Migrate lazy_loader_spec to Jest

- Introduces mock_dom_observer which can be used
  to manually trigger MutationObserver or
  IntersectionObserver

app/assets/javascripts/lazy_loader.js

@@ -15,7 +15,7 @@ export default class LazyLoader {
   }
 
   static supportsIntersectionObserver() {
-    return 'IntersectionObserver' in window;
+    return Boolean(window.IntersectionObserver);
   }
 
   searchLazyImages() {

spec/frontend/helpers/mock_dom_observer.js

+/* eslint-disable class-methods-use-this, max-classes-per-file */
+import { isMatch } from 'lodash';
+
+/**
+ * This class gives us a JSDom friendly DOM observer which we can manually trigger in tests
+ *
+ * Use this in place of MutationObserver or IntersectionObserver
+ */
+class MockObserver {
+  constructor(cb) {
+    this.$_cb = cb;
+    this.$_observers = [];
+  }
+
+  observe(node, options = {}) {
+    this.$_observers.push([node, options]);
+  }
+
+  disconnect() {
+    this.$_observers = [];
+  }
+
+  takeRecords() {}
+
+  // eslint-disable-next-line babel/camelcase
+  $_triggerObserve(node, { entry = {}, options = {} } = {}) {
+    if (this.$_hasObserver(node, options)) {
+      this.$_cb([{ target: node, ...entry }]);
+    }
+  }
+
+  // eslint-disable-next-line babel/camelcase
+  $_hasObserver(node, options = {}) {
+    return this.$_observers.some(
+      ([obvNode, obvOptions]) => node === obvNode && isMatch(options, obvOptions),
+    );
+  }
+}
+
+class MockIntersectionObserver extends MockObserver {
+  unobserve(node) {
+    this.$_observers = this.$_observers.filter(([obvNode]) => node === obvNode);
+  }
+}
+
+/**
+ * Use this function to setup a mock observer instance in place of the given DOM Observer
+ *
+ * Example:
+ * ```
+ * describe('', () => {
+ *   const { trigger: triggerMutate } = useMockMutationObserver();
+ *
+ *   it('test', () => {
+ *     trigger(el, { options: { childList: true }, entry: { } });
+ *   });
+ * })
+ * ```
+ *
+ * @param {String} key
+ */
+const useMockObserver = (key, createMock) => {
+  let mockObserver;
+  let origObserver;
+
+  beforeEach(() => {
+    origObserver = global[key];
+    global[key] = jest.fn().mockImplementation((...args) => {
+      mockObserver = createMock(...args);
+      return mockObserver;
+    });
+  });
+
+  afterEach(() => {
+    mockObserver = null;
+    global[key] = origObserver;
+  });
+
+  const trigger = (...args) => {
+    if (!mockObserver) {
+      return;
+    }
+
+    mockObserver.$_triggerObserve(...args);
+  };
+
+  return { trigger };
+};
+
+export const useMockIntersectionObserver = () =>
+  useMockObserver('IntersectionObserver', (...args) => new MockIntersectionObserver(...args));
+
+export const useMockMutationObserver = () =>
+  useMockObserver('MutationObserver', (...args) => new MockObserver(...args));

spec/frontend/helpers/scroll_into_view_promise.js

-export default function scrollIntoViewPromise(intersectionTarget, timeout = 100, maxTries = 5) {
-  return new Promise((resolve, reject) => {
-    let intersectionObserver;
-    let retry = 0;
-
-    const intervalId = setInterval(() => {
-      if (retry >= maxTries) {
-        intersectionObserver.disconnect();
-        clearInterval(intervalId);
-        reject(new Error(`Could not scroll target into viewPort within ${timeout * maxTries} ms`));
-      }
-      retry += 1;
-      intersectionTarget.scrollIntoView();
-    }, timeout);
-
-    intersectionObserver = new IntersectionObserver(entries => {
-      if (entries[0].isIntersecting) {
-        intersectionObserver.disconnect();
-        clearInterval(intervalId);
-        resolve();
-      }
-    });
-
-    intersectionObserver.observe(intersectionTarget);
-
-    intersectionTarget.scrollIntoView();
-  });
-}

