Add counterReducer spec

Author: alexandr-g

src/features/counter/counterReducer.spec.ts

@@ -0,0 +1,37 @@
+import { INCREMENT_COUNTER, DECREMENT_COUNTER } from './actionTypes'
+import counterReducer from './counterReducer'
+import { CounterActionTypes } from './types'
+
+describe('features > counter > counterReducer', () => {
+  it(`increments value, if ${INCREMENT_COUNTER} action is provided`, () => {
+    const initialState = {
+      value: 0,
+    }
+
+    const expectedState = {
+      value: 1,
+    }
+
+    const action: CounterActionTypes = {
+      type: INCREMENT_COUNTER,
+    }
+
+    expect(counterReducer(initialState, action)).toEqual(expectedState)
+  })
+
+  it(`increments value, if ${DECREMENT_COUNTER} action is provided`, () => {
+    const initialState = {
+      value: 0,
+    }
+
+    const expectedState = {
+      value: -1,
+    }
+
+    const action: CounterActionTypes = {
+      type: DECREMENT_COUNTER,
+    }
+
+    expect(counterReducer(initialState, action)).toEqual(expectedState)
+  })
+})