test: add unit tests for usePrevious hook

packages/hooks/src/usePrevious/__tests__/index.spec.ts

@@ -0,0 +1,270 @@
+import { ref, computed, nextTick } from 'vue'
+import usePrevious from '../index'
+
+describe('usePrevious', () => {
+  describe('Basic functionality', () => {
+    it('should be defined', () => {
+      expect(usePrevious).toBeDefined()
+    })
+
+    it('should return undefined as initial previous value', () => {
+      const state = ref(0)
+      const previousValue = usePrevious(state)
+      expect(previousValue.value).toBeUndefined()
+    })
+
+    it('should return readonly ref', () => {
+      const state = ref(0)
+      const previousValue = usePrevious(state)
+      expect(previousValue.value).toBeUndefined()
+
+      // Should not be able to modify the returned value directly
+      // readonly refs in Vue log warnings but don't throw errors
+      expect(() => {
+        // @ts-expect-error - testing readonly behavior
+        previousValue.value = 1
+      }).not.toThrow()
+
+      // But the value should remain unchanged
+      expect(previousValue.value).toBeUndefined()
+    })
+  })
+
+  describe('State tracking', () => {
+    it('should track previous value when state changes', async () => {
+      const state = ref(0)
+      const previousValue = usePrevious(state)
+
+      // Initial previous value should be undefined
+      expect(previousValue.value).toBeUndefined()
+
+      // Update state
+      state.value = 1
+      await nextTick()
+
+      // Previous value should now be 0
+      expect(previousValue.value).toBe(0)
+
+      // Update state again
+      state.value = 2
+      await nextTick()
+
+      // Previous value should now be 1
+      expect(previousValue.value).toBe(1)
+    })
+
+    it('should work with string values', async () => {
+      const state = ref('hello')
+      const previousValue = usePrevious(state)
+
+      expect(previousValue.value).toBeUndefined()
+
+      state.value = 'world'
+      await nextTick()
+
+      expect(previousValue.value).toBe('hello')
+
+      state.value = 'vue'
+      await nextTick()
+
+      expect(previousValue.value).toBe('world')
+    })
+
+    it('should work with object values', async () => {
+      const state = ref({ a: 1 })
+      const previousValue = usePrevious(state)
+
+      expect(previousValue.value).toBeUndefined()
+
+      const newObj = { a: 2 }
+      state.value = newObj
+      await nextTick()
+
+      expect(previousValue.value).toEqual({ a: 1 })
+
+      state.value = { a: 3 }
+      await nextTick()
+
+      expect(previousValue.value).toStrictEqual(newObj)
+    })
+
+    it('should work with computed ref', async () => {
+      const baseValue = ref(10)
+      const computedValue = computed(() => baseValue.value * 2)
+      const previousValue = usePrevious(computedValue)
+
+      expect(previousValue.value).toBeUndefined()
+
+      baseValue.value = 20
+      await nextTick()
+
+      expect(previousValue.value).toBe(20) // previous computed value (10 * 2)
+
+      baseValue.value = 30
+      await nextTick()
+
+      expect(previousValue.value).toBe(40) // previous computed value (20 * 2)
+    })
+  })
+
+  describe('Custom shouldUpdate function', () => {
+    it('should use custom shouldUpdate function', async () => {
+      const state = ref(0)
+      const shouldUpdate = vi.fn((prev, next) => {
+        if (prev === undefined) return true
+        return Math.abs(prev - next) > 1
+      })
+      const previousValue = usePrevious(state, shouldUpdate)
+
+      expect(previousValue.value).toBeUndefined()
+
+      // First change should always update (from undefined)
+      state.value = 1
+      await nextTick()
+
+      expect(previousValue.value).toBeUndefined()
+      expect(shouldUpdate).toHaveBeenCalled()
+
+      // Change by 1, should not update due to custom logic
+      state.value = 2
+      await nextTick()
+
+      expect(previousValue.value).toBe(0)
+
+      // Change by 2, should update
+      state.value = 4
+      await nextTick()
+
+      expect(previousValue.value).toBe(2)
+      expect(shouldUpdate).toHaveBeenCalledTimes(4) // Initial + 3 updates
+    })
+
+    it('should handle undefined values in custom shouldUpdate', async () => {
+      const state = ref<number | undefined>(undefined)
+      const shouldUpdate = vi.fn((prev, next) => prev !== next)
+      const previousValue = usePrevious(state, shouldUpdate)
+
+      expect(previousValue.value).toBeUndefined()
+
+      state.value = 1
+      await nextTick()
+
+      expect(previousValue.value).toBeUndefined()
+      expect(shouldUpdate).toHaveBeenCalledWith(undefined, 1)
+
+      state.value = undefined
+      await nextTick()
+
+      expect(previousValue.value).toBe(1)
+      expect(shouldUpdate).toHaveBeenCalledWith(1, undefined)
+    })
+
+    it('should not update when shouldUpdate returns false', async () => {
+      const state = ref(0)
+      const shouldUpdate = vi.fn(() => false)
+      const previousValue = usePrevious(state, shouldUpdate)
+
+      expect(previousValue.value).toBeUndefined()
+
+      state.value = 1
+      await nextTick()
+
+      expect(previousValue.value).toBeUndefined()
+      expect(shouldUpdate).toHaveBeenCalledWith(undefined, 1)
+
+      state.value = 2
+      await nextTick()
+
+      expect(previousValue.value).toBeUndefined()
+      expect(shouldUpdate).toHaveBeenCalledWith(undefined, 2)
+    })
+  })
+
+  describe('Default shouldUpdate behavior', () => {
+    it('should use Object.is for comparison by default', async () => {
+      const state = ref(0)
+      const previousValue = usePrevious(state)
+
+      // Same value should not update
+      state.value = 0
+      await nextTick()
+
+      expect(previousValue.value).toBeUndefined()
+
+      // Different value should update
+      state.value = 1
+      await nextTick()
+
+      expect(previousValue.value).toBe(0)
+    })
+
+    it('should handle NaN values correctly', async () => {
+      const state = ref(NaN)
+      const previousValue = usePrevious(state)
+
+      expect(previousValue.value).toBeUndefined()
+
+      // NaN should equal NaN with Object.is
+      state.value = NaN
+      await nextTick()
+
+      expect(previousValue.value).toBeUndefined()
+
+      // Change to a different value
+      state.value = 1
+      await nextTick()
+
+      expect(previousValue.value).toBeNaN()
+    })
+  })
+
+  describe('Edge cases', () => {
+    it('should handle rapid state changes', async () => {
+      const state = ref(0)
+      const previousValue = usePrevious(state)
+
+      // Rapid changes
+      state.value = 1
+      state.value = 2
+      state.value = 3
+      await nextTick()
+
+      // Should track the previous value before the final change
+      expect(previousValue.value).toBe(0)
+    })
+
+    it('should handle boolean values', async () => {
+      const state = ref(true)
+      const previousValue = usePrevious(state)
+
+      expect(previousValue.value).toBeUndefined()
+
+      state.value = false
+      await nextTick()
+
+      expect(previousValue.value).toBe(true)
+
+      state.value = true
+      await nextTick()
+
+      expect(previousValue.value).toBe(false)
+    })
+
+    it('should handle null and undefined values', async () => {
+      const state = ref<string | null | undefined>(null)
+      const previousValue = usePrevious(state)
+
+      expect(previousValue.value).toBeUndefined()
+
+      state.value = undefined
+      await nextTick()
+
+      expect(previousValue.value).toBeNull()
+
+      state.value = 'hello'
+      await nextTick()
+
+      expect(previousValue.value).toBeUndefined()
+    })
+  })
+})