spec/frontend/helpers/wait_for_attribute_change.js

-export default (domElement, attributes, timeout = 1500) =>
-  new Promise((resolve, reject) => {
-    let observer;
-    const timeoutId = setTimeout(() => {
-      observer.disconnect();
-      reject(new Error(`Could not see an attribute update within ${timeout} ms`));
-    }, timeout);
-
-    observer = new MutationObserver(() => {
-      clearTimeout(timeoutId);
-      observer.disconnect();
-      resolve();
-    });
-
-    observer.observe(domElement, { attributes: true, attributeFilter: attributes });
-  });

spec/frontend/lazy_loader_spec.js

+import { noop } from 'lodash';
+import LazyLoader from '~/lazy_loader';
+import { TEST_HOST } from 'helpers/test_constants';
+import waitForPromises from './helpers/wait_for_promises';
+import { useMockMutationObserver, useMockIntersectionObserver } from 'helpers/mock_dom_observer';
+
+const execImmediately = callback => {
+  callback();
+};
+
+const TEST_PATH = `${TEST_HOST}/img/testimg.png`;
+
+describe('LazyLoader', () => {
+  let lazyLoader = null;
+
+  const { trigger: triggerMutation } = useMockMutationObserver();
+  const { trigger: triggerIntersection } = useMockIntersectionObserver();
+
+  const triggerChildMutation = () => {
+    triggerMutation(document.body, { options: { childList: true, subtree: true } });
+  };
+
+  const triggerIntersectionWithRatio = img => {
+    triggerIntersection(img, { entry: { intersectionRatio: 0.1 } });
+  };
+
+  const createLazyLoadImage = () => {
+    const newImg = document.createElement('img');
+    newImg.className = 'lazy';
+    newImg.setAttribute('data-src', TEST_PATH);
+
+    document.body.appendChild(newImg);
+    triggerChildMutation();
+
+    return newImg;
+  };
+
+  const createImage = () => {
+    const newImg = document.createElement('img');
+    newImg.setAttribute('src', TEST_PATH);
+
+    document.body.appendChild(newImg);
+    triggerChildMutation();
+
+    return newImg;
+  };
+
+  beforeEach(() => {
+    jest.spyOn(window, 'requestAnimationFrame').mockImplementation(execImmediately);
+    jest.spyOn(window, 'requestIdleCallback').mockImplementation(execImmediately);
+    jest.spyOn(LazyLoader, 'loadImage');
+  });
+
+  afterEach(() => {
+    document.body.innerHTML = '';
+  });
+
+  describe.each`
+    hasIntersectionObserver | trigger
+    ${true}                 | ${triggerIntersectionWithRatio}
+    ${false}                | ${noop}
+  `(
+    'with hasIntersectionObserver=$hasIntersectionObserver',
+    ({ hasIntersectionObserver, trigger }) => {
+      let origIntersectionObserver;
+
+      beforeEach(() => {
+        origIntersectionObserver = global.IntersectionObserver;
+        global.IntersectionObserver = hasIntersectionObserver
+          ? global.IntersectionObserver
+          : undefined;
+
+        lazyLoader = new LazyLoader({
+          observerNode: 'foobar',
+        });
+      });
+
+      afterEach(() => {
+        global.IntersectionObserver = origIntersectionObserver;
+        lazyLoader.unregister();
+      });
+
+      it('determines intersection observer support', () => {
+        expect(LazyLoader.supportsIntersectionObserver()).toBe(hasIntersectionObserver);
+      });
+
+      it('should copy value from data-src to src for img 1', () => {
+        const img = createLazyLoadImage();
+
+        // Doing everything that happens normally in onload
+        lazyLoader.register();
+
+        trigger(img);
+
+        expect(LazyLoader.loadImage).toHaveBeenCalledWith(img);
+        expect(img.getAttribute('src')).toBe(TEST_PATH);
+        expect(img.getAttribute('data-src')).toBe(null);
+        expect(img).toHaveClass('js-lazy-loaded');
+      });
+
+      it('should lazy load dynamically added data-src images', async () => {
+        lazyLoader.register();
+
+        const newImg = createLazyLoadImage();
+
+        trigger(newImg);
+
+        await waitForPromises();
+
+        expect(LazyLoader.loadImage).toHaveBeenCalledWith(newImg);
+        expect(newImg.getAttribute('src')).toBe(TEST_PATH);
+        expect(newImg).toHaveClass('js-lazy-loaded');
+      });
+
+      it('should not alter normal images', () => {
+        const newImg = createImage();
+
+        lazyLoader.register();
+
+        expect(LazyLoader.loadImage).not.toHaveBeenCalled();
+        expect(newImg).not.toHaveClass('js-lazy-loaded');
+      });
+
+      it('should not load dynamically added pictures if content observer is turned off', async () => {
+        lazyLoader.register();
+        lazyLoader.stopContentObserver();
+
+        const newImg = createLazyLoadImage();
+
+        await waitForPromises();
+
+        expect(LazyLoader.loadImage).not.toHaveBeenCalledWith(newImg);
+        expect(newImg).not.toHaveClass('js-lazy-loaded');
+      });
+
+      it('should load dynamically added pictures if content observer is turned off and on again', async () => {
+        lazyLoader.register();
+        lazyLoader.stopContentObserver();
+        lazyLoader.startContentObserver();
+
+        const newImg = createLazyLoadImage();
+
+        trigger(newImg);
+
+        await waitForPromises();
+
+        expect(LazyLoader.loadImage).toHaveBeenCalledWith(newImg);
+        expect(newImg.getAttribute('src')).toBe(TEST_PATH);
+        expect(newImg).toHaveClass('js-lazy-loaded');
+      });
+    },
+  );
+});

