.detail.observer).toBe(id);
+        });
+
+        observer.trigger();
+      },
+    );
+
+    it('should always emit an Update event with the entry and the observer', () => {
+      const target = document.createElement('div');
+      const entry = { target };
+
+      observer.addEntry(entry);
+
+      target.addEventListener('IntersectionUpdate', (e) => {
+        expect(e.detail.observer).toBe(id);
+        expect(e.detail.entry).toStrictEqual(entry);
+      });
+
+      observer.trigger();
+    });
+  });
+});