Use `Set` data structure instead of `Array`

`Set` is a better data structure because it has a faster search: `Set.has` has a O(1) while `Array.includes` has a O(n).

src/annotator/frame-observer.js

 import debounce from 'lodash.debounce';
 
-/** @typedef {(frame: HTMLIFrameElement) => void} FrameCallback */
-
-// Find difference of two arrays
-let difference = (arrayA, arrayB) => {
-  return arrayA.filter(x => !arrayB.includes(x));
-};
-
 export const DEBOUNCE_WAIT = 40;
 
+/** @typedef {(frame: HTMLIFrameElement) => void} FrameCallback */
+
 export default class FrameObserver {
   /**
    * @param {Element} element - root of the DOM subtree to watch for the addition
@@ -20,8 +15,8 @@ export default class FrameObserver {
     this._element = element;
     this._onFrameAdded = onFrameAdded;
     this._onFrameRemoved = onFrameRemoved;
-    /** @type {HTMLIFrameElement[]} */
-    this._handledFrames = [];
+    /** @type {Set<HTMLIFrameElement>} */
+    this._handledFrames = new Set();
 
     this._mutationObserver = new MutationObserver(
       debounce(() => {
@@ -50,7 +45,7 @@ export default class FrameObserver {
         frameWindow.addEventListener('unload', () => {
           this._removeFrame(frame);
         });
-        this._handledFrames.push(frame);
+        this._handledFrames.add(frame);
         this._onFrameAdded(frame);
       });
     } else {
@@ -63,22 +58,22 @@ export default class FrameObserver {
    */
   _removeFrame(frame) {
     this._onFrameRemoved(frame);
-
-    // Remove the frame from our list
-    this._handledFrames = this._handledFrames.filter(x => x !== frame);
+    this._handledFrames.delete(frame);
   }
 
   _discoverFrames() {
     let frames = findFrames(this._element);
 
     for (let frame of frames) {
-      if (!this._handledFrames.includes(frame)) {
+      if (!this._handledFrames.has(frame)) {
         this._addFrame(frame);
       }
     }
 
-    for (let frame of difference(this._handledFrames, frames)) {
-      this._removeFrame(frame);
+    for (let frame of this._handledFrames) {
+      if (!frames.has(frame)) {
+        this._removeFrame(frame);
+      }
     }
   }
 }
@@ -122,8 +117,8 @@ export function isDocumentReady(iframe, callback) {
  * can do that. See https://github.com/hypothesis/client/issues/530
  *
  * @param {Element} container
- * @return {HTMLIFrameElement[]}
+ * @return {Set<HTMLIFrameElement>}
  */
 export function findFrames(container) {
-  return Array.from(container.querySelectorAll('iframe[enable-annotation]'));
+  return new Set(container.querySelectorAll('iframe[enable-annotation]'));
 }

src/annotator/test/frame-observer-test.js

@@ -22,13 +22,13 @@ describe('findFrames', () => {
   it('should return valid frames', () => {
     let foundFrames = findFrames(container);
 
-    assert.deepEqual(foundFrames, []);
+    assert.lengthOf(foundFrames, 0);
 
     const frame1 = _addFrameToContainer();
     const frame2 = _addFrameToContainer();
     foundFrames = findFrames(container);
 
-    assert.deepEqual(foundFrames, [frame1, frame2]);
+    assert.deepEqual([...foundFrames], [frame1, frame2]);
   });
 
   it('should not return frames that have not opted into annotation', () => {
@@ -37,6 +37,6 @@ describe('findFrames', () => {
     frame.removeAttribute('enable-annotation');
     const foundFrames = findFrames(container);
 
-    assert.deepEqual(foundFrames, []);
+    assert.lengthOf(foundFrames, 0);
   });
 });