supplement unit test cases

__tests__/scheduler.test.tsx

@@ -0,0 +1,68 @@
+import { addTask, initiateWorkLoop } from '../src/workLoop'
+import { NORMAL_PRIORITY, IMMEDIATE_PRIORITY, LOW_PRIORITY } from '../src'
+
+describe('Scheduler', () => {
+  beforeEach(() => {
+    jest.useFakeTimers()
+  })
+
+  it('executes normal priority tasks in the order they are scheduled', async () => {
+    const log: string[] = []
+
+    addTask(NORMAL_PRIORITY, () => {
+      log.push('A')
+    })
+    addTask(NORMAL_PRIORITY, () => {
+      log.push('B')
+    })
+    addTask(NORMAL_PRIORITY, () => {
+      log.push('C')
+    })
+
+    expect(log).toEqual([])
+    initiateWorkLoop()
+    jest.runAllTimers()
+    expect(log).toEqual(['A', 'B', 'C'])
+  })
+
+  it('executes low priority tasks in the order they are scheduled', async () => {
+    const log: string[] = []
+
+    addTask(LOW_PRIORITY, () => {
+      log.push('A')
+    })
+    addTask(LOW_PRIORITY, () => {
+      log.push('B')
+    })
+    addTask(LOW_PRIORITY, () => {
+      log.push('C')
+    })
+
+    expect(log).toEqual([])
+    initiateWorkLoop()
+    jest.runAllTimers()
+    expect(log).toEqual(['A', 'B', 'C'])
+  })
+
+  it('executes callbacks in order of priority', async () => {
+    const log: string[] = []
+
+    addTask(NORMAL_PRIORITY, () => {
+      log.push('A')
+    })
+    addTask(NORMAL_PRIORITY, () => {
+      log.push('B')
+    })
+    addTask(IMMEDIATE_PRIORITY, () => {
+      log.push('C')
+    })
+    addTask(IMMEDIATE_PRIORITY, () => {
+      log.push('D')
+    })
+
+    expect(log).toEqual([])
+    initiateWorkLoop()
+    jest.runAllTimers()
+    expect(log).toEqual(['C', 'D', 'A', 'B'])
+  })
+})