Reduce svg icon requests to one

To avoid duplicate requests this adds a memoized promise
that resolves to a DOM that contains GitLab icons in svg format.

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

-/* eslint-disable import/prefer-default-export */
-
+import { memoize } from 'lodash';
 import axios from '~/lib/utils/axios_utils';
 
 /**
- * Retrieve SVG icon path content from gitlab/svg sprite icons
- * @param {String} name
+ * Resolves to a DOM that contains GitLab icons
+ * in svg format. Memoized to avoid duplicate requests
  */
-export const getSvgIconPathContent = name =>
+const getSvgDom = memoize(() =>
   axios
     .get(gon.sprite_icons)
-    .then(({ data: svgs }) =>
-      new DOMParser()
-        .parseFromString(svgs, 'text/xml')
-        .querySelector(`#${name} path`)
-        .getAttribute('d'),
-    )
+    .then(({ data: svgs }) => new DOMParser().parseFromString(svgs, 'text/xml'))
+    .catch(() => {
+      getSvgDom.cache.clear();
+    }),
+);
+
+/**
+ * Clears the memoized SVG content.
+ *
+ * You probably don't need to invoke this function unless
+ * sprite_icons are updated.
+ */
+export const clearSvgIconPathContentCache = () => {
+  getSvgDom.cache.clear();
+};
+
+/**
+ * Retrieve SVG icon path content from gitlab/svg sprite icons.
+ *
+ * Content loaded is cached.
+ *
+ * @param {String} name - Icon name
+ * @returns A promise that resolves to the svg path
+ */
+export const getSvgIconPathContent = name =>
+  getSvgDom()
+    .then(doc => {
+      return doc.querySelector(`#${name} path`).getAttribute('d');
+    })
     .catch(() => null);

spec/frontend/lib/utils/icon_utils_spec.js

 import MockAdapter from 'axios-mock-adapter';
 import axios from '~/lib/utils/axios_utils';
-import * as iconUtils from '~/lib/utils/icon_utils';
+import { clearSvgIconPathContentCache, getSvgIconPathContent } from '~/lib/utils/icon_utils';
 
 describe('Icon utils', () => {
   describe('getSvgIconPathContent', () => {
     let spriteIcons;
+    let axiosMock;
+    const mockName = 'mockIconName';
+    const mockPath = 'mockPath';
+    const mockIcons = `<svg><symbol id="${mockName}"><path d="${mockPath}"/></symbol></svg>`;
 
     beforeAll(() => {
       spriteIcons = gon.sprite_icons;
@@ -15,45 +19,63 @@ describe('Icon utils', () => {
       gon.sprite_icons = spriteIcons;
     });
 
-    let axiosMock;
-    let mockEndpoint;
-    const mockName = 'mockIconName';
-    const mockPath = 'mockPath';
-    const getIcon = () => iconUtils.getSvgIconPathContent(mockName);
-
     beforeEach(() => {
       axiosMock = new MockAdapter(axios);
-      mockEndpoint = axiosMock.onGet(gon.sprite_icons);
     });
 
     afterEach(() => {
       axiosMock.restore();
+      clearSvgIconPathContentCache();
     });
 
-    it('extracts svg icon path content from sprite icons', () => {
-      mockEndpoint.replyOnce(
-        200,
-        `<svg><symbol id="${mockName}"><path d="${mockPath}"/></symbol></svg>`,
-      );
-
-      return getIcon().then(path => {
-        expect(path).toBe(mockPath);
+    describe('when the icons can be loaded', () => {
+      beforeEach(() => {
+        axiosMock.onGet(gon.sprite_icons).reply(200, mockIcons);
       });
-    });
 
-    it('returns null if icon path content does not exist', () => {
-      mockEndpoint.replyOnce(200, ``);
+      it('extracts svg icon path content from sprite icons', () => {
+        return getSvgIconPathContent(mockName).then(path => {
+          expect(path).toBe(mockPath);
+        });
+      });
 
-      return getIcon().then(path => {
-        expect(path).toBe(null);
+      it('returns null if icon path content does not exist', () => {
+        return getSvgIconPathContent('missing-icon').then(path => {
+          expect(path).toBe(null);
+        });
       });
     });
 
-    it('returns null if an http error occurs', () => {
-      mockEndpoint.replyOnce(500);
+    describe('when the icons cannot be loaded on the first 2 tries', () => {
+      beforeEach(() => {
+        axiosMock
+          .onGet(gon.sprite_icons)
+          .replyOnce(500)
+          .onGet(gon.sprite_icons)
+          .replyOnce(500)
+          .onGet(gon.sprite_icons)
+          .reply(200, mockIcons);
+      });
+
+      it('returns null', () => {
+        return getSvgIconPathContent(mockName).then(path => {
+          expect(path).toBe(null);
+        });
+      });
 
-      return getIcon().then(path => {
-        expect(path).toBe(null);
+      it('extracts svg icon path content, after 2 attempts', () => {
+        return getSvgIconPathContent(mockName)
+          .then(path1 => {
+            expect(path1).toBe(null);
+            return getSvgIconPathContent(mockName);
+          })
+          .then(path2 => {
+            expect(path2).toBe(null);
+            return getSvgIconPathContent(mockName);
+          })
+          .then(path3 => {
+            expect(path3).toBe(mockPath);
+          });
       });
     });
   });