alrert-reducer.test.js

import deepFreeze from "deep-freeze";
//importing deep-freeze, an npm module which helps to make sure I am not mutating state as that is the rule for a reducer

describe("Alert reducer", () => {
  it("Adds an error", () => {
    const initialState = {
        type:"",
        message:""
    };
    
    const action = {
      type: ADD_ERROR,
      payload: { type: "error", message: "oops, something went wrong" }
    };
    //declaring my action variable to let redux know what action to dispatch
    
    deepFreeze(initialState);
    //calling the deepFreeze function on the initialState to make its value “frozen” and ensure it is not been mutated.
    
    const updatedState = alertReducer(initialState, action);
    //updateState variable is the returned value of calling the alertReducer function declared earlier

    const expectedState = {
      type: "error",
      message: "oops, something went wrong"
    };
    
    expect(updatedState).toEqual(expectedState);
    //comparing the values of the updatedState with that of the expected state
  });
});