spec/javascripts/helpers/scroll_into_view_promise.js

-export default function scrollIntoViewPromise(intersectionTarget, timeout = 100, maxTries = 5) {
-  return new Promise((resolve, reject) => {
-    let intersectionObserver;
-    let retry = 0;
-
-    const intervalId = setInterval(() => {
-      if (retry >= maxTries) {
-        intersectionObserver.disconnect();
-        clearInterval(intervalId);
-        reject(new Error(`Could not scroll target into viewPort within ${timeout * maxTries} ms`));
-      }
-      retry += 1;
-      intersectionTarget.scrollIntoView();
-    }, timeout);
-
-    intersectionObserver = new IntersectionObserver(entries => {
-      if (entries[0].isIntersecting) {
-        intersectionObserver.disconnect();
-        clearInterval(intervalId);
-        resolve();
-      }
-    });
-
-    intersectionObserver.observe(intersectionTarget);
-
-    intersectionTarget.scrollIntoView();
-  });
-}

spec/javascripts/helpers/wait_for_attribute_change.js

-export default (domElement, attributes, timeout = 1500) =>
-  new Promise((resolve, reject) => {
-    let observer;
-    const timeoutId = setTimeout(() => {
-      observer.disconnect();
-      reject(new Error(`Could not see an attribute update within ${timeout} ms`));
-    }, timeout);
-
-    observer = new MutationObserver(() => {
-      clearTimeout(timeoutId);
-      observer.disconnect();
-      resolve();
-    });
-
-    observer.observe(domElement, { attributes: true, attributeFilter: attributes });
-  });

