Actually broke the unobserve - change method, so it works in IE11 and other browsers.

Add a basic test to ensure calling observe/unobserve doesn't trigger an error

Author: Daniel Schmidt

src/intersection.js

@@ -45,12 +45,16 @@ export function unobserve(element) {
       observerInstance.unobserve(element)
     }
 
-    const itemsLeft = INSTANCE_MAP.values().some(
-      item => item.threshold === instance.threshold,
-    )
+    // Check if we are stilling observing any elements with the same threshold.
+    let itemsLeft = false
+    INSTANCE_MAP.forEach(item => {
+      if (item.threshold === instance.threshold) {
+        itemsLeft = true
+      }
+    })
 
     if (observerInstance && !itemsLeft) {
-      // No more elements to observe, disconnect
+      // No more elements to observe for threshold, disconnect observer
       observerInstance.disconnect()
       OBSERVER_MAP.delete(instance.threshold)
     }

tests/intersection.test.js

@@ -0,0 +1,19 @@
+import { observe, unobserve } from '../src/intersection'
+
+global.IntersectionObserver = jest.fn(() => ({
+  observe: jest.fn(),
+  unobserve: jest.fn(),
+  disconnect: jest.fn(),
+}))
+
+const el = { el: 'htmlElement' }
+
+it('should observe', () => {
+  const cb = jest.fn()
+  observe(el, cb)
+})
+
+it('should unobserve', () => {
+  observe(el, jest.fn())
+  unobserve(el)
+})