Add some Jest tests

Author: Daniel Schmidt

src/intersection.js

@@ -33,15 +33,19 @@ export function observe(
     if (observerId) OBSERVER_MAP.set(observerId, observerInstance)
   }
 
-  INSTANCE_MAP.set(element, {
+  const instance = {
     callback,
     visible: false,
     options,
     observerId,
     observer: !observerId ? observerInstance : undefined,
-  })
+  }
+
+  INSTANCE_MAP.set(element, instance)
 
   observerInstance.observe(element)
+
+  return instance
 }
 
 /**

tests/intersection.test.js

@@ -12,17 +12,67 @@ const el = { el: 'htmlElement' }
 
 it('should observe', () => {
   const cb = jest.fn()
-  observe(el, cb)
+  const instance = observe(el, cb)
+
+  expect(instance).toMatchObject({
+    observerId: '0',
+    visible: false,
+    options: {
+      threshold: 0,
+    },
+  })
+})
+
+it('should observe with options', () => {
+  const cb = jest.fn()
+  const instance = observe(el, cb, { threshold: 0 })
+
+  expect(instance).toMatchObject({
+    observerId: '0',
+    visible: false,
+    options: {
+      threshold: 0,
+    },
+  })
 })
 
 it('should observe with threshold', () => {
   const cb = jest.fn()
-  observe(el, cb, 1)
+  const instance = observe(el, cb, { threshold: 1 })
+
+  expect(instance).toMatchObject({
+    observerId: '1',
+    visible: false,
+    options: {
+      threshold: 1,
+    },
+  })
 })
 
 it('should observe with Array threshold', () => {
   const cb = jest.fn()
-  observe(el, cb, [0.3, 0.6])
+  const instance = observe(el, cb, { threshold: [0.3, 0.6] })
+
+  expect(instance).toMatchObject({
+    observerId: '0.3,0.6',
+    visible: false,
+    options: {
+      threshold: [0.3, 0.6],
+    },
+  })
+})
+
+it('should observe with rootId', () => {
+  const cb = jest.fn()
+  const instance = observe(el, cb, { threshold: 0, root: {} }, 'window')
+
+  expect(instance).toMatchObject({
+    observerId: 'window_0',
+    visible: false,
+    options: {
+      root: {},
+    },
+  })
 })
 
 it('should unobserve', () => {