spec/javascripts/lazy_loader_spec.js

-import LazyLoader from '~/lazy_loader';
-import { TEST_HOST } from './test_constants';
-import scrollIntoViewPromise from './helpers/scroll_into_view_promise';
-import waitForPromises from './helpers/wait_for_promises';
-import waitForAttributeChange from './helpers/wait_for_attribute_change';
-
-const execImmediately = callback => {
-  callback();
-};
-
-describe('LazyLoader', function() {
-  let lazyLoader = null;
-
-  preloadFixtures('issues/issue_with_comment.html');
-
-  describe('without IntersectionObserver', () => {
-    beforeEach(function() {
-      loadFixtures('issues/issue_with_comment.html');
-
-      lazyLoader = new LazyLoader({
-        observerNode: 'foobar',
-      });
-
-      spyOn(LazyLoader, 'supportsIntersectionObserver').and.callFake(() => false);
-
-      spyOn(LazyLoader, 'loadImage').and.callThrough();
-
-      spyOn(window, 'requestAnimationFrame').and.callFake(execImmediately);
-      spyOn(window, 'requestIdleCallback').and.callFake(execImmediately);
-
-      // Doing everything that happens normally in onload
-      lazyLoader.register();
-    });
-
-    afterEach(() => {
-      lazyLoader.unregister();
-    });
-
-    it('should copy value from data-src to src for img 1', function(done) {
-      const img = document.querySelectorAll('img[data-src]')[0];
-      const originalDataSrc = img.getAttribute('data-src');
-
-      Promise.all([scrollIntoViewPromise(img), waitForAttributeChange(img, ['data-src', 'src'])])
-        .then(() => {
-          expect(LazyLoader.loadImage).toHaveBeenCalled();
-          expect(img.getAttribute('src')).toBe(originalDataSrc);
-          expect(img).toHaveClass('js-lazy-loaded');
-          done();
-        })
-        .catch(done.fail);
-    });
-
-    it('should lazy load dynamically added data-src images', function(done) {
-      const newImg = document.createElement('img');
-      const testPath = `${TEST_HOST}/img/testimg.png`;
-      newImg.className = 'lazy';
-      newImg.setAttribute('data-src', testPath);
-      document.body.appendChild(newImg);
-
-      Promise.all([
-        scrollIntoViewPromise(newImg),
-        waitForAttributeChange(newImg, ['data-src', 'src']),
-      ])
-        .then(() => {
-          expect(LazyLoader.loadImage).toHaveBeenCalledWith(newImg);
-          expect(newImg.getAttribute('src')).toBe(testPath);
-          expect(newImg).toHaveClass('js-lazy-loaded');
-          done();
-        })
-        .catch(done.fail);
-    });
-
-    it('should not alter normal images', function(done) {
-      const newImg = document.createElement('img');
-      const testPath = `${TEST_HOST}/img/testimg.png`;
-      newImg.setAttribute('src', testPath);
-      document.body.appendChild(newImg);
-
-      scrollIntoViewPromise(newImg)
-        .then(waitForPromises)
-        .then(() => {
-          expect(LazyLoader.loadImage).not.toHaveBeenCalledWith(newImg);
-          expect(newImg).not.toHaveClass('js-lazy-loaded');
-          done();
-        })
-        .catch(done.fail);
-    });
-
-    it('should not load dynamically added pictures if content observer is turned off', done => {
-      lazyLoader.stopContentObserver();
-
-      const newImg = document.createElement('img');
-      const testPath = `${TEST_HOST}/img/testimg.png`;
-      newImg.className = 'lazy';
-      newImg.setAttribute('data-src', testPath);
-      document.body.appendChild(newImg);
-
-      scrollIntoViewPromise(newImg)
-        .then(waitForPromises)
-        .then(() => {
-          expect(LazyLoader.loadImage).not.toHaveBeenCalledWith(newImg);
-          expect(newImg).not.toHaveClass('js-lazy-loaded');
-          done();
-        })
-        .catch(done.fail);
-    });
-
-    it('should load dynamically added pictures if content observer is turned off and on again', done => {
-      lazyLoader.stopContentObserver();
-      lazyLoader.startContentObserver();
-
-      const newImg = document.createElement('img');
-      const testPath = `${TEST_HOST}/img/testimg.png`;
-      newImg.className = 'lazy';
-      newImg.setAttribute('data-src', testPath);
-      document.body.appendChild(newImg);
-
-      Promise.all([
-        scrollIntoViewPromise(newImg),
-        waitForAttributeChange(newImg, ['data-src', 'src']),
-      ])
-        .then(waitForPromises)
-        .then(() => {
-          expect(LazyLoader.loadImage).toHaveBeenCalledWith(newImg);
-          expect(newImg).toHaveClass('js-lazy-loaded');
-          done();
-        })
-        .catch(done.fail);
-    });
-  });
-
-  describe('with IntersectionObserver', () => {
-    beforeEach(function() {
-      loadFixtures('issues/issue_with_comment.html');
-
-      lazyLoader = new LazyLoader({
-        observerNode: 'foobar',
-      });
-
-      spyOn(LazyLoader, 'loadImage').and.callThrough();
-
-      spyOn(window, 'requestAnimationFrame').and.callFake(execImmediately);
-      spyOn(window, 'requestIdleCallback').and.callFake(execImmediately);
-
-      // Doing everything that happens normally in onload
-      lazyLoader.register();
-    });
-
-    afterEach(() => {
-      lazyLoader.unregister();
-    });
-
-    it('should copy value from data-src to src for img 1', function(done) {
-      const img = document.querySelectorAll('img[data-src]')[0];
-      const originalDataSrc = img.getAttribute('data-src');
-
-      Promise.all([scrollIntoViewPromise(img), waitForAttributeChange(img, ['data-src', 'src'])])
-        .then(() => {
-          expect(LazyLoader.loadImage).toHaveBeenCalledWith(img);
-          expect(img.getAttribute('src')).toBe(originalDataSrc);
-          expect(img).toHaveClass('js-lazy-loaded');
-          done();
-        })
-        .catch(done.fail);
-    });
-
-    it('should lazy load dynamically added data-src images', function(done) {
-      const newImg = document.createElement('img');
-      const testPath = `${TEST_HOST}/img/testimg.png`;
-      newImg.className = 'lazy';
-      newImg.setAttribute('data-src', testPath);
-      document.body.appendChild(newImg);
-
-      Promise.all([
-        scrollIntoViewPromise(newImg),
-        waitForAttributeChange(newImg, ['data-src', 'src']),
-      ])
-        .then(() => {
-          expect(LazyLoader.loadImage).toHaveBeenCalledWith(newImg);
-          expect(newImg.getAttribute('src')).toBe(testPath);
-          expect(newImg).toHaveClass('js-lazy-loaded');
-          done();
-        })
-        .catch(done.fail);
-    });
-
-    it('should not alter normal images', function(done) {
-      const newImg = document.createElement('img');
-      const testPath = `${TEST_HOST}/img/testimg.png`;
-      newImg.setAttribute('src', testPath);
-      document.body.appendChild(newImg);
-
-      scrollIntoViewPromise(newImg)
-        .then(waitForPromises)
-        .then(() => {
-          expect(LazyLoader.loadImage).not.toHaveBeenCalledWith(newImg);
-          expect(newImg).not.toHaveClass('js-lazy-loaded');
-          done();
-        })
-        .catch(done.fail);
-    });
-
-    it('should not load dynamically added pictures if content observer is turned off', done => {
-      lazyLoader.stopContentObserver();
-
-      const newImg = document.createElement('img');
-      const testPath = `${TEST_HOST}/img/testimg.png`;
-      newImg.className = 'lazy';
-      newImg.setAttribute('data-src', testPath);
-      document.body.appendChild(newImg);
-
-      scrollIntoViewPromise(newImg)
-        .then(waitForPromises)
-        .then(() => {
-          expect(LazyLoader.loadImage).not.toHaveBeenCalledWith(newImg);
-          expect(newImg).not.toHaveClass('js-lazy-loaded');
-          done();
-        })
-        .catch(done.fail);
-    });
-
-    it('should load dynamically added pictures if content observer is turned off and on again', done => {
-      lazyLoader.stopContentObserver();
-      lazyLoader.startContentObserver();
-
-      const newImg = document.createElement('img');
-      const testPath = `${TEST_HOST}/img/testimg.png`;
-      newImg.className = 'lazy';
-      newImg.setAttribute('data-src', testPath);
-      document.body.appendChild(newImg);
-
-      Promise.all([
-        scrollIntoViewPromise(newImg),
-        waitForAttributeChange(newImg, ['data-src', 'src']),
-      ])
-        .then(() => {
-          expect(LazyLoader.loadImage).toHaveBeenCalledWith(newImg);
-          expect(newImg).toHaveClass('js-lazy-loaded');
-          done();
-        })
-        .catch(done.fail);
-    });
-  });